Inside My Old Tuner

In a recent post, I wrote about an old antenna tuner I built about 25 years ago. Although a description of it has been online for decades, I never posted pictures of it. So, here it is. 

I originally posted an article about this tuner on my QSL.net website under the title: A Simple and Flexible Tuner for QRP. Once my go-to transmatch for portable use, it had been on the shelf for quite a while. I hadn’t opened the case in 20 years, so it was a nostalgic walk down Memory Lane for me. 

Schematic of the "Simple and Flexible Tuner for QRP."
Schematic of the “Simple and Flexible Tuner for QRP.”

All of the parts used for this project came from my junk box or were re-purposed from other projects. This is the second tuner to inhabit this enclosure, so the variable capacitor and rotary switch were already in place. 

The coil is consists of 40 turns of enameled copper wire on a plastic 35mm canister. The wire appears to be 22 AWG. I wasn’t shooting for any particular inductance value; I just started winding turns. Based on the dimensions of the coil, the total inductance appears to be approximately 31 uH. I tapped it in 8 places and wired it to a rotary switch. I used two-sided foam tape to secure it to the bottom of the enclosure. I left the cap on the film canister so that the lid would press down slightly on it. This helps to securely hold the coil in place.

Inside view of the tuner
Inside view of the tuner

The variable capacitor was salvaged from an old radio by a friend of mine. It’s a two-section capacitor, totaling about 365 pf, according to my notes. I added a switch to select between one or both of the sections. Because the capacitor is sometimes in series with the coil, I used some thin fiberglass material to insulate it from the chassis. 

Front panel of my old antenna tuner. The switch selects one or both sections of the variable capacitor.
Front panel of my old antenna tuner. The switch selects one or both sections of the variable capacitor.

To the best of my recollection, I purchased the aluminum box at Radio Shack back in the day. I finished off the project with some embossed labels made on an old Dymo label maker. They look tacky, but they’re still holding up after all these years.

Rear view of my old tuner. The slide switch on the left selects the confiuration. In the "LO" position, the coil and capacitor are in series. In the "HI" position, the tuner is configured as an L-match tuner.
Rear view of my old tuner. The slide switch on the left selects the configuration. In the “LO” position, the coil and capacitor are in series. In the “HI” position, the tuner is configured as an L-match tuner.

After spending 15 or more years on the shelf, this funky-looking tuner has been seeing a lot more use lately. I mostly use it as an L-Match for end-fed wires. (I’ve only used the low impedance, series connection a few times over the years.) It’s a great portable tuner for QRP when weight isn’t a consideration. 

I have the parts on hand to build a lighter L-match when I need to carry a tuner in my backpack. Until I find the time to put it together, I’ll keep using this funky old tuner.

73, Craig WB3GCK

Speaker Wire EFHW Antennas

A while back, I challenged myself to see what kind of antennas I could make from a cheap 50-foot roll of two-conductor speaker wire. This time I made a couple of end-fed halfwave wires for the 40M and 20M bands. 

My aim with these projects is to make (nearly) full use of the 50 feet of speaker wire. I figured that would be enough for 66-foot and 34-foot radiators for the 40M and 20M bands, respectively. These dimensions work with the Hendricks SOTA Tuner (now sold by Pacific Antenna) I planned to use with them. 

Construction couldn’t be more simple:

  • Starting with 50 feet of speaker wire, separate the conductors.
  • Cut one of the wires into two lengths, 34 and 16 feet.
  • Splice the 16-foot wire onto the 50-foot wire. Now you have wires that are approximately a halfwave on 40M (66 feet) and 20M (34 feet).
  • I added spade lugs to one end of each wire. 
  • I used pieces of a used gift card to make end insulators that would allow for adjustments if needed. (See photo)
Speaker Wire EFHW Antenna diagram
Speaker Wire EFHW Antenna diagram

Of course, you’ll need an antenna coupler to match these wires to your rig. The SOTA Tuner I used worked fine, but each wire operated only on a single band. I cheated a bit and used some other scrap wire to make two short counterpoise wires, 5 feet for 40M and 3 feet for 20M. Of course, you could always use the 34-foot wire as a counterpoise for the 66-foot wire if you’d like. 

An improvised end insulator made from an old gift card. I used this so I could shorten the wire, if needed, by folding it back on itself.
An improvised end insulator made from an old gift card. I used this so I could shorten the wire, if needed, by folding it back on itself.

I haven’t tried it yet, but an L-network transmatch should allow the 66-foot wire to work on 40M, 20M, and 10M. A 49:1 transformer might also give you multiple bands with the 66-foot wire. You’ll likely need to adjust the length to obtain a match. You’re on your own here.

In the field, the SOTA Tuner provided a good match on both wires. I used the 66-foot wire as an inverted vee and the 34-foot wire as a sloper. I had no trouble making contacts on both bands with 5 watts.

Of course, you could build these antennas with any old wire. After all, it’s just wire. But, I enjoy the challenge of being constrained by the 50 feet of speaker wire. 

I have more speaker wire and more antenna ideas, so you’re going to be subjected to more of these crazy projects in the future.

73, Craig WB3GCK

Weather-Resistant UnUn

When camping or on vacation, one of my go-to antennas is a simple 29.5-foot wire and 9:1 unun. In these situations, the antenna is usually up for days, and I have to use plastic shopping bags to protect the unun from the elements. For this project, I attempted to build an unun that can withstand the elements.

I had been thinking about this for a while. I wanted something that would protect the internal parts and provide some protection for the coax connection. Eventually, my stash of PVC pipe odds and ends caught my attention. I figured if this stuff could keep water in, it should be able to keep water out. What I came up with is somewhat weird-looking, but it should do the job

This is the completed 9:1 UnUn.
This is the completed 9:1 UnUn.

Parts

Here are the major parts I used:

  • About 2.5 inches of 1.5-inch PVC pipe
  • (1) 1.5-inch PVC end cap (slightly rounded top)
  • (2) 1.5-inch PVC end caps with flat tops
  • (1) SO-239 panel-mount connector (along with some #4 hardware for mounting)
  • A 9:1 unun wound on a T130-2 toroid
  • (1) 10-24×3/4″ stainless steel screw (along with some #10 flat washers, nuts, wingnut, and lock washer)

I have to mention a few things about the parts. The PVC end-caps with flat tops are hard to find. If you search online for furniture-grade end caps, you might find some. For winding the toroid, the Emergency Amateur Radio Club in Hawaii (EARCHI) has excellent instructions you can download. 

Construction

I wasn’t sure how I was going to put this together until I started building it. So, these won’t be detailed, step-by-step instructions. They should, however, give you a general idea of how I ended up assembling it. 

  • First, I glued the two flat end caps together, end-to-end. 
  • While the glue was drying, I wound the unun. I left the leads a little longer than the EARCHI instructions, but I cut them back as needed during assembly. I used some #22 gauge solid hookup wire for the windings.
  • I drilled a 5/8-inch hole through the two attached end caps and installed the SO-239 connector. To keep things simple, I only used two screws to mount it. So, I only drilled two holes for the #4 machine screws for mounting. I also created a couple of weep holes to allow any condensation to drain out. I don’t know if these are needed or not, but they won’t hurt. 
  • I drilled a hole in the rounded end cap for the #10 screw. I made this hole a snug fit for the screw.
  • Next, I soldered the toroid input and ground connections to the SO-239. I left the toroid leads about 1.5 inches long. I used a small lug to attach the gound lead to one of the SO-239 mounting screws.
  • I then soldered a ring lug onto the end of the output wire (antenna connection) and attached it to the stainless steel bolt. I made sure that this output lead was just long enough to make the connection to the bolt. (You probably noticed a splice in this wire. I cut it by mistake, while installing the toroid. Stuff happens!)
  • I squeezed in some foam packing material on both sides of the toroid to hold it in place.
  • Finally, I press-fitted the top end cap. The end caps are on pretty tight, so I decided not to glue the parts together. With a little effort, I can still get inside of it if needed.
This is a view of the toroid. Before I closed it up, I wedged pieces of packing foam on either side of the toroid to hold it in place.
This is a view of the toroid. Before I closed it up, I wedged pieces of packing foam on either side of the toroid to hold it in place.

I don’t typically use radials with this setup, so I didn’t provide for an external ground connection. I rely on the coax shield for the necessary counterpoise. Should I ever need to, I can easily add a ground stud. 

This is a view of the bottom of the UnUn. I added two "weep holes," in case there's any condenstation inside. These probably aren't necessary.
This is a view of the bottom of the UnUn. I added two “weep holes,” in case there’s ever any condensation inside. These probably aren’t necessary.

Field Testing

I took the unun out for a test drive, and it performed as expected. With a 29.5-foot radiator and 25 feet of RG-8x coax, the internal tuner in my KX3 was able to load it up from 80M through 6M.  (This type of antenna is certainly compromised on 80M and 60M, but I have made lots of contacts with them.)

This is the weather-resistant unun in use. I used an adjustable bungee cord to strap it to the Jackite pole. The recessed connector helps to protect the coax connection from the elements.
This is the weather-resistant unun in use. I used an adjustable bungee cord to strap it to the Jackite pole. The recessed connector helps to protect the coax connection from the elements.

The Straight Key Century Club (SKCC) Weekend Sprintathon (WES) was in progress while I was out, so I made a few contest contacts. Running my usual 5 watts, I worked two French stations on 20M. I was also pleasantly surprised to have a station in Hawaii come back to my 5-watt CQ on 15M. So, it looks like it’s working. 

I also inadvertently tested the unun’s mechanical integrity. I accidentally dropped it twice before using it for the first time. No problems.

Conclusion

I admit I might have over-engineered this thing, but it was a fun project, nonetheless. Our first camping trip of the season is two weeks away. Hopefully, we won’t have any rain. But, if we do, my antenna will be ready for it.

73, Craig WB3GCK

More Fun with the Dollar Store Special

In a previous post, I mentioned an antenna of mine that went missing. The antenna in question was a variation of my old Dollar Store Special. After I built a replacement, I found the original in my truck. No problem; as the name suggests, it wasn’t a huge monetary investment. This antenna is just another example of what can happen with some extra speaker and too much time on my hands.

The original Dollar Store Special (circa 2005) was the first of several projects to see if I could build a usable antenna from a 50-foot length of inexpensive speaker wire. The resulting antenna was a 50-foot radiator and some counterpoise wires configurable for 40M, 30M, and 20M. I used one of these for years as a backup antenna. As with all random wire antennas, it requires a tuner and, of course, some way to get one end up in the air.

For this version, I went with a 50-foot radiator and two 25-foot radials. Besides being more simple to construct, it adds a little more flexibility. Space permitting, I can use the 50-foot wire in an inverted L, inverted V, or sloper configuration. When I need a quick way to get on the air, I can use a 25-foot radiator with a 25-foot counterpoise. (Elecraft documentation often recommends the 25-foot wires as a simple field antenna. [1][2])

My updated Dollar Store Special. In addition to this configuration, I sometimes use one of the 25-foot wires as the radiator and the other as a counterpoise.
My updated Dollar Store Special. In addition to this configuration, I sometimes use one of the 25-foot wires as the radiator and the other as a counterpoise.

I refer to this antenna—with tongue firmly planted in cheek—as the Dollar Store Special 2.0. That makes it sound like a bigger deal than it actually is. I should also note that I can no longer get speaker wire at my local dollar store. I have to spend a few dollars more now, but I kept the name anyway.

Construction is as easy as it gets:

  • Get a 50-foot length of two-conductor speaker wire. I use some inexpensive 24 gauge wire.
  • Separate the two conductors.
  • Cut one of the 50-foot wires in half.
  • I added a spade lug on one end of each wire and made a small loop in the other end.
  • I also added some Goop® sealant/adhesive to hold the end loops together and provide some strain relief to the spade lugs.

The 50-foot radiator and two 25-foot radials cover 60M through 10M using my KX3’s internal tuner. Feeding it through a 4:1 unun, I can cover 80M through 10M. A 9:1 unun works well with this length also. 

With a 25-foot radiator and a single 25-foot radial, my KX3 covers 40M through 10M with no problems. Adding in a 4:1 unun makes this a Rybakov 806 antenna that covers 60M through 10M. If you’re so inclined, you could partially unroll the 50-foot wire and use it as a second radial. 

These results, of course, are highly dependent on the tuner you’re using. There’s nothing special about the 50-ft length. You can trim the radiator back to a length that provides an easier match. I stayed with the 50-foot length since I wanted to make use of the entire pool of speaker wire for these projects. Go with whatever works for you.

I’ve had good results with both configurations, and I have been impressed with the 25-foot radiator and 25-foot radial configuration. Although it’s slightly compromised on 40M, it seems to get out pretty well. 

There’s nothing at all magical about this antenna; after all, it’s just three pieces of cheap wire. However, it makes a decent backup—or even a primary—antenna kit for portable use.

As I was writing this, I jotted down two more ideas for speaker wire antennas. Somebody stop me!

73, Craig WB3GCK

References:
[1] Elecraft KX3 manual, “Antennas,” p. 6
[2] Elecraft AX-Line Owner’s Manual, “Operating Tips,” p. 4

Speaker Wire Half-Square Antenna

I’ve been intrigued by the half-square antenna for some time now. I don’t have the real estate to put one up at home, so I built one for portable use. Like my other speaker wire projects, this antenna is built from a 50-foot length of cheap, two-conductor wire.

You can think of the half-square as two quarter-wave verticals spaced a half-wavelength apart. It provides some gain over a quarter-wave vertical and has a low take-off angle. The half-square has a bi-directional pattern with lobes broadside to the antenna and nulls off of the ends.

Normally, the half-square is fed with coax at the top of one of the vertical elements and functions as a single-band antenna. The coax should be kept perpendicular to the vertical leg, to avoid interaction. That arrangement, however, would be somewhat awkward for a portable antenna.

For expediency in the field, I went in a different direction. I decided to feed it at the bottom of one of the vertical legs, which is a high impedance point. I use a 9:1 unun to reduce the high input impedance to something easier for a tuner to handle.

I designed this antenna for the 20M band, but I wanted to use it on other bands as well. By using the 9:1 unun to feed the bottom of the antenna, I’m able to squeeze some more bands out of it. A tuner is required, of course.

Speaker Wire End-Fed Half-Square Antenna
Speaker Wire End-Fed Half-Square Antenna

Materials

Here’s what I used to build it:

Construction

Refer to the accompanying diagram to help make sense of the following steps.

  • Separate the speaker wire into two 50-ft wires
  • On one of the wires, install a spade lug at one end. This will be the connection to your matching device)
  • From the spade lug, measure up 16′ 7.2″ and make a small loop using two small zip-ties. 
  • From the second wire, cut a length that is about 16′ 9″ or so. 
  • Strip and splice the smaller wire to the end of the larger wire. After soldering it, I covered the splice with heat-shrink tubing. 
  • Next to the splice, make another small loop, using two zip-ties.
  • At the end of that wire, twist the wire to form an attachment loop. When you do this, make sure you have 16′ 7.2″ from the splice to the attachment loop.
  • I applied some Goop® adhesive to the loop at the end of the wire to hold it together. I also added Goop® to each of the other attachment loops.
  • As is my usual practice, I added some Goop® to where the wire enters the spade lug to add some strain relief.
  • At this point, the antenna is finished. You can, however, cut the leftover wire in half to make two radials for 20M (approximately 16 feet, give or take). I installed a spade lug on each of these wires and twisted the other ends to make a small loop. You guessed it; I put Goop® on these wires, as well.
This photo shows the splice and one of the mounting loops used in the Speaker Wire Half-Square.
This photo shows the splice and one of the mounting loops used in the Speaker Wire Half-Square.

Matching

[Update (6/17/2020) – After initially publishing this post, I received some great feedback from readers. As a result, I have updated, clarified, and expanded this section.]

For my first couple of outings with this antenna, I used a 9:1 unun as a quick and dirty way to get it on the air. I run about 18 feet of RG-8x coax from the unun to the radio. There’s nothing particularly critical about the coax length, but I would recommend a minimum of 16-feet for 40M and up. The exact length of the radials isn’t critical either since they’re laying on the ground. In fact, you can probably use the antenna without them. In this case, you’re relying on the coax shield for the counterpoise.

While the 9:1 worked fine, there are more efficient ways to match this antenna. I plan to continue experimenting with other methods to match the high-impedance input on 40M and 20M.

I haven’t tested them myself, but the end-fed halfwave tuners from Pacific Antenna and QRPGuys should work on 20M and 40M. They use a parallel resonant circuit and are designed to match an end-fed halfwave (EFHW) antenna. 

An EFHW transformer, like the ubiquitous 49:1 transformer, should also work. You will likely need to do some pruning on the antenna to get the SWR where you want it.

Finally, a simple L-Match antenna tuner with a tapped inductor in series and a variable capacitor across the output looks like it may be the best solution for me. It should handle the high impedances on 40M and 20M, and work on other bands like a random wire tuner. This will definitely be part of my next round of experiments.

Deployment

Deploying this antenna is a snap and takes me about 5 minutes. I use two collapsible poles to support it. I attach one corner to a partially-extended 28-foot Jackite pole. The feed point of the antenna is about 3 feet off the ground. 

These are the two poles I used to support the Speaker Wire Half-Square. Unfortunately, the wire is too thin to be visible in this picture.
These are the two poles I used to support the Speaker Wire Half-Square. Unfortunately, the wire is too thin to be visible in this picture.

I use a 20-foot Black Widow pole (actual length about 19.5 feet) to support the other end. I support this pole with an appropriately-sized screwdriver shoved in the ground. The handle of the screwdriver fits snugly inside the bottom section of the pole. After attaching the other corner of the antenna to top of this pole, I extend the pole and remove the bottom cap. Next, I walk the pole back until the horizontal section is taut. Then, I just shove the screwdriver in the ground and place the pole over it.

With appropriate trees nearby, you might be able to eliminate one or both of the poles. I’m not usually that lucky.

Results of Field Testing

I was pleased with the results of my initial field tests with the half-square. The internal tuner in my Elecraft KX3 was able to load the antenna from 80M through 6M. (Since the antenna’s input impedance is low on 80M, I wouldn’t recommend using the 9:1 there.) The SWR was 1.2:1 or better on all bands with the tuner. 

During my first outing with the half-square, I was able to make contacts on 40M, 20M, and 15M at 5 watts with no difficulty. The antenna is a half-wavelength on 40M, and it appears to play well on that band. I had numerous Reverse Beacon Network spots on 40M showing a signal-to-noise of 20db or better.

I also used it in the field during a recent QRP contest with similar results. Signals were strong on 40M, and I worked Georgia and Quebec on 20M.

This was hardly a rigorous scientific evaluation, but I’m happy with this antenna so far. One of these days, I’d like to do some modeling to see what the radiation patterns look like on the various bands. In the meantime, I’ll do some more experimenting with impedance matching.

Wrap-Up

This was an easy and fun project. It certainly made good use of a roll of cheap speaker wire. After using this antenna in the field a couple times, I have officially added it to my arsenal of portable antenna options.

73, Craig WB3GCK

My Activities of Late

I haven’t been posting much here lately. The COVID-19 pandemic and other family obligations have been cutting into my ham radio activities. Nevertheless, I do have a few projects in the works.

A few weeks ago, I started another project in my ongoing series of speaker wire antennas. This one will be a variant of the bi-square antenna. This antenna has the potential to be a little more field-friendly than the delta loop I tested last month. It’s all built; I just need to get out somewhere to set it up and see how it works.

I’ll file my next project under the category of Old Dogs/New Tricks. Back in December, I bought a Kenwood TH-D74a HT. That gave me the ability to reach a nearby D-Star repeater. This week, I purchased an MMDVM hotspot to go along with it. I plan to spend some time in the coming days getting it set up. I’m hoping to be able to eventually connect to the DMR talk groups used by my ARRL section and local ARES-RACES groups. Fortunately, my local group has some experienced hotspot users I can consult if I run into any snags. Wish me luck.

Sadly, our camping season with our little QRP Camper is off to a late start. State park campgrounds in our area have been closed due to pandemic. We have reservations at a state park in Maryland next month, however, and it looks that might be our first trip of the year. I’m looking forward to a little QRP-portable operating from the camper.

My local QRP club has started making plans for Field Day. We have a set of social-distancing guidelines we’ll be following this year. We’ll be limiting the number of participants, keeping our tents at least 10 feet apart, and eliminating common eating areas. Also, we won’t be sharing stations and equipment. This year’s Field Day will be different, for sure. 

Other than that, I’ve been active on our local ARES-RACES nets, and I have been checking into the Pennsylvania NBEMS Net on Sunday mornings. 

You can also find me on 40M or 80M CW in the evening. I usually hang out around the SKCC watering holes.

I’ll be posting more on all of this stuff in the coming weeks. Until then, stay safe, and I’ll see you on the air. 

73, Craig WB3GCK

A Homebrew Z-Match Transmatch

[This is an updated version of an article that originally appeared on my QSL.NET website. Although it’s twenty years old, I still occasionally hear from people who have built similar tuners.]

Antenna tuners (more accurately referred to as “transmatches”) make great homebrew projects; they are reasonably simple to build and, when finished, provide a useful piece of equipment. Every shack should have (at least) one. I built this one a couple of decades ago, and it’s still in use.

WB3GCK Z-Match Tuner - front panel
WB3GCK Z-Match Tuner – front panel

For this project, I decided to try my hand at building a Z-Match tuner from scratch. This type of tuner has been around for a while. While the Z-match can take on several variations, what distinguishes it from other circuits is that it is a resonant circuit that uses a fixed inductor. 

WB3GCK Z-Match Tuner - rear panel
WB3GCK Z-Match Tuner – rear panel

Z-Match tuners became very popular within the QRP community years back, thanks primarily to articles in QRP journals by Charlie Lofgren W6JJZ and the emergence of Z-Match tuners in kit form. Emtech sold its wildly popular ZM-2 kit commercially and the NorCal QRP Club began selling their BLT tuner kit (a W6JJZ design) like hotcakes.

Some Pros and Cons

Why the popularity? Here are some advantages that the Z-match design offers:

  • Matches balanced loads without the use of lossy baluns.
  • Being a parallel resonant circuit, the Z-match can provide some band-pass filtering for your receiver and harmonic attenuation for your transmitter.
  • A well-designed Z-match tuner has a high Q and is more efficient (less lossy) than other types of tuners.
  • The fixed inductor simplifies construction (no taps or rollers needed).
  • Using a toroid inductor and some small poly-film variable capacitors, the Z-match can be built into a very compact package. This sort of thing usually appeals to QRPers.

There is, of course, no free lunch here. Here are some disadvantages of the Z-match design:

  • Tuning is usually very narrow and can be a bit touchy sometimes to tune up
  • The range of impedances that can be matched is not as great as in other designs, such as the “T” configuration.

Design and Construction

I make no claims of originality for anything in my version of the Z-match. I based it on a classic design which was first appeared in SPRAT #84 (see the G3YCC web site for a schematic of the original design). This design, by the way, is similar to the one used in the Emtech ZM-2.

I incorporated a few modifications in my version, based on an article by W6JJZ (“The Z-Match: An Update”, QRP Quarterly, July 1995, pp 10-11). First, instead of the T-200-2 toroid specified in the SPRAT article, I used a T-200-6 core. W6JJZ recommends the Type-6 core over the Type-2 because it provides a higher Q over most of the HF range. The number of turns has to be adjusted for the Type-6 core, due to differences in permeability. Here again, I went with W6JJZ’s suggested turns count. Another reason for choosing the T-200-6 core was that I happened to have one in my junk box. How convenient!

Z-Match Tuner Schematic
Z-Match Tuner Schematic

The coil was wound using some #22 solid hookup wire (from Radio Shack) which I had laying around. The secondary winding is wound between the turns of the primary to ensure tight coupling. I added a toggle switch to ground one side of the secondary winding to accommodate single-ended loads (like a random wire). A piece of styrofoam was glued to the bottom of the enclosure to provide some support for the toroid and to keep it away from metal surfaces.

WB3GCK Z-Match Tuner - Inside view
WB3GCK Z-Match Tuner – Inside view

Another W6JJZ modification I used was the inclusion of a DPDT (center off) toggle switch to provide some flexibility with the input capacitor. Using this switching arrangement, I can select between one section of the capacitor, both sections in parallel, or both sections in parallel with a fixed 470pF mica capacitor. The extra input capacitance can sometimes be helpful on the lower frequencies.

The capacitors are poly film variable capacitors (2 sections @ 365pF each), which were originally purchased from Mouser Electronics. Unfortunately, Mouser no longer carries them, and I don’t know of another commercial source. I should have purchased a truckload of them when they were available! Similar capacitors with smaller values are still available if you look around.

The SWR bridge I used is a Dan Tayloe LED SWR indicator from a kit that was offered years ago by the Arizona scQRPions. It uses a resistive bridge circuit with a single LED to indicate a null when the bridge is balanced. For the 50-ohm resistors in the bridge, I substituted 2 100-ohm, 1-watt resistors. The bridge will handle a typical 5-watt QRP rig without flinching and could probably handle a bit more than that.

SWR Bridge Schematic
SWR Bridge Schematic

The whole thing was packaged in an enclosure which measures 3 x 5 x 2 inches. It certainly could have been built into a smaller package, but I had this enclosure on hand and decided to put it to use.

On the Air

To use the Z-Match, adjust the capacitors for a null in the background noise in your transceiver. That will get you close to a match. Then, switch in the SWR bridge, apply some RF, and tweak the capacitors for minimum brightness on the LED. There may be some interaction between the two capacitors, so you might have to go back and forth between them a time or two.

For an initial test, I hooked it up to the famous—in my mind, at least—WB3GCK Downspout Antenna. The little Z-match loaded up the downspout on 40 through 10 meters with no problems. On most bands, I could get the LED indicator to go completely out. On one or two bands, I couldn’t get it completely extinguished but it did give a definite null. Double-checking with a second SWR bridge indicated that the SWR was 1.5:1 or less in this condition. While tuned up on 40 meters, I had a quick QSO with a station near Chicago from here in southeastern Pennsylvania with 3 watts.

Wrap-Up

This little Z-Match tuner was one of my favorite—and most useful—projects. It’s a great accessory for QRP rigs that lack an internal tuner or SWR meter.

73, Craig WB3GCK

©2000-2020 Craig A. LaBarge

Speaker Wire Delta Loop

Here’s an example of what can happen when you have a hunk of cheap wire and a little too much time on your hands.

Years back, I did a write-up on a simple, random wire antenna made from a 50-foot roll of speaker wire from a local dollar store. I nick-named it the Dollar Store Special. I had a similar roll of wire in my junk box, so I set out to see if I could build another useful portable antenna from it.

This time out, I wanted to build something more elaborate than a random wire. After some sketching with a pencil and paper, I came up with this simple portable delta loop.

There are certainly better ways to construct a delta loop. However, I just wanted to see if I could build a functional antenna using only cheap speaker wire. So, with that in mind, here’s how I did it.

The Design

The antenna I built was inspired by a portable delta loop designed by Doug DeMaw, W1FB. [1] Doug’s multiband delta loop was designed for the 40M band and used a 300-ohm balanced feeder. 

According to Doug’s book, this type of antenna should work well on the fundamental frequency and higher. For the next band below the fundamental, he suggests connecting the feeder wires together and using it like a random wire. I figured I’d just try loading it up as is to see what happens.

Given that I constrained myself to a 50-foot roll of speak wire, I scaled my antenna for the 20M band. Using the formula, 1005 divided by the frequency in megahertz, I calculated a total length of 71 feet (21.6 meters) for the center of the 20M band. That would leave some of the two-conductor wire for an improvised balanced feeder.

Feeding the delta loop in a corner (with the apex of the loop pointing up), gives the antenna vertical polarity with a low take-off angle.[2] As with most antennas, higher is better. However, this antenna is still quite useful at practical heights in the field.

Since a tuner will always be necessary, I expended no effort trying to optimize the design.

Construction

Schematic diagram of the delta loop antenna
Schematic diagram of the delta loop antenna

If you’re a visual person like me, refer to the diagram to help make sense of the directions below.

  1. Measure off 35.5 feet from one end of the speaker wire. Place a small zip-tie around the wire at this point.
  2. Separate the 35.5-foot end of the speaker wire into two separate wires.
  3. Strip and solder the loose ends of the 35.5-foot wires together. Put some electrical tape or shrink tubing over the splice.
  4. Make 3 small loops in the wire, as shown in the diagram. You can see an example in the accompanying photo. These are going to be the attachment points. I used some Goop® adhesive on the zip-ties to help hold things in place.
  5. Finally, install some spade terminals on the ends of the shorter conductors. These will be used to attach the antenna to your tuner or balun.
Example attachment point. This is the feedpoint of the antenna. The two wires to the right are part of the loop antenna, while the wires towards the bottom serve as the balanced feedline. I used some Goop® adhesive on the zip-ties to help hold them in place.
Example attachment point. This is the feedpoint of the antenna. The two wires to the right are part of the loop antenna, while the wires towards the bottom serve as the balanced feedline. I used some Goop® adhesive on the zip-ties to help hold them in place.

Deployment

For my initial tests, I used a 28-foot Jackite pole to support the antenna. I only partially-extended the pole, such that the bottom of the antenna was about 4 to 5 feet off the ground. I used some nylon twine and a couple of tent stakes to tie off the two bottom corners.

This is the delta loop set up for my intial testing. The light-colored wires were difficult to photograph, so I enhanced them for visibility.
This is the delta loop set up for my intial testing. The light-colored wires were difficult to photograph, so I enhanced them for visibility.

The setup was somewhat more complicated than most portable antennas I use. It took me about 20 minutes to get it set up, but I suppose that wasn’t too bad for my first time. 

I used a couple of large tent stakes to keep the feedline off the ground. I connected the antenna to my KX3 using a 4:1 balun and a 1-foot piece of coax. 

I used a couple of large tent stakes to keep the balanced feedline portion of the antenna off the ground.
I used a couple of large tent stakes to keep the balanced feedline portion of the antenna off the ground.

Results

I first did a quick check to see what bands the KX3’s internal antenna tuner would handle. I found that I could load it up on every band from 60M through 6M, although I couldn’t get the SWR below 2:1 in the low end of 40M. That’s not surprising for a 20M loop, I suppose. I did have a usuable match between 7.030 and 7.060, where I normally operate.

I was only about 50 yards away from some powerlines, but the loop seemed quiet on receive. 

On 20M, a French station answered my third CQ. I also made contacts with Missouri and wrapped up with yet another French station. 

From the signal report the last station gave me, this antenna appears to do reasonably well with DX on 20M running QRP. It was a chilly and windy day, so I didn’t stay out there to try for contacts on other bands. 

Wrap-Up

Although my initial outing with this antenna was promising, I need to spend some more time using it on bands other than 20M. In any event, it was a fun—and cheap—antenna project.

73, Craig WB3GCK

References:

[1] DeMaw, D. (1991). Technical Bits & Pieces. In W1FB’s QRP Notebook (2nd Edition, pp. 157–161). Newington, CT: QST.
[2] DeMaw, D., & Aurick, L. (1984, October). The Full-Wave Delta Loop at Low Height. QST, 24–26.

Doublets I Have Known and Loved

In a recent post, I covered some (very) basic information about the venerable doublet antenna. This time around, I’ll cover some practical examples. These are antennas I have used and one unique design I know of.

Doublet Fed with TV Twinlead

My go-to portable antenna for several years was a simple doublet fed with 25 feet of that cheap, brown TV twin-lead. For the radiating elements, I used some #22 stranded hookup wire.

I first built the antenna as a 40M dipole fed with RG-174 coax. After a while, I wanted to cover multiple bands, so I removed the coax and replaced it with the twin-lead. I used a small piece of fiberglass perf board for the center insulator.

I have often used my homebrew Z-match tuner to load it up, although a 4:1 balun and a short run of coax to my rig’s internal tuner works fine, too. The whole antenna weighs next to nothing, and fits in a sandwich-sized Ziplock® bag.

Nothing fancy but it works great.

This is the center connector for my 66-foot doublet. The feedline is the old, cheap TV twin-lead.
This is the center connector for my 66-foot doublet. The feedline is the old, cheap TV twin-lead.

Up and Outer

The Up and Outer is simply a doublet with one vertical leg and one horizontal leg. I had done some experimenting with this old-time antenna and decided to build one to use while on vacation in the Outer Banks of North Carolina. 

I planned to support the vertical leg with a 28-foot Jackite pole, so I made a simple modification to a 44-foot doublet I had on the shelf. I spliced 6 feet of additional wire to each of the elements down to 28 feet each, and I was in business. Like my 40M doublet, the Up and Outer is fed with TV twin-lead and uses a perf board center insulator.

This antenna always goes with me on our annual Outer Banks vacation. I’ve used it from numerous beach rental houses, and it’s perfect for use on a second story deck. I used it last summer with great results, connecting it directly to my KX3. And, if I need to, I can use it as a normal horizontal doublet.

Appalachian Trail (AT) Dipole

This design is the brainchild of my friend, Ed Breneiser WA3WSJ, and goes back about 20 years. Rich Arland K7SZ, wrote about it in his QRP column in QST [1] back in 2001 and devoted a few pages to it in one of his books [2].

In simplest terms, it’s a 40M doublet made from #26 copper-clad stealth wire. Ed used a 3/4-inch PVC end cap for the center insulator (see photo). After soldering wires to an SO-239 socket and routing the wires through the end cap, the inside of the end cap is potted with epoxy. This makes it pretty much bomb-proof. 

The antenna is fed with 300-ohm ladder line, which is soldered to a PL-259 UHF connector. The PL-259 probably causes a slight imbalance, but in the field, you’ll never notice it. You can also feed it with coax and use it as a normal 40M dipole. Pretty cool, huh? 

When I built mine, I went with some #22 stranded hookup wire I had on hand. Although I departed from Ed’s design a bit, this doublet has been a reliable portable antenna over the years. 

My version of the WA3WSJ AT Dipole. The discoloration on the PVC end cap is from a mishap I had while potting it with epoxy. On the right is a PL-259 connector used with 300-ohm ladder line. As you can probably tell, this antenna has seen years of heavy use.
My version of the WA3WSJ AT Dipole. The discoloration on the PVC end cap is from a mishap I had while potting it with epoxy. On the right is a PL-259 connector used with 300-ohm ladder line. As you can probably tell, this antenna has seen years of heavy use.

WV0H Park Doublet

Myron WV0H designed a unique doublet that he dubbed The WVØH Park Portable DoubletHe uses two 50-foot pieces of wire to create a 60-foot doublet fed with a built-in open-wire feeder. I won’t attempt to offer a detailed description here; Myron’s blog post provides all the details you need to build one. Go check it out.

While I’ve never used Myron’s unique antenna, I can vouch that it works. I worked Myron a few years back while he was out in a park with his doublet. I can attest that it puts out a great QRP signal.

Wrap-Up

Well, that’s about it. If you need a reliable, easy-to-build, multi-band antenna, give the time-tested doublet a try.

73, Craig WB3GCK

References:
[1] Arland, R. (2001, July). QRP Power – Antenna Time. QST, p. 100.
[2] Arland, Richard K7SZ, Low Power Communication – The Art and Science of QRP, The American Radio Relay League, 2nd Edition, 2004, Chapter 6, pp. 6-36, 6-37

The Doublet – Revisiting a Classic Antenna

I was recently going through my stash of portable wire antennas and came across one of my old favorites—the doublet. I don’t see too many references to this type of antenna these days, but the doublet provides a great portable antenna option.

What is it?

The doublet, simply put, is just a dipole. The difference is that you feed it with a balanced feeder, rather than coax. 

Hams have been using doublets for many years. The earliest reference I could find in the ARRL QST archives was from September of 1929.[1] In this write-up, Clair Foster W6HM describes a 40M doublet fed with twisted wire lamp cord used for receiving. 

Advantages of the Doublet

The balanced feeder provides some advantages. Depending on how it’s constructed, it can withstand higher SWR with lower losses than coax. Because of the low losses at high SWR, you can use the doublet as a multi-band antenna. 

As a portable antenna, it’s hard to beat, especially as an inverted vee. Use a tree branch or telescopic pole to hoist up the center, tie off the ends, and you’re in business. [2]

Some Disadvantages

This multi-band capability comes with some disadvantages, though. Fortunately, none of them are insurmountable.

First, you’ll need a transmatch that can handle balanced feedlines. Typical commercially-available feedlines have either 300 or 450-ohm characteristic impedances. For open-wire feedlines, the impedance can sometimes be 600 ohms or more.  

I’ve had success using a homebrew Z-match tuner. Companies like Pacific Antenna and QRP Guys sell Z-match tuner kits that will handle balanced lines. They offer tuners that are small and light enough for QRP-portable use. 

You can also use a balun to transition from the balanced feedline to 50-ohm coax. While this isn’t an optimum approach, it works. Textbooks often recommend a 4:1 balun, and that’s a good starting point. If you go this route, I recommend keeping the coax as short as practical. If you run into matching problems on some bands, try another balun ratio (e.g., 1:1), or change the length of your feedline. 

The second drawback is that you need to be careful of how you route balanced lines. You need to avoid getting it too close to metal or laying it on the ground. Both can upset the line’s balance. This can cause it to radiate or introduce losses. If you have excess line, don’t coil it up. Operating outdoors, I found these restrictions aren’t very difficult to work around; you just need to be mindful of them. 

Construction

Building the doublet is pretty simple. Many folks suggest making the doublet a half-wavelength long at the lowest band you intend to use. I’ve built one for 40M and it worked well on 40M and higher. Depending on your tuner, it may also be usable on the next lower band.

L. B. Cebik W4RNL (SK) popularized the 44-foot doublet for 40M-10M. According to Cebik’s analysis, this length produces a more consistent radiation pattern across the bands. 

Doublet diagram. One rule-of-thumb suggests avoiding combinations of feedline electrical length (L1) and one leg of the radiator (L2) that are odd multiples of an eighth-wavelength.
Doublet diagram. One rule-of-thumb suggests avoiding combinations of feedline electrical length (L1) and one leg of the radiator (L2) that are odd multiples of an eighth-wavelength.

Regardless of the size of the doublet, you should try to avoid certain feedline lengths. One rule-of-thumb suggests avoiding combinations of feedline electrical length plus one leg of the radiator that are odd multiples of an eighth-wavelength. [3] If you run into matching problems, you can try adjusting the length of either the feedline or the radiating elements.

Feedline Options

You have several options here:

  • Commercial 450-ohm or 300-ohm ladder line. These are commonly available, and they work great. 
  • Homebrew open-wire feeders. This is the most efficient option. If you do some Internet searching, you’re likely to find lots of ways to build open-wire feeders. SOTABEAMS has a great example on their website.
  • TV twin-lead. I’ve used the cheap, brown stuff quite a bit for portable doublets. Unfortunately, it’s nearly impossible to find these days. If you come across it somewhere, stock up!
  • Lamp cord or speaker wire. This works and I’ve seen folks use it for portable antennas. However, it can be lossy, compared to window line or open-wire feeders.

More Later

If you want more technical details on this antenna, information abounds on the Internet and in antenna books. In particular, Cebik wrote some great articles that are worth searching for. 

In a future post, I’ll cover some practical examples that I have come across or used in the field.

73, Craig WB3GCK

References:
[1] Foster, C. W6HM (1929, September). Experimenters’ Corner: The “Doublet” for Receiving. QST, p. 39.
[2] DeMaw, D. W1FB (1991). Technical Bits & Pieces. In W1FB’s QRP Notebook (2nd Ed., pp. 157–161). Newington, CT: QST.
[3] Heys, John D., G3BDQ (1989). Center-fed antennas using tuned feedlines. In Practical Wire Antennas. Bedford, UK: Radio Society of Great Britain, p 7.