homebrew – WB3GCK QRP Amateur Radio

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
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!

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.

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

“Stationary-Mobile” with My 19-foot Vertical

Earlier this year, I built a lightweight, 19-foot vertical. Intended for tripod or ground mounting, I did the initial tuning and pruning of the vertical in that configuration. Today, I thought I’d see if it would work mounted on my pickup truck.

I have this plastic crate that I keep in the bed of the pickup truck. I use it to hold parts for my drive-on antenna mount, along with some tools and miscellaneous “stuff.” I hold the crate in place using bungee cords attached to a cargo bar that spans the width of the bed. I decided to make use of the crate as a quick and dirty antenna mount.

I took some 1-inch PVC pipe with a female threaded coupler from a previous antenna project and attached it to an inside corner of the crate with heavy-duty zip ties. I kept this part short enough to fit underneath the tonneau cover when traveling. To mount my 20-foot Black Widow pole, I used a 1-inch PVC male coupler and a reducer to go down to a 3/4-inch PVC pipe. The 3/4-inch PVC pipe fits nicely up inside the Black Widow pole. I went with the Black Widow pole rather than the lighter weight pole I normally use with this antenna since I already had all the PVC parts I needed to mount it.

My makeshift mount. The PVC mount is attached to the plastic crate, while the crate is attached to a cargo bar using bungee cords. The random junk I store in the crate keeps things stable.

I headed out to a local park today to give it a try. It only took a few minutes to get it set up. From the antenna, I ran some RG-8x coax through a window and into the cab of the truck. I connected the antenna ground to the body of the truck using a short piece of braid to a metal plate used to latch the tonneau cover closed.

The Black Widow pole installed on my makeshift mount.

I fired up my antenna analyzer and the SWR was off the charts. On closer inspection, I found the plate I was using for my ground wasn’t actually attached to the body of the truck. Instead, I connected two radials and ran them off the back of the truck. This time the SWR on 40 and 30 was much better. The resonant frequencies in this configuration were higher than when ground-mounted but my KX3’s internal tuner easily handled the minor mismatches.

I started out on 20 meters where this antenna operates as a random wire. I heard N5PHT doing a Parks on the Air (POTA) activation (KFF-3023) down in Texas. I gave him a call and exchanged reports. Moving down the band, I worked XE1XR in Mexico. So, the antenna seemed to be working fine. I checked 30 meters but it was devoid of activity.

Down on 40 meters, I had a nice ragchew with Bernard VE9BEL. Bernard was operating a club station (VE9CRM) in New Brunswick, Canada. He gave me a 599 and said I was “booming” into New Brunswick. Not bad for 5 watts into a 19-foot loaded vertical. I last worked Bernard a few years ago from Mt. Misery in Valley Forge National Park. We had strong signals both ways on that day, too.

So, it looks like this antenna is usable from the truck. I still need to find a way to connect the ground to the body of the truck. If possible, I’d like to avoid drilling holes in my new truck. This antenna is a little easier to deploy than my usual “Bike Rack Vertical.” The downside is I have to exit the truck to change bands. Life is a series of trade-offs, I guess.

73, Craig WB3GCK

Memorial Day Antenna Testing

Some time ago, I bought a small, lightweight telescopic fishing pole from a Chinese vendor on eBay. It’s about 19.5 feet tall and collapses down to about 26 inches. It’s a great size for backpacking or transporting on my bike. It weighs practically nothing. In fact, it’s too light for supporting anything but a lightweight vertical wire. Although I have used it a few times to support various antenna configurations, I never really found one that was a “keeper.”

Since I had some time over the long holiday weekend, I scratched out a quick design for yet another vertical antenna and cobbled it together with parts I had on hand in my junk box. I designed it to operate as a base-loaded resonant vertical on both 40 and 30 meters. On 20 meters and higher, it operates as a non-resonant wire; thus, an antenna tuner is required on those bands. Along with the loading coil, the matching unit contains a 1:1 choke balun to isolate the feedline. Both the choke balun and tapped loading coil are wound on toroids and mounted in a small, plastic enclosure. The radiator is a 19-foot piece of #28 wire. I could have shortened the radiator to make it resonant on 20 meters also, however, I went with the longer radiator for better performance on 40 meters. I used four 12.5-foot radials that I made from a 25-foot roll of cheap speaker wire.

The antenna I was testing. The white piece between the telescopic pole and the tripod is an adapter I made from PVC pipe.

Normally, I like to use the “build a little, test a little” approach. Since I don’t have the luxury of space at home for antenna testing, I just took my chances and built the whole thing. I headed out to a local park yesterday to give it the “smoke test” and see how close I came with my loading coil design.

My operating location on a cloudy and rainy morning

It took less than 5 minutes to set it up. I used an antenna analyzer to take some initial measurements. On both 20 and 30 meters, the resonant frequencies were low and fell outside the band. I still have some work to do there. On 20 meters and up, the KX3’s tuner loaded it up easily.

The antenna matching unit. The red jumper is used to change bands.

Next, I wanted to put it on the air. I started on 40 meters and used the KX3’s tuner to tweak the SWR. I called CQ a few times and eventually got a call from K4ALE in Virginia. Bevin said I was 559 with QSB. Despite the poor band conditions, we had a nice chat.

After I signed with Bevin, I set the antenna for 30 meters and kicked in the KX3’s tuner. I called CQ and was quickly answered by NN4NC in North Carolina. Jim gave me a 569. At times, the band would fade to just about nothing. As I was chatting with Jim, some drizzle started blowing in under the pavilion where I was sitting. So I signed with Jim and quickly packed up.

I’ll be doing some adjustments to the antenna over the coming weeks. It looks, though, that this could be a useful portable antenna, once I get the loading coil straightened out.

Since this is a work in progress, I left out the details for now. After I get the antenna working as intended, I’ll provide a detailed description, schematic and parts list in a future post.

72, Craig WB3GCK

Drive-on Portable Antenna Support

[This is an updated description of the drive-on antenna support that I have been using for many years. This version originally appeared in the July 2016 edition of QRP Quarterly in the “Idea Exchange” column. You can still find the older article here.]

Here’s a simple, inexpensive drive-on mast support that I have been using for more than ten years now. It’s been particularly handy for quick trips to the field, such as National Parks on the Air (NPOTA) activations.

Over the years, telescopic fiberglass poles have become popular as portable supports for lightweight antennas. Two popular suppliers of these collapsible poles are Jackite (http://www.jackite.com/) and SOTABeams (http://www.sotabeams.co.uk/). I typically use my 31-foot Jackite pole to support a vertical wire along the outside of the pole. I have also used them to support lightweight dipoles and a variety of end-fed wire antennas.

One trip to your local hardware store will get you everything you need for this project. To support a 31-foot Jackite pole, here’s what you’ll want to buy:

1-1/4 inch floor flange
18-inch length of 1-1/4 inch threaded steel pipe
(4) 1/4-20 x 1-1/2-inch flathead bolts
(4) 1/4-20 nuts
(4) 1/4-inch flat washers
(4) 1/4-inch lock washers
18 to 24-inch length of 1×8 lumber (I used a piece of maple. A piece of 1×6 lumber would also work)

[NOTE: I’ve heard that the dimensions of newer Jackite poles may be different. Use the dimensions given here as a general guide, and be sure to double-check the dimensions of your particular pole before buying materials.]

Figure 1. Drill 4 countersunk holes for the floor flange at the end of the board.

Assembly is pretty straightforward. Drill four holes to mount the flange to the board. The flathead bolts go in from the bottom. You need to countersink the bolts so they will flush with the bottom of the board. Attach the flange with the flat washers, lock washers and nuts. That’s about it.

Figure 2. Here is the floor flange mounted on the board.

To use the mount, I just set it on the ground and run one of my vehicle’s tires up on it. Next, I screw the threaded pipe into the flange. Once the pole is fully extended and the bottom cap removed, I just slide the pole over the pipe. For my 31-foot Jackite pole, I use a little electrical tape on the pipe to give a snug fit.

Figure 3. Drive onto the mount and screw in the pipe.

Figure 4. Drive-on mast in use supporting a vertical wire.

You can also adapt this for other size poles. For my 28-foot Jackite pole, for example, I use a 1-inch pipe. For my 20-foot Black Widow pole (https://www.bnmpoles.com/), I use a 3/4-inch pipe. You can buy reducers (adapters) in the plumbing department that will allow you to use the smaller diameter pipes with the 1-1/4 inch flange. If you only use one particular pole, you can always buy a smaller flange and build your mount with that.

This design is more than sufficient for a lightweight, telescopic fiberglass mast. If you need to support something heavier, like a steel mast, you’ll need a more robust support than this.

DE WB3GCK

MS2 Straight Key Magnetic Mount

As I mentioned in an earlier post, I bought the little American Morse MS2 straight key intending to somehow magnetically attach it to the clipboard I use for portable operating. It took some thinking but I came up with a workable solution. I might come up with a better solution in the future but, for now, it should suffice.

What I set out to do was build a wooden mount that could attach the MS2 that held two magnets that lined up with the steel washers on the clipboard. I had a couple of “super magnets” that I planned to use. The problem I ran into is that the magnets are almost too strong to attach directly to the washers. My solution was to enclose the magnets within the wood base.

Super magnets used for the MS2 straight key magnetic mount. Boy, these things are powerful! — Super magnets. Boy, these things are powerful!

I cut a 1×3.25-inch piece of 1/8-inch plywood. Then I drilled two 3/4-inch holes just deep enough to fit the magnets. After placing the magnets in the holes, I glued on a thin wood veneer. This puts some extra spacing between the magnets and the washers on the clipboard. After drilling a mounting hole for the MS2, I sprayed on a couple of coats of paint.

MS2 straight key magnet mount wood pieces prior to assembly — Wood pieces prior to assembly

After letting the paint dry, I went to attach the key to the base. Oops! I drilled the mounting hole from the wrong side of the mount. My first inclination was to putty it in and repaint. However, I decided to leave it there as a constant reminder to always measure twice and drill once!

MS2 straight key attached to the magnetic base — MS2 attached to the magnetic base. Don’t look too closely or you might see the drilling mistake I made.

The mount actually works well. The concealed super magnets hold the key firmly to the clipboard without the need for excessive force to remove it. Once I free up some time, I’ll give it a thorough test out in the field.

MS2 straight key mount attached to the clipboard — The finished product

72, Craig WB3GCK

Li-Ion Battery Pack

Here’s a little battery pack I put together for use as an external, portable power source for my YouKits HB-1B. I wanted something relatively lightweight and inexpensive that would put out at least 13 volts. This solution has fit the bill, so far.

There isn’t too much to it. I already had some Li-Ion cells on hand, so I wanted to make use of them. They are 18650 cells with a 6000 maH rating. I haven’t actually verified the claimed capacity but most cells tend to be somewhat over-rated. These particular cells are the “protected” type; each cell contains some circuitry that prevents overcharge and over-discharge. There are much cheaper unprotected cells but I’d rather be safe than sorry.

To put it together, I bought a 4-cell battery holder for 18650-size cells. With 4 fully charged cells, the voltage can exceed 16 volts. To keep the voltage below 14 volts (the maximum for my HB-1B), I put 3 silicon diodes in series with the output. This brings the voltage down to about 13.7 volts with fully charged cells. I also added a 2-amp fuse and an Anderson Powerpole connector.

To package it, I had a sandwich-sized Rubbermaid container that wasn’t being used. It turned out to be the perfect size to hold everything snugly. When not in use, everything is neatly tucked inside the container. In use, I lift one corner of the lid to bring out the connector.

For charging, I remove the cells from the holder and charge them with a Nitecore D4 charger. This is a 4-bay smart charger. It automatically detects the type of battery inserted and applies the proper charging method. Each bay works independently, so balanced charging is not an issue. The D4 works with a variety of battery types (Ni-Cad, NiMH, Li-Ion, etc.) so it is a handy accessory in the shack.

Nitecore D4 smart charger. Each cell is charged independently.

I haven’t done any formal testing of this battery arrangement, but it has provided adequate power for an afternoon of portable operating. For extended operating sessions, I throw 4 extra cells in my backpack that I can swap in if needed.

It’s not the most elegant solution but it works fine.

72, Craig WB3GCK

Line Isolator

One of my favorite portable antennas is a 30-ft wire fed through a 9:1 unun. This type of antenna generally the uses coax feeder as a counterpoise, since the 9:1 unun configuration provides no line isolation. Most of the time, this has worked well for me with no issues with stray RF getting back into the equipment.

On a couple of occasions, my Elecraft T1 auto tuner began to act up, refusing to load up on one or more bands. (Running through the T1’s diagnostic mode always seems to restore operation to normal.) I’ve also had one of my keyers behave erratically once or twice. Since this has only happened when using the 9:1 unun, my suspicion is that common-mode RF currents on the coax shield are the culprit.

My proposed solution for this is to use a line isolator between the tuner and the coax feeder. (Note: Using a line isolator at the antenna end of the coax would defeat the purpose in using the coax as a counterpoise.) A quick survey of my junk box stash of parts showed I had everything I need to build a line isolator from scratch.

Parts List

RG-174/U coax (approximately 24 inches)
FT-140-43 ferrite core
(2) BNC-F chassis mount connectors
Hammond Manufacturing 1591MSBK Enclosure (2.2 x 3.3 x 0.8 inches)

Construction

This is a very simple project. You can build one in well under an hour.

The RG-174 coax is wound on the FT-140-43 core for a total of 10 turns. Take note of how the 5th turn goes across the core. This makes installation in the case a little easier. I used a couple of small nylon tie-wraps to hold the windings in place.
Drill the holes for each of the BNC connectors and wired up the choke, as shown. I used a 5/64-inch drill bit and had to use a reamer to get the holes to the right size for the BNC connectors I used.
Solder the coax to the BNC connectors.
To mechanically secure the core, I used a piece of two-sided foam mounting tape to mount the choke to the bottom of the case. As an additional precaution, I put a piece of packing foam on top of the choke before attaching the lid. This foam provides a slight downward pressure on the choke to prevent it from shaking loose in the case during handling.

isolator_core_installed — Core installed in case

Testing

I don’t have access to the equipment necessary to do any type of exhaustive testing of the line isolator. In lieu of that, I hooked it up to a 50-ohm dummy load and checked the SWR. It was basically flat from 160M through 6M. While that tells me nothing about how effective it is in reducing common-mode currents, I at least know I didn’t make any serious screw-ups in building it.

2015-11-08 13.45.50 — Completed line isolator

In Operation

Well, this part will have to wait until I have a chance to get out for some portable operating. I want to make sure that the line isolator doesn’t affect the T1’s ability to tune my antenna. Since the initial problems were very intermittent, only time will tell if I solved those problems or not. I’ll be sure to update this post with any new insights I gain.

Update 5/16/2017:

Since this article seems to get a lot of traffic, I figured it was time for a long-overdue update. Not long after this post was published, I tested this 1:1 unun in line with the coax to my 30-foot wire and 9:1 unun. As I suspected it might, it affected the tuning of the antenna. One or two bands wouldn’t load up properly. This made sense to me, since this antenna configuration relies on the shield of the coax for the counterpoise. So, there’s some RF on the coax shield by design. This device obviously is blocking some RF, as it should. I haven’t pursued it further and I still have done any measurements to determine its effectiveness. With a change of connectors on the output side, it could definitely be useful as a 1:1 balun, I suppose.

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