According to the weather prognosticators, today is the start of a 4- or 5-day heatwave here in southeastern Pennsylvania. So, I wanted to get out early for some antenna testing before things heated up too much.
I planned to play around with the speaker wire end-fed halfwave antenna I built recently. To do this, I went back to my favorite antenna test range—my daughter and son-in-law’s property.
I set up the formerly 66-foot wire (now about 63 feet) in an inverted-V configuration. Since the last time I used this antenna, I trimmed off a couple of feet to see if I could get my little Hendricks SOTA tuner to load it on 20M.
I started with the SOTA tuner on the 40M band and worked a POTA activator in Indiana. I moved up to 20M, but the SOTA tuner wouldn’t tune below a 2.2:1 SWR.
Before I left the house today, I had the forethought to pack an old antenna tuner I built about 25 years ago. I used it extensively years ago, but I relegated it to the shelf when fancier equipment came along. I switched to the old tuner, which I configured as an L-match. It loaded up nicely on 40, 20, and 15 meters. As a bonus, the old-school tuner gave a good match on 30 and 17 meters.
After I finished experimenting with the antenna, I set out to make a few contacts. It was a busy day for Parks-on-the-Air (POTA) activators. I worked 10 of them in 8 states plus Puerto Rico. Three of the QSOs were on 17 meters. It was nice to hear POTA activity up there.
All in all, I was pleased with how my old homebrew antenna tuner performed. I don’t think it will be spending as much time on the shelf in the future.
I mentioned in a previous post my obsession with bags and cases for equipment. Well, this post is further evidence of that.
Over the past year or so, I purchased a couple of new HTs. It was a long-overdue upgrade. I first acquired a Kenwood TH-D74. About six months later, I came across a deal on an AnyTone AT-D878UV I couldn’t resist. Accessory-wise (batteries, chargers, antennas, etc.), these radios are very different. So, I wanted a way to organize these accessories and pack everything for travel and ARES-RACES events.
After looking at available options, I settled on an electronics travel organizer from a company called Bagsmart. They weren’t very expensive, so I bought one for each HT. I purchased my bags on Amazon for well below the list price shown on the manufacturer’s website. The specific models available on Amazon, however, seem to come and go.
The bag measures 9.4″L x 7.5″W x 2.8″H and is constructed of water-resistant—not waterproof—nylon. It weighs a mere 0.25kg/ 0.55 pounds. There three padded partitions that attach with Velcro that can be repositioned or removed. The bag also has a zippered mesh compartment under the lid that is great for storing cables, adapters, etc. There’s also a small compartment intended for memory cards or thumb drives. Despite its small size, it has sufficient room for everything I use for each radio.
These bags have been perfect for my needs, but they do have their limitations. While they offer some protection for your radios, we aren’t talking Pelican cases here. If you need something water-tight that you can bang around on a rock, these bags aren’t for you. These are light-duty bags, to be sure.
My only complaint with these bags is that the partitions are somewhat flimsy. Something a bit more rigid would be more to my liking. They do, however, keep things separated inside the bag.
There’s nothing earth-shattering here, but if you need an inexpensive way to organize your gear, this bag (or something similar) might do the trick. The usual disclaimer applies: I have no financial interest in this company or their products. I’m just a satisfied customer.
While trying mobile DMR last month, my (relatively) cheap hotspot experienced an untimely death. I set the hotspot aside as a future troubleshooting project.
While the Pi-Zero board appeared to be working, the MMDVM board was dead as a doornail. The OLED display was blank, and the RF side of the hotspot was non-responsive. It also wasn’t interacting with the Internet at all.
The other day, I came across a posting on a Facebook MMDVM page that shed some light on my problem. Some users reported shorts between the internal boards and the aluminum case. This tidbit of information prompted some further investigation.
I removed the top of the case, reinstalled the antenna, and applied power. To my surprise, the hotspot booted up and came back to life. I let it run for a while without the case to verify that all was well. It was.
I noticed a solder connection that extended a little beyond the edge of the MMDVM board. It appeared to be a power connection, so I suspect that might have been what shorted. I also noticed that a single screw and the header pins are all that secure the top board. I’m guessing the board shifted a small amount while mobile causing the short.
To remedy this, I applied electrical tape inside the top cover of the hotspot. Not being 100 percent certain where the problem was, I covered everything except the vent openings. I used a razor blade to trim the tape around the OLED display and antenna jack openings. I also took a file to the little solder blob I had noticed. (I exercised extreme caution in doing this since I’m sometimes prone to creating new problems.)
I reassembled the case and powered it up. Voila! It still worked. Hopefully, this fix will avoid any reoccurrences and make it withstand the vibrations of mobile use.
Although you’re far more likely to find me on CW on the HF bands, I do monitor Brandmeister TG-3142 (Pennsylvania State-wide) on DMR. And, on occasion, I listen to D-Star REF20A.
I haven’t bought any new ham radio toys lately, so I decided to upgrade the homebrew passive CW interface I use for contesting. I had been looking at the K1EL WKmini USB keyer for a while. I recently bought one, and it fit my needs exactly.
I’m not a big contester, but for Field Day, Winter Field Day, and some POTA activations, I key the radio using macros in the logging software. For years, I used a passive interface built into a DB-9 connector, along with a USB-to-RS-232 adapter. The interface consists of a resistor and a 2N2222 transistor. It served me well, but occasionally, there were some hiccups. With this simple interface, the logging software on the laptop is doing all the CW work. Once in a while, I noticed some timing issues in the code sent.
The WKmini is based on the WinKeyer 3 chip and designed for use with contesting logging software. The WKmini takes on the work of generating the CW, so it eliminates those timing issues. The logging software sends commands and data to the keyer, and the keyer does the rest.
The other nice feature is the paddle input. This feature allows me to instantly send CW manually when needed. I was able to do this in my previous setup, but it was a bit more complicated. The WKmini keyer is a more simple, elegant approach. Its small form factor makes it ideal for portable operating.
The WKmini was incredibly easy to set up. I connected the keyer to my laptop, and Windows immediately recognized it. I used the free K1EL WKscan utility to determine which COM port the keyer was using. I connected the keyer to my KX3 using a stereo patch cable with 1/8-inch connectors. Using the K1EL WK3demo utility, I was successful in keying up the radio and sending some code.
The last thing I needed to do was to configure my N3FJP logging programs to use the WKmini instead of the old passive interface. The WKmini doesn’t have any external controls; the logging software provides the necessary settings. There is a long list of software that supports WinKey keyers, including the N3FJP suite of software. The User Manual covers the N3FJP software, which was helpful. So, with a few mouse clicks, I was in business. All of this testing and setup took less than 15 minutes.
Like other K1EL keyer products I own, the WKmini is a solid performer. I’m hoping to give this little gem a workout during Winter Field Day later this month.
My XYL says I’m obsessed with bags, cases, and containers. She might be right.
During normal years, I participate in several public service events with my local ARES-RACES group. For a couple of those events, I’m often out on foot away from my truck with an HT. I wanted a convenient way to carry a few essentials for those situations.
I put together this little kit last Winter, but it hasn’t seen much use. Sadly, the pandemic forced the cancellation of our public service events this year. This year has been anything but normal.
I wanted something to carry the following items:
Spare battery for the HT
Small notepad and pencil
Minimal first aid kit (a few antiseptic wipes and bandages)
A few snacks
To carry everything, I bought a no-name water bottle carrier on eBay for about $13. I looked at lots of bottle carriers, but this inexpensive one was best suited to my needs. I’d provide a specific link, but these eBay offerings tend to quickly come and go. A search for “tactical military molle system water bottle bag” should should yield lots of options. I found some for less than $10. Of course, there are name brands out there that cost much more.
Here are the particulars of the one I bought:
The bag is constructed of 600D nylon. The specs say it’s waterproof, but I haven’t verified that.
The main compartment is 10.6″ tall and 4.3″ in diameter. It’s large enough to hold a 2-liter bottle. If I use a smaller water bottle, there’s room in this compartment for some other gear, too. It also has a zippered lid that will keep your gear from falling out.
There’s a 5.9″ x 4.3″ x 2″ gadget pouch on the front. This pouch is large enough for a couple of HT batteries, notepad, pencil, first aid kit, etc.
It has plenty of MOLLE webbing. A couple of them have Velcro for attaching patches. You could use the straps on the rear of the bag to attach it to another larger bag or your belt.
This bag certainly has room enough to carry everything I plan to carry in it. Heck, I’m sure I could carry a complete HF QRP station in it.
To hold my HT, I tried out several MOLLE-compatible pouches. The one I plan to use is a no-name item I bought from a Chinese seller on eBay. It only cost me about $4.00 (shipping included), but it works well with my current collection of HTs. I attached it to the side of the bottle carrier using the MOLLE webbing. An Internet search for “radio pouch” will bring you a dizzying assortment from which to choose.
As a final touch, I added a patch with my callsign on it. I ordered the 4″ x 1″ custom embroidered Velcro patch from a shop on Etsy. This little bit of vanity cost almost as much as the bag, but it looks good.
So far, I’ve only used this bag for a few short hikes. I haven’t used this bag for its intended purpose yet, but I’m hoping that will change next year. (Fingers crossed)
I mentioned in a previous post that I was going to venture into the digital voice modes. Well, I did. I was successful in getting a hotspot up and running, although I did hit a snag along the way.
Yeah, I know; as usual, I’m late to the party on this stuff. Up until now, I’ve had only limited experience with D-Star. Years ago, I used a borrowed Icom ID-1 for 1.2GHz data during a couple of ARES-RACES exercises. I hadn’t used D-Star again until I bought a Kenwood TH-D74A six months ago. Even then, I had only used D-Star to check into a local ARES-RACES net over a nearby D-Star repeater.
My local ARES-RACES group here in Chester County, PA, held one of our monthly Tech Rallies using Zoom. Jim WA3NOA gave an interesting talk on digital voice hotspots. Inspired by Jim’s talk, I went ahead and ordered an MMDVM hotspot from Amazon.
While waiting for the hotspot to arrive, I started studying up. I found lots of great articles and videos out there. W6GPS’s videos on the TH-D74 and D-Star were particularly helpful. Following one of his videos, I picked a hotspot frequency and programmed it into my radio.
The hotspot arrived a few days later. I was immediately struck how tiny this thing is. The instructions that came with it were sparse but clear enough to get started. I was able to successfully connect my laptop to the hotspot’s internal WiFi hotspot and access the Pi-Star configuration screens.
Long story short… Before too long, I had the hotspot connected to my home WiFi network and had the D-Star section configured. I keyed up my radio and received the expected D-Star response from the hotspot. Then, I tried to connect to some reflectors. No matter which one I tried, I got a “reflector is busy” message on my radio. Hmmm…
I sent screenshots of my hotspot configuration screens to Jim, but he didn’t see anything out of wack. I next focused on my WiFi router. I tried connecting the hotspot to other WiFi networks. No joy.
Finally, I went out and checked the status of my D-Star registration. That showed that my registration from way-back-when was missing a Terminal ID. An email exchange with WA3NOA confirmed that was likely my issue. My D-Star registration password no longer worked, so I reached out to Jim W3BIF, the admin for the W3EOC D-Star repeater.
The next morning I had an email from W3BIF saying he had straightened out my incomplete D-Star registration. Within minutes, I was connected to a reflector and having a QSO with a ham in Georgia. He reported that my audio sounded great.
I used to be one of those guys who thought this stuff isn’t real ham radio. I guess I’ve come around a bit. I won’t be giving up CW any time soon, but I will admit it has been fun playing around with this new (to me)mode over the past week.
Over time, I might branch out to try some other digital voice modes. In the meantime, you can often find me monitoring D-Star reflector, REF20A or REF30C.
[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.
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.
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!
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.
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.
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.
Like many of you around the world, I’m under a stay-at-home order, due to the pandemic. Taking advantage of my new-found spare time, I decided to take care of some overdue antenna maintenance.
I’ve been using my rainspout as my main antenna at home since 1993. It’s a compromise antenna, but it has given me a way to get on the air from my real estate-challenged home. Normally, I do some routine maintenance on it annually, but I’ve been lax lately. It’s been about two years or more since I’ve cleaned up the connections. This was the perfect time to get caught up.
My normal maintenance routine is to re-do the connections at the downspout. The connections are subject to exposure to the elements on the outside and rainwater coming down the inside. The downspout is on a south-facing side of the house, so the sealant on the connections also takes a beating from the sun.
Sometimes, my maintenance reminder is when I notice a change in antenna performance. Typically, it’s a change in the tuning. I might notice, for example, problems loading up on one or more bands. This time, it was an increase in noise levels.
So, I headed out to the downspout and removed the screws I use to make the connection. I had to scrape off enough of the sealant I use to get a screwdriver in there. Next, I took some light sandpaper and sanded around the connection points. A few times over the years, I drilled new holes for the connections. This time, I just needed to clean things up a bit.
I use two ring lugs, jumpered together, to make the connections (see photo). My rationale is that two connections are better than one. If needed, I replace the ring lugs and the screws, but this time, they were still in good shape. I just cleaned them up a bit and reinstalled them. I finished up by resealing everything with a generous amount of Goop® sealant/adhesive.
Next, I moved indoors and turned my attention to the ground connections.
I have a 1/2-inch copper pipe that I use as a ground bus for my station. The pipe runs across the back of my radio desk to an incoming cold water pipe about 6 feet away. I use hose clamps and braided strap to make the connections to it. The 1:1 unun that feeds the wire out to the downspout is also grounded to the pipe with a braided strap and a hose clamp. The pipe itself is connected to the incoming water pipe with hose clamps and a short braided strap.
I couldn’t remember the last time I cleaned up these connections, so, it was time to get busy. I removed the connections to the copper pipe ground bus and cleaned up the pipe with light sandpaper and steel wool. I reinstalled everything, making sure the connections were tight.
Turning on my transceiver, I was pleasantly surprised with the fruits of my efforts. My local noise levels have always been high, and 40M has been the worst lately. The noise was S5 or S6 at times, but now it was down to S1 or S2. The noise on 80M was also an S unit or two lower.
I wish I had taken care of this maintenance sooner. Next time, I’ll try not to procrastinate.
Stay safe in these difficult times. I’ll see you on the air.
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.
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  back in 2001 and devoted a few pages to it in one of his books .
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.
WV0H Park Doublet
Myron WV0H designed a unique doublet that he dubbed The WVØH Park Portable Doublet. He 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.
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:  Arland, R. (2001, July). QRP Power – Antenna Time. QST, p. 100.  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
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. 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. 
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.
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.
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.
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.  If you run into matching problems, you can try adjusting the length of either the feedline or the radiating elements.
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.
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:  Foster, C. W6HM (1929, September). Experimenters’ Corner: The “Doublet” for Receiving. QST, p. 39.  DeMaw, D. W1FB (1991). Technical Bits & Pieces. In W1FB’s QRP Notebook (2nd Ed., pp. 157–161). Newington, CT: QST.  Heys, John D., G3BDQ (1989). Center-fed antennas using tuned feedlines. In Practical Wire Antennas. Bedford, UK: Radio Society of Great Britain, p 7.