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QRP End-fed Trapped Half Wave Antenna

This antenna is a variation of several versions I've seen online, with credit going to Adam Kimmerly K6ARK, Dan Koellen AI6XG, Dave Redfearn, Fred Maas KT5X, WG0AT, and other smart people along the way.  If I missed a name or call and you feel they should be cited, drop me a note.

Some of the previously mentioned designs were for me a tad tedious due to the tight work involved.  Their awesome work and guidance was the stimulus for me to create something that I could build while maintaining the electrical properties of their designs.

Design criteria dictated the finished product needed to be small, light, relatively easy-to-build, and durable. I wanted to have the option to build it with either a male or female BNC connector. This allows for direct connect to my MTR-3B, or via a small jumper.

Following is a list of parts employed in the build:

Transformer:

  • Male or female PCB mount BNC connector
  • PCB board
  • FT50-43 toroid
  • 100 pF, 500v SMD ceramic capacitor
  • 26 ga magnet wire
  • Heat shrink tubing
  • Five minute epoxy


Traps:

  • PCB board
  • (2) T37-2 Toroids
  • (1 ea.) 39 pF and 56 pF 500v SMD ceramic capacitors
  • 28 ga magnet wire
  • 5/8" and 1/4" heat shrink tubing


Antenna:

  • 26 ga Poly Stealth antenna wire


The Transformer
With parts in hand, I started with the PCB's shape by cutting to length and rounding sharp edges. Two holes (not shown) were drilled in the center of the narrow part to create stress relief for the radiating wire.

IMG 2685

Next up, was winding the the toroid. The primary consisted of 3 turns and the secondary (after tuning) was 26 turns, if I recall. Not pretty to look at, but it got the job done. I would like to hone my skills in this area.

IMG 2644

Then, I soldered the capacitor in shunt on the bottom of the board where the BNC center and ground pins connected.

IMG 2687

The toroid was soldered to the top of the board in front of the BNC; connections to tap and ground went underneath. Pro tip: Ensure the enamel is removed from the wire and TINNED. The reason for my emphasis is that all of my failed attempts resulted from poor tinning of the magnet wire. I found the best way to accomplish this was to turn up my iron's temperature to 400 C, place a nice ball of solder on the tip and dip the portion I wanted stripped and tinned into it. Easy peasy. I would then drop the temperature to my solder's requirements, clean/re-tin, and keep going.

IMG 2688

I built this transformer for 30 meters to be used in a 20 - 40 meter trapped antenna. Tuning was accomplished by cutting a 30m segment of wire and hoisting it to the top of my SOTABeams pole and trimming to resonance. Then, turns of the toroid were removed until I achieved a near flat VSWR as close to 50 Ohms as possible. For this process I used my RigExpert AA-35 antenna analyzer. You can also get away with using a $45 Nano VNA.

Once satisfied with the functionality of the circuit, I potted it to the board with epoxy and covered the unit with heat shrink tubing. It wound-up looking like this:

IMG 2690


The Traps
As this antenna was designed to be resonant on 3 bands, traps were employed to mitigate having to drop the pole to change bands via links, switches, etc.

The first and - most important - step to this process is to perform the math. If someone says to use this capacitor and that toroid with X windings for success, you may get lucky and have a resonant circuit but the odds are against you. Determine the LC resonance and toroid winding calculations with the proper formulas or go online to obtain the correct calculator. (I have added pertinent information at the bottom of the page).

As suggested by K6ARK Adam on his YouTube channel regarding this build, ideally you want to come in between 2-5 uH, so I targeted 3uH. With that in mind after performing calculations, I arrived at the following: For 20 meters, I calculated for a 39pF cap @ 14 mHz to equal 3.34uH. With a T37-2 toroid, this gave me 29 turns. Similarly, for 30 meters, I calculated for a 56pF cap @ 10.1 mHz to equal 4.43uH. With a T37-2 toroid, this gave me 33 turns.

Armed with this guidance, I set out to make the traps. First, with a Dremel tool. I cut down PCB board into single, six hole strips. Made a bunch of them for future use. Who doesn't like whittling on stuff with a rotary tool?

IMG 2536

The six holes are important. In the center 2, I soldered the cap. The next two out are where the radiant wire will be soldered to the cap with the last 2 furthest from center drilled out to provide stress relief for the wire itself. (Remember to use the proper value capacitor based on the frequency of the trap) Here's step 2, soldering the cap:

IMG 2598

Wind the toroid:

IMG 2602

Once that step is complete, tack the long ends of the toroid in preparation for tuning the circuit. To tune, place a short piece of wire (2-4") through the toroid and connect the wire's ends to a cobra head (BNC-to-dual banana plug). Place the assembly on your network analyzer, antenna tuner, oscilloscope, etc. and check the resonant frequency. You want to dip the circuit by removing turns of wire until you are at the low end of the band for which you are building. In this case, it's 14.0 mHz for one trap and 10.1 mHz for the other. Fine tuning can be obtained by compressing or spacing the wire turns on the toroid. Be very careful to not move the windings or you can change the resonance of the circuit.

IMG 2600

Once satisfied with the tuning, insert the board with the cap inside the toroid. Re-test one more time... Trim and solder the wires to the ends of the caps. This is delicate work and requires a steady soldering hand with a fine tipped soldering iron. When this is complete, pot the toroid with epoxy so that it does not move.

IMG 2601

Lastly, place a small ring of heat shrink tubing around the toroid to add resilience. Here is one already finished and on the wire:

IMG 2689


Putting it all together
Assuming you now have a properly tuned transformer on 30 meters and traps for 20 and 30 meters, we'll assemble the antenna. Anecdotally, I thought this would be a slam-dunk; no, it was not.

Start with a length of 26 gauge Poly Stealth wire cut a little longer than a half wave on 20 meters and connect your transformer to it. Then attach your analyzer. Check the resonant frequency and begin to trim (I did 6" at a time - very tedious) and measure. I continued to trim until I attained the lowest SWR (1.09:1) at 14.0 mHz. The 20 meter trap was soldered on at the end. Then, I took another 5 meters or so and trimmed for resonance at 10.1 mHz for 30 meters. Lastly, I added 5 meters and trimmed back until the wire was resonant at 7.0 mHz for 40 meters. During the construction, I noted that 20 and 40's SWR were good, but 30 climbed above 2.0. This required me cut out the 30 meter section and reintroduce another length. Trim, measure, trim, measure... Walking back and forth from end-to-end to trim and measure earned me a beer at day's end. I drank three for good measure.

All spooled-up and ready for deployment:

IMG 2691


Conclusions
SWR results were 1.35 - 1.55 on 20m, 1.23 - 1.45 on 30m, and 1.55 - 1.65 on 40m. It was the best I could get, but sufficient for the CW portion of each band.

I was close to throwing in the towel and prostituting myself to offer someone $100 to build me one of these things. That would have gotten me on the air sooner with the QRP rig in the field, but I wouldn't have learned anything, nor conquered my doubts. This was the first antenna project I have done in a long, long time. My eyes are a little weaker and hands a bit shaky, but with perseverance, I can still do anything.

Thanks to those who offered guidance and inspiration and to you for stopping by to read this.

Addendum
These two videos on YouTube were the framework for this project:
K6ARK's DIY Micro End-fed Halfway Matching Unit
K6ARK's HF Antenna Micro-Traps - DIY Build Project