Many hams look forward to ARRL Field Day every year. It’s their chance to get outside and operate. Some local radio clubs sponsor group events where they get together, eat, socialize and sometimes do a little bit of radio.

Being an introvert, I’ve always skipped field day. I do a lot of POTA activating, which is pretty much the same thing. I figured I wasn’t missing much.

Hays-Caldwell ARC

I spend a lot of time activating Lockhart State Park US-3033. Over the last year or so, I’ve run into a number of hams there. They are usually getting to the park about the same time I’m leaving. It’s been fun to stop and chat with them. They all belong to the Hays-Caldwell ARC.

Side story: For a long time I was the POTA park leader there. I was a little bit like a ghost because none of them had ever seen me.

Anyway, they had plans to hold a field day event at Lockhart State Park. Even better, they were planning to use the Recreation Hall (built by the CCC) and the right introductions had been made so that they could use a portable tower being brought in by the TP&W Emergency Management folks.

So I decided I could suck it up and try to have some fun [not by myself, but with other people].

Camping

My son and I have been experimenting with camping in our Subaru Outback. I thought it would be fun to bring it out to the park and do a little radio the night before field day and just spend the night.

Bad Idea.

We really only camp during the cooler seasons. In South Texas, this usually means mid-to-late fall, early spring, etc. It turns out the Subaru is not a good place to spend the night at the end of June. I ended up spending the night in a Best Western, but only after drenching myself with sweat getting the sleeping area inside the car set up.

Lesson learned.

The Actual Field Day

I wish that field day started earlier in the day. I only planned to participate on Saturday. Waiting until 1 was kind of a pain in the neck. I did get to help the club up with setup though.

The tower was impressive. We didn’t end up putting it up all the way. I spent some time operating the station on 40 meters, getting about 15 contacts or so (all in Texas, except one from Louisiana).

When I do POTA, I log straight to paper. I use N3FJP for logging at home. I’d never done any contesting, so getting used to the field day exchange and using N3FJP in contest mode took a bit of work. It was fun though.

The Introvert Curse

Unfortunately I ran out of social energy about two hours in. I packed it in.

The takeaway for me is that I will need to start later next year. Either that means help less with set up, or just show up at 1 or 3—I dunno.

Still, it’s the most fun I’ve had with any club. (I’ve participated with three clubs at this point.) I’ve been joining their Tuesday night nets via my Allstar node (probably the subject of a future post).

Also, it was pretty cool finally getting to see the inside of the recreation hall. I’ve done a lot of reading about the CCC involvement with Texas State Parks, and find the history fascinating.

More Photos of the Area

Many of the structures at Lockhart State Park were constructed by the Civilian Conservation Corps (CCC) in the 1930s. The CCC was a depression-era work program that gave jobs to young men. Most of the workers were not local. They were organized into “companies” and stayed in camps.

First Experiences

Like a lot of hams, I felt overwhelmed surveying the world of antennas when I came to HF. And like a lot of hams, I built a dipole right off the bat. In fact it was a linked dipole for 40, 20, 15 and 10 based on the SOTA plans. That experience alone was enough to convince me that building, testing and tweaking antennas was fun.

Then I got into POTA. My starting antenna there was a base-loaded vertical that involved one of those ubiquitous 17ft whips. It got me on to 40 meters. That coil and the whip remains as the only antenna system I’ve ever bought (excluding my MFJ-1866 because it’s listen-only). It also continues to be my POTA go-to because it’s easy to: put up, take down, change bands.

Enter the Buddipole

During those early days I came across the buddipole plans by W3FF. I was getting a lot of enjoyment out of building antennas, so I thought “why not?”. I got the stuff and built one.

And it worked. It worked well in fact. My first DX contacts were using that dipole. I’d hoist it up on my 12m Spiderbeam and be off to the races.

My primary complaint was that it was kind of a pain in the neck to change bands. I’d bring it down, then essentially disassemble it to replace the loading coils. It was cumbersome and a little finicky.

KI5GRD Buddipole Variant

I figured I could improve on this design by using coils that would slide over the dipole arms and reattach using power poles. The coils themselves were just 14 AWG copper with heat-shrink. I designed and 3D printed a “puck tee” that sent the arms out at 5º angles to account for inevitable arm sag.

I was pretty happy with this design. Coil swap outs were quicker. I still use it regularly.

Two Whips and a Hub

Ever the tinkerer, I recently decided I could simplify this setup by just suing two 17ft whips. So I ordered a few from Aliexpress and set about to design a hub that I could slide on to my mast.

To keep the build simple, I decided to let friction keep them in place and have them overlap a little bit. Given that I’m using cheap imported whips, I figured it would be wise to let the print include a some support for the whips. I am tempted to add trusses to help support the whip.

The results are pretty good. I conducted preliminary tests in my back yard last night. I had a sideband QSO with a ham in Colorado using only 10 watts. I plan to take it out during my next POTA excursion.

Anyway, I’m including a build list here so you can make your own if you want.

73, Gary KI5GRD.

Two-Whip No Load Dipole

• 2x 5.6m whips. I wouldn’t order these ones again. They are M10 threaded, and each was attached to the brass thread a little differently.

• 3D printed hub - Tinkercad link.

• 2x M10 nuts. I got mine from Lowes.

• 1:1 CMC. Mine is built from a mix 31 toroid and some coax.

• Wire to connect things.

Picture Dump - KI5GRD Buddipole Variant

I thought I’d include pictures here. I was very proud of this build at the time.

• 3D printed puck tee - Tinkercad link.

** Kids growing up…

We’ve had a high school graduation this year, bringing the number of children attending college or otherwise pursing their careers to four. I write a lot of checks and don’t have time for much else. We decided to get into the landlord business by purchasing a rental at one of the kids universities. That’s been a huge time-sink over the spring and summer.

I hope things will slow down this fall. I’d like to get out and do more radio.

I had just finished building a moxon antenna for 10 meters and wanted to test it out. Actually, I needed to test several things out. I had never placed a mast on my homebrew hitch mount attached to this new car.

My pattern over the last year has been to simplify my kit. I had been using U-bolts to attach the mast to the long threaded bolt extending from the hitch. This was problematic for two reasons. First, I was concerned the U-bolts would eventually damage the mast. Second, they took time to tighten. My solution was to use zip ties.

I set up everything in my driveway the night before to see if the antenna needed any adjustments. The nano-VNA showed the antenna was just a little long, so I removed about an inch from each side of the driven element, as well as an inch from each side of the reflector. It showed the Moxon to be centered right around 28.300 MHz at with SWR at 1.15, which is right were I wanted it.

I headed out to Lockhart State Park the next morning, arriving around 7:30 am. The sun had been up for about 30 minutes and my car reported it was about 65º outside. Perfect weather.

The Setup

My plan was to set up in the parking lot (Google Maps) on the hill where the refectory is. The stalls are long, leaving plenty of space for setting up the antenna and a chair. It took about 15 minutes to get the antenna up. I’ve learned that it is best to store the wires left-to-right on a wire winder. This is so when you set up, you can just move from one side to the other for the 12-14 ft length of the antenna. The antenna went up without much fuss. I was even able to leave the Subaru hatch open during setup. (This was not possible with my VW.) I estimate the antenna was elevated about 15 feet above the ground.

BTW, here’s the equipment I’m using today:

• Rig: Yaesu FT-891

• Antenna: Homebrew 10 meter moxon with a RG-316 pigtail.

• Feed line: 35’ RG-58 with a homebrew common mode choke attached to the pigtail.

• Battery: Homebrew 100 Ah camping monster, charged to about 30 Ah. I’ve been testing it at the house. This was its first time out in the field.

• Other stuff: Yamaha CM-500 headphones, homebrew mic spliltter and PTT grip.

The Activation

In the past, the most European DX I’ve had was about 5 contacts. I’m pretty sure it was there at Lockhart and I would have been using my homebrew buddie-pole on 20 meters. Knowing that 10 meters has been a hot band lately, I was hoping improve on that. I would have been over the moon with 10 DX contacts.

Tuning around the band at 8 am I could hear lots of activity. I found an open spot at 28368 kHz, spotted myself and keyed up. My first contact was a UK station. So was my 3rd and 5th. In fact, 9 out of the first 20 QSOs were DX. I was pretty excited. Things went on like this for the next 40 minutes or so.

Other Hams

Around 8:45 a few other cars came into the parking lot. One of them had what looked like a 2 meter whip. I had ran into other hams here before, but usually as I was leaving. I could see them coming over to chat, so I announced I would be QRT for break and took my headphones off.

The other individuals were indeed hams. I had the pleasure of meeting Jim KA6J and Gary W5GW. We chatted for about 15 minutes. They come up to Lockhart regularly, but often during the week. We’d never met but were aware of each other from the park leader board.

They let me know about the Hays Caldwell club so I could get connected there.

The Rest of the Activation

The next 40 minutes were much like the first. Plenty of domestic stations, but lots of DX too.

Some notable [near] QSOs…

• There was a QRP station from Equador I tried to complete a QSO with for several minutes. I could make out everything but his call sign. At the time I didn’t know where he was. If I did I could have rotated the antenna so that he wasn’t on my backside. Maybe next time.

• The other was with a ham in New Hampshire operating at 1/4 watt. Nice!

Aftermath

I ended up with 89 QSOs for the day, 34 of them were DX. Pretty amazing. I had set the antenna up pointing abuot 40º from my azimuth location, which would put Europe in my crosshair. The map of QSOs certainly agrees. All the QSOs were in that direction.

Here are the entities I ended up working:

• Austria

• Bulgaria

• Canada

• Croatia

• Dominican Republic

• England

• France

• Germany

• Greece

• Ireland

• Italy

• Netherlands

• Sweden

I never expected to have a morning like this. It exceeded my expectations by an order of magnitude.

This antenna is a keeper! Be on the lookout for another post containing build instructions in the next week or so.

The Venerable Wolf River Coil

One of the first pieces of kit I bought after acquiring a Yaesu FT-891 and building a linked dipole was a Wolf River Coil setup consisting of their 17ft whip, a Silver Bullet Mini and whatever they call their 24 inch tripod (TIA or something?). To date it is the ONLY antenna I have ever bought*.

It has served me well. The whip is going on four years. I bought its replacement two years ago thinking it was about to go any day.

The coil is rock-solid too. Except I haven’t really used it that much. I used to do 40 meter POTA activations regularly. I’d set up and run on 40 before switching over to 20. At some point I stopped doing that and just started each activation on 20 meters.

I’ve been wanting to get back into 40 meters. And while I could just start using the coil again (and I have!), I’d kind of like something I could just screw in that gives me 40 meters without any adjustment.

The folks at WRC noticed this demand and began offering something called the “Sporty 40”. It’s reasonably priced at around $30 plus shipping. Unfortunately, they’ve temporarily shut the store down due to health problems. I don’t know when they’ll open back up.

Since I’ve got money to burn, I decided I would try my hand at building one.

* I’m not counting the MFJ-1886 mounted on the shed.

Supplies

Here’s what you’ll need:

• A 1 inch PVC coupling.

• Two 1 inch PVC plugs.

• 2x 3/8-24 stainless steel lock washers.

• 2x 3/8” stainless steel washers.

• 3x 3/8-24 stainless steel nuts.

• 2x 1.5 inch 3/8-24 stainless steel bolts.

• 14 AWG enameled magnet wire. About 9 ft.

• A 1-1/8 inch 3/8-24 stainless steel coupling nut.

• About 2.5” of 2” ID shrink wrap (wide enough to slide over the PVC coupling)

Other Items:

• a drill with

  • a 1/8” bit and

  • a step bit

• a few adjustable crescent wrenches

• possibly a mallet

• needle nose pliers

• something to cut heavy wire.

• loctite (optional)

NOTE: I’ve created some 3D-printable templates that make it easy to mark up the PVC parts for drilling. You can find them on Tinkercad.

Step 1: Prepare the Ends

You’ll do this twice…

Drill a 1/8” starter hole in the center of two PVC plugs (template). Next use a step bit to drill a hole large enough to pass a 3/8” bolt through the center of each PVC plug. Take care to ensure you get it dead-on center.

Place a regular washer on a bolt and pass it through so that it comes out of the top of the plug. (The head of the bolt will be on the inside of the plug.)

Add a lock washer and nut to the bolt shaft and tighten it as hard as you can. Use some Loctite if you want.

Do it gain for the other plug. You should have two plugs that are more or less identical.

Step 2: Prepare the Coupling

You’ll end up drilling 4 holes in the coupling that the wire will pass through. The holes should be opposite each other and on each end about 9.5 mm from the end of the coupling. Using the template will help tremendously.

In any event, you’ll want 30 mm between the edges of the holes so that there is enough space for the coils coils to be wrapped. (17 turns of 14 AWG means you’ll need 1.15 inches at the very least, but usually a bit more.)

Step 3: Attach the Plugs

Attach a plug to the coupling and re-drill the wire holes so that they go all the way through the plug that is currently blocking the wire from going all the way through. Then attach the second plug and do the same thing. You may need to use a vise or a mallet to get the job done—the fitting is pretty tight. Because of this I do not recommend using glue.

Also, if you’re going to apply force, apply it to the PVC, not the screws.

You are now in possession of a pretty decent coil form.

Step 4: Wind the Coil

I use a measurement template for this part. Mine is made from a 3x5 card.

Remove the enamel from about 1.3 (measurement B) inches of the end of your magnet wire. Thread it all the way through one end of the coil form so that 2.5 inches are poking out (including the bit you just stripped, see measurement A). This excess will need to be bent up and wrapped clockwise around the protruding bolt. Take a regular nut and hand tighten it over the wire.

Start winding the other end of the wire around the coil form. Orientation matters a little here. I like to start with the side I just threaded down and wind from the bottom up, rotating the form clockwise. (This gives you a “right-handed” turn.)

You need to make 17 full revolutions (“turns” in coil parlance). When that is done, send the remaining wire all the way through your holes, pull tight with your pliers, and bend the wire up. Measure 4.25 inches of wire and cut (measurement C). This will be enough fold up and around the screw after you’ve scraped the enamel off the last 1.3 inches of it.

Wrap it clockwise around the bolt and add a coupling nut.

If your coils are loose, this is your opportunity to tighten them. There’s no easy way to do this. I just hold the coil with both hands and twist in the direction of the coils.

Once you’re happy with the windings, add a little loctite to each end and tighten the regular nut and the coupling nut with wrenches. We don’t want them going anywhere.

Your coil is functionally complete.

Step 5: Testing

If you have the capability, I recommend testing the inductance of your coil. I’ve made several of these now. They usually measure somewhere between 10.7μH and 11.1μH when measured from screw to nut (end to end). JAN 2025 UPDATE: As I’ve gotten better at building and measuring these I find that inductance is closer to 15μH. I find it a little easier to measure inductance before attaching the coil to the plug/bolt assemblies.

Finally, go test it with your whip. Make sure your radials are laid out. I’ve found that when I connect it to my CHA SS17 whip that it will tune the low end of 40 meters fully extended. To reach the phone portion I have to tuck in 6-8” of the whip. Your mileage may vary.

Step 6: Shrink Wrap (Optional)

This isn’t required, but gives the coil a nice finished look and will protect it from getting scratched. It also keeps the coils in place.

Cut 2.5 inches of 2-inch ID (51 mm) shrink wrap. I like using the marine grade type that has glue on the inside, but you can use whatever you have.

Working with this stuff is a little tricky because it can shift as it’s heated. Here’s the technique I’ve adopted:

1. Get the tubing centered on your coil.

2. Hold down in the middle gently with a pair of needle-nosed pliers. This prevents any shifting.

3. Begin applying heat the entire length of the tubing.

4. Once that side feels like it’s not going to shift, roll the coil 90 degrees and repeat these steps.

Remember: PVC can soften or melt at high temperatures, so don’t over do it.

That’s it. You’re done. Have fun with your creation!

Shameless Plug

I’ve bought enough hardware to make about 50 of these during cold and wet weekends this winter. I’ve set up a storefront at https://southtxradio.square.site/ and will be selling them there. $40 will get one of these shipped right to your door.

If you’re adventurous and would prefer a kit, please reach out to gary@ki5grd.org. I have enough parts leftover to build a few.

Unfortunately they haven’t been taking orders due to illness.

I had most of the hardware laying around, so I decided to build my own. I’ve got a post coming out soon that documents the build, so will be no secrets. That said, I’ve decided to put a few up for sale.

You can buy them over on my Square Storefront if you’re interested. I’ll also stick other items there too as needed.

I picked up a set of CM500 headphones when I brought my Flexradio station online. They came recommended from a number of folks on the Flexradio forums. At the time the price was only $60 representing a good combination of price and performance.

I ended up building an adapter recommended by those good folks had had a good experience using the headset with the Flex.

The Yaesu FT-891

By that point I had already begun doing a good number of POTA activations. For audio I had been using a set of earbuds connected to the rig and the stock mic. However, winter was coming and I wanted something that would allow me to more easily keep my hands warm.

I surmised that a proper headset and a PTT foot switch would do the trick. But given the voltage differences between the mic connector on the FT-891 (+5v) and the Flex (+3v) I knew I would have to wire up something different.

The internet is full of schematics from people creating adapters FT-891 adapters for their Heil headsets. I built one of those first, but ran into problems (can no longer find the design), so I began tweaking. This post serves to document what I ended up with.

The Schematic

Notes

• You can fiddle with resistor values. I think calculations originally called for for 415 and 275 ohms for R3 and R4, but I didn’t have any of those sitting around. Likewise, I would have preferred a single 4.7 uF cap, but I didn’t have one of those either.

• My schematic includes a momentary switch mounted to the perf board so that I don’t have to rely on an external switch to be connected.

• You’re really dealing with two circuits here. The bottom is for PTT. The top is for the microphone.

• You can chose the wiring for the right hand side of the PTT connector (which wires to assign to T, R or S). Just make sure your switch wiring matches when you wire up the male end.

• The switch is completely optional. I just wanted an easy way to test PTT without having to plug something in. I also figure it might be handy in the field. (To date, I have never used it. 😁)

The Build

I ended up putting the whole thing on a perf board, then fitted the perf board into a custom designed 3D printed case.

I had the following on hand that I wanted to incorporate:

• RJ45 right angle solder chassis mount - Inconvenient because the pins are offset and DO NOT line up with the holes on the perfboard. You end up having to force them to fit. Because of that, you need to MAKE SURE to solder down the anchor tabs to the PCB or use some kind of adhesive.

• 1/8” TRS connectors - these fit right into the perf board.

The 3D printed case is designed so that you just snap the PCB in. No tabs, screws or anything like that. (Tinkercad case)

Here are some pictures of the build:

Apologies for the sloppiness of the wiring. I still haven’t figure out how everybody else manages such tidy builds.

I’ve been using this for over a year now with dozens of activations. It does the job well.

Backstory

I’ve spent the summer putting together a QRP kit that will let me do SO2R digital activations using low-cost QDX radios (4-5 watts). My last experiment had me burning out the finals of my original QDX low-band kit for the second time! I made an attempt to rewire it for 9 volts, but I think it has succumbed to it’s injuries and my successive attempts to repair it.

So I ordered another. This time a v6 board (old radio had a v2 board).

At the same time I’ve been putting together a trapped vertical built for 12-17-30 meters (my WARC special!) that matches my existing 10-15-20 meter trapped vert.

Putting It All Together

Today was my latest attempt at bringing them all together. I drove out to Lockhart State Park (US-3033) to give things a try. Honestly, the activation got off to a rough start. First, it took me about 20 minutes to realize I forgot to plug the coax into the low-band QDX. I figured I had broken something in transit. However, the hi-band QDX was getting things done at a good clip on 20 meters.

Eventually I recognized the problem with the low-band QDX, plugged in the coax and began making QSOs on 30 meters. Interestingly, I noticed that the 20 meter QSOs began to dry up as the 30 meter QSOs began to roll in. I’ve conducted enough tests that I knew that I’d be able to carry on QSOs on multiple bands. I noticed that RBN was POTA-spotting me on both bands and began to suspect a problem there. If people are relying on the spotting page as their sole source of POTA hunting, that probably did me in.

This was my first activation using a 12/17/30 trapped vertical I’ve been working on. I had it connected to the low band rig. It’s OK, but I think I’ll continue to use my QRP guys 20/30/40 tri-bander that switches in loads since it gives me more bands.

The high-band setup with a tri-band vertical trap continues to work very well. After getting a mess of QSOs on 20 meters and went ahead and worked a few stations on 10 and 15 meters.

This was also the first time using my homebrew 12v regulator. I spent some time thinking about using a 3D printed case, but realized that heavy-duty heat shrink would be a lot easier and more compact.

The good news, though, is that the finals on my low band QDX are still intact. Whew!

I ended up with about 30 contacts. The only things I’ll change next time is to a) ensure my coax is plugged in, and b) use an antenna on the low bands that is matched to the rig.

Todays Equipment

• Lo-band QDX (80-40-20-20 meters)

• Hi-band QDX (20-17-15-12-10 meters)

• Homebrew tri-band trapped vertical (30-17-12 meters)

• Homebrew tri-band trapped vertical (20-15-10 meters)

• Homebrew common mode chokes (7x 1.125” toroids on RG58)

• 20ft Shakespeare fishing pole

• Sotabeams Carbon-6 portable telescoping mast.=

• Pole holder clamps.=

• Homebrew-ish 12v regulator (built from one of these)

• 16 Ah Miady LiFePO4 battery

• 2022 Macbook Air (M2)

Here’s all you need if you want to do the build and you’re the adventurous type:

The bill of materials is the same except that you’ll need roughly 52 feet of wire this time around. Remember: 30 ft is just for the radials.

Note that you get 30pF by using 3x 10pF capacitors in parallel. The same way, you get 47pF by using a 27pF capacitor and adding 2x 10pf capacitors in parallel.

Some Caveats

• This build will not fit on my 20 ft fishing pole. I’m tempted to add some loading to the 30 meter section to make the overall length shorter.

• I haven’t taken this out into the field yet, but I suspect I’ll need to have a better ground plane.

• I’m tempted to remove a single winding from the 17 meter trap. I felt like I had everything dialed in with the prototype (built from recycled wire). When I went to build it with my “good” wire, I had a hard time getting resonance low enough, even with the coils as pinched as I could get them. It’s also possible that the wire itself isn’t long enough. So just keep that in mind if you build it.

I ended the last post with the general idea for how to build a trapped vertical:

1. Start with a ground plane.

2. Cut and add a vertical segment for the band you want,

3. Add a trap for that band.

4. Measure and adjust for resonance.

5. Go back to step 2 if you want more bands.

Remember: the bands we’re interested in are 10, 15 and 20 meters.

Supplies

1. Some kind of winder to store the thing on (see my ideas below).

2. A chassis mount BNC connector that has a solderable tab for the ground connection.

3. 2x 3/4” #6 screws.

4. 4x #6 washers.

5. 2x #5 lock washers.

6. 2x #6 nuts

7. 2x #6 butterfly nuts.

8. Roughly 44 feet of 24 gauge antenna wire. (Length is going to vary depending one the velocity factor of the wire you choose. This antenna was built using BNTECHGO 24 gauge stranded silicone coated wire.

9. 2x T50-6 toroids (yellow on one side)

10. 32 inches of 20 AWG enameled magnet wire. I like this stuff from Remington Wire.

11. 1x 30pF 1kv mica capacitor (I used 3x 10pF caps in parallel).

12. 1x 37pF 1kv mica capacitor (I used a 27pF wired in parallel with a 10pF cap).

13. 4x 14-16 (blue collar) AWG crimp ring terminals with the collar removed.

14. As much shrink tube of various sizes that you think you’ll need to build things to your standard. I didn’t keep track of the sizes, but most makers have tons of this stuff on hand.

You’ll also need a few tools: wire cutter, wire stripper, solder iron and something to measure SWR (antenna analyzer or Nano VNA).

Here’s a diagram for those of you too impatient to read the full instructions:

NOTE: This size of toroid and wire suggests you shouldn’t plan on sending more than 10 watts through it.

Storage - My Very Own Wire Winder

Up until now I’ve been using a variant of this wire winder I found on Thingiverse. It’s adequate, but it’s tricky to scale and there are one or two things about its construction that annoy me (the reinforcing rails don’t line up 100%).

I decided to create my own. It’s posted here on Tinkercad where I designed it, and also uploaded to here on Thingiverse. It’s dimensions are roughly 200x75mm, but you can scale it however you like. (I recommend only scaling X and Y and leaving Z alone.)

It contains the following holes:

• standard BNC chassis mount

• #6 screws for adding posts to connect your elements

• additional holes for hanging

• some flat holes intended for velcro straps.

The design is CC BY-SA, so please give me credit if you publish a derivative.

Connect the BNC ground tab directly to a ring terminal and solder it. You need to make sure the ring terminal lines up with the hole in your wire winder. It’s tricky, but not impossible.

Attach hardware for posts. I use 2 regular washers and 1 lock washer per post.

Crimp and solder a 1.25” piece of #20 wire to a ring terminal and then to the center pin of your BNC connector. Measure for fit before you crimp and solder. If you use a bit of your enabled magnet wire make sure to remove enamel from the ends.

Attach hardware for posts.

The winder is done. Let’s move on.

The Ground Plane

Cut of three 10 ft pieces of your antenna wire. Crimp them into a single ring terminal. Solder and add shrink tubing for stability.

I decided to keep the ground plane simple. I know more is better, but this setup is intended to be lightweight and portable. I want to make sure it works using a minimal amount of radials.

10 Meter Radiator

Online calculators suggest a radiator for 28.1 MHz should be 8 ft 3.9 in long. For this particular wire, I only needed 7 ft 5 in, suggesting its velocity factor is around 0.76.

If you don’t know the velocity factor of your wire, just cut it long (8.5 ft or so) and trim until your antenna analyzer says you’ve got it dialed in. (Make sure you measure with the radials attached.)

Attach a ring terminal and any shrink tubing you might want.

10 Meter Trap

The normal approach to building a trap is to use a few calculators to determine how many windings you need for your toroid to get to your desired inductance.

I’m going to do the math once for 10 meters, then leave it as an exercise for you for every other band. Remember these two formulas?

We’re going to use the heck out of them.

First, assume an ideal X of 200 ohms. This allows you to solve for L and C of 1.13uH and 28.32pF respectively (I’m playing fast and loose with uF and pF here). Since I don’t have a 28.32 pF cap, but I do have a 27pF, I’m going to recalculate my reactance by solving the bottom equation for X_C with C=0.000027, giving me 209.77 ohms. Close enough.

The rest of the math here isn’t hard since you know that X_L=X_C=209.77, f=28.1 and C=27pF.

Plug that into L=X_L/(2𝜋f) and we arrive at an inductance of 1.188uH (not far off from our original optimal value of 1.13uH).

The calculator from the kind folks at kitsandparts.com tells us we need 17.2 turns on our toroid. (The math for calculating turns isn’t hard either, but assumes you know a thing or two about the characteristics of the toroid you’re using, namely that it’s A_L is 4).

I will tell you this now: I only needed 16 turns. Read on to learn how to figure that out…

What you’re looking at is a T50-6 toroid with 16 windings of 20 AWG magnet wire and a single 27pF capacitor.

Tuning a Trap

There is often a difference between the theoretical values derived from our equations ant the actual values that work best in the field. In this case, it’s the difference between the 17.2 turns called for and the 16 turns I actually used.

Note: you can measure the resonant frequency of a trap using a small loop and your NanoVNA, but I’ve found that only gets you in the neighborhood. It’s more effective to take the number of windings called for by the math, round up, attach to the antenna, measure, then adjust.

In this case, the first reading was lower than 28.1 suggesting I had too much inductance. My options at this point were to a) remove a turn from the windings or 2) spread the windings apart (either of these will move the resonant frequency up). (Conversely, tightening the windings increases inductance and will lower your resonant frequency—this comes in handy during tuning.)

From a practical standpoint, it makes sense to first try adjusting the spacing of the windings. In my case, it didn’t get me up to 28.1, so I removed one winding. That ended up being too much (antenna now resonated too high), so I tightened the windings a bit. Eventually I got it dialed in at 28.1 MHz. 10 meters was now perfectly dialed in.

In theory, you shouldn’t have to adjust the lower bands once you’ve got a trap dialed in.

15 Meters

The next band was 15 meters. I selected 21.1 MHz as my target.

I added wire to the last toroid that brought an additional resonance dip at 21.1 MHz. While I had the antenna analyzer attached, I measured again for 10 meters (28.1 MHz) just to make sure things didn’t go whacky. I needed 1 ft 10.5 in of wire to get to 21.1 MHz.

Next was the 15 meter trap. Math suggested my optimal L and C of 1.32uH and 33.02pf respectively. I settled for C=37pF and eventually arrived at 19 turns around the same T50-6 toroid. You can see the trap I ended up with below which uses a 27pF and a 10pF cap connected in parallel.

This trap was tuned to 21.1 MHz using the same trial and error approach I described for 10 meters.

20 Meters

All that remains for 20 meters is to add enough wire to bring it into resonance for my targeted frequency (14.1 MHz). That took exactly 4 ft.

That’s it. You’re done. Go play radio. Read on if you want to learn about some of the pitfalls…

Some Pitfalls

Air Coils

My first attempt at building this antenna called for me using air coils. I never did figure out what I was doing wrong. Perhaps the coil was making the antenna look longer. Perhaps I just hadn’t figured out the trap tuning process yet. I don’t know. Of course, this was all before I learned about the ideal 200 ohm reactance, so that could have been part of my problem.

Bad Reactance

If you don’t have the right combination of capacitors to get somewhere close to the ideal C, you may be tempted to stick in 100pF and recalculate for L. Do this at your peril. You’ll have a more difficult time tuning the trap as your reactance gets further and further from 200 ohms.

Measure, Measure, Measure

Once I settled on using toroidal traps I quickly realized that if I did a step and didn’t go back and measure everything (including for the frequencies I had already verified), something would invariably get out of whack. Take it easy, go slow, and measure everything after you make any kind of adjustment. You’ll a) learn more and b) save time. And make sure to keep that NanoVNA charged!

Next Steps

This antenna can be easily deployed using my 20 ft fishing pole.

By the time you’ve read this, I will have already completed a similar antenna for 12, 17 and 30 meters. Six bands with two antennas and no tuner is a potent setup in my opinion. Based on how I operate, the only band it lacks is 40 meters which could be solved by adding an extension to either antenna and run in an inverted V configuration.

If testing goes well, I’m going to build one each for 100 watts as well. I’ll adjust the resonant frequencies up a bit to target the phone portion of each band, use a bigger toroid (T90-6) to account for the extra power, and heavier wire (20 ga). It’ll be fun.

I now own two QDX digital transceivers. One is built for the low bands, the other is built for the high bands. I thought it might be really cool to go out and do a SO2R (single operator, two radios) digital POTA activation using these rigs.

For the low band QDX I have a nice tri-band vertical antenna for 30, 40 and 20 meters from QRP guys that is only 17-ish feet tall. It operates by switching one of two loading coils in or out of the antenna. It’s nice because it works and it’s compact. But messing around with those switches requires me to get up and makes it difficult to do unattended multi-band WSPR beaconing and the like. And heaven knows I didn’t get into ham radio for the exercise…

For the high band QDX, I wanted something a little different. My goals were modest. I wanted a single antenna that:

1. Could work on 10, 15 and 20 meters.

2. Would not need a tuner.

3. Would not require adjustment to change bands.

4. Can handle 10 watts.

5. Could be deployed using a fishing pole.

Simple ask, right?

It’s a Trap!

One of the first [good] antennas I ever built was a trap dipole for 40 and 20 meters. It lived outside in a permanent deployment. It used stainless steel screws and heavy duty THHN 14 gauge stranded copper. The traps used air-coil inductors made from the same wire around 3/4” PVC. The capacitors were monstrous heavy duty 6kv 27pF ceramic film jobs. They looked scary.

That thing sat out in the Texas sun logging QSOs for about a year before one of the caps died. I replaced that and put it back into service. It’s been a great performer. I still have it, though it’s no longer in use.

Bottom line: I knew that traps worked, and that it should lend itself well to a vertical. So there… I was going to build a trapped vertical.

A trap is simply a capacitor (C) wired in parallel with an inductor (L). Depending the L and C values a trap will resonate at a particular frequency. At frequencies below the resonant frequency the trap acts as a inductor (loading coil, or inductive reactance). At frequencies above the resonant frequency, the reactance is capacitive, which has the effect of blocking RF.

Remember that: resonant frequency, inductive (lengthening) below, capacitive (blocking) above.

I’ve been told that you can simply add traps to the end of antennas to simply block lower and lower frequencies. There are some caveats, sure, but this isn’t a bad way to build a multi-band no-tune antenna.

So I set out to ask the Google how to build a 10/15/20 meter trapped vertical antenna. Well… if someone has already done this, they sure didn’t do a good job of writing it up.

Determining L and C

I had a difficult time initially using the internet how to come up with optimal values of L and C for a given frequency. In fact, consensus seemed to be that you could pick any capacitor you had laying around in your junk drawer. (I don’t think this is true in theory or practice.)

I decided to consult my oracle (a 1988 edition of the ARRL Handbook). There on page 33-7 it lists the formulas required to compute optimal L and C. However, it starts with this little gem before giving the formulas: “The rule of thumb for choosing the coil and capacitor values for traps is based on a reactance of approximately 200 ohms…”, Then:

We know that at resonance XL = XC, so solving for L and C should be pretty straightforward. It turns out this calculator does it for you. (Remember that these formulas give you micro henries and micro farads. So you want pico, you need to multiply by one million.)

Here’s a table of all the optimal L and C values for the low sides of each band 20 meters and up:

It turns out that in my junk box I have a bunch of 27pF capacitors and a bunch of 10pf capacitors, so I ended up selecting values of 37pF for 15 meters and 27pF for 10 meters. Both are reasonably close, but yielded slightly different reactances:

Using Toroids

Knowing that a few websites sell them (sotabeams, qrpguys), I decided to create my inductors using T50-6 toroids. (Actually, I have a long and sordid story in which I try to create some air-coil inductors, but that will be the subject of another post.) That last link contains a good calculator that will get you in the right ballpark for number of turns required to achieve the right inductance.

Wires and Such

All that’s left is wiring things up. I’m going to leave that for another post. But the general idea is to:

1. Start with a ground plane.

2. Cut and add a vertical segment for the band you want,

3. Add a trap for that band.

4. Measure and adjust for resonance.

5. Go back to step 2 if you want more bands.

Stay turned for Part 2: the next post will have full build instructions for my antenna.

I am Gary, KI5GRD. I've decided to start a ham radio blog to document some of my builds and portable activations.

I've been thinking about doing this for a while. What got me to finally do it was my most recent build. I wanted a trap vertical antenna that would work on 10, 15 and 20 meters, but I couldn’t find suitable plans anywhere.

It was difficult for me to find good and reliable information about building air-form and toroid traps (especially with regard to capacitor selection). And not a single website seemed to put things together in a way that I simply had to measure and cut (it would have been nice).

I ended up doing the research and making a build over the course of about three weeks. I was very happy with the result. It’s worth sharing, but I don’t really have an avenue for that.

So here we are…

Are You Qualified?

No. Absolutely not. In fact, I put the “amateur” in “amateur radio”.

I’ve been a ham since 2019, but I didn’t get serious about it until 2020 when my Dad and I decided we wanted to get on HF. So I got my general ticket, scored a Yaesu FT-891, built a linked dipole and got on the air.

Then started a phase of intense antenna building and experimentation, which continues.

What Can I Expect?

Well, I like amateur radio—homebrew, QRP, portable operations, antenna building. There’s a lot of fodder there.

And then there’s electronics. And Arduino. 3D printing. Raspberry Pis. Software hacking. Weather satellites. ADSB. The list goes on.

Frequency of posts will be commensurate with my free time. I may go months in between updates. So maybe consider subscribing—your call.

Thanks for reading. Let’s get this show on the road.

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