Sam builds a tiny tabletop HF antenna

Many thanks to Sam Duwe (WN5C) who shares the following guest post:


A (surprisingly good) tabletop HF antenna

by Sam Duwe, WN5C

I recently built a tabletop QRP HF antenna for 17 and 20 meters, in the spirit of the Elecraft AX-1, so I could operate at lunchtime on the campus where I teach. My wants were something small, that would fit in my work bag, that didn’t require a tuner, and could work on a couple of different bands. But on a lark I decided to attempt a POTA activation at Lake Thunderbird State Park (K-2792) pairing this antenna with my Penntek TR-35 QRP CW transceiver. I figured I’d maybe get one or two QSOs and then switch to a long wire in a tree. But what happened amazed me.

I talked to seemingly everyone. Beginning at 9:00 AM September 26th I worked both 17 and 20 meters for an hour and a half and made 37 contacts from across the country. I even had a Swiss guy call me back on 17 but he faded before we could finish. This antenna, at least as a CW POTA activator, works. Granted conditions were very good, but I’ve replicated this multiple times in the past few weeks, just recently at a picnic table in the parking lot of the Route 66 Museum (K-8644) in Clinton, OK (there is quite a thrill in urban activations).

It has also reasonably low profile and very quick to setup and take down. It is also quite a conversation piece when I set it up at school. I elevated the counterpoise by attaching it to a nearby oak and an interested undergrad sheepishly asked if I was listening to the tree!

The build is pretty simple. Physically the antenna consists of a small painters pole from Walmart and an old tabletop camera tripod. I found a nut that fit the screw portion of the tripod and hot glued it into the orange connecting section of the pole. That way the tripod can then be screwed onto the pole. The RF parts of the antenna consist of a 38” telescoping whip that I scavenged from the rabbit ears antenna that came with my RTL-SDR. It connects using the original connector which was hot-glued into a hole I drilled into the top of the painters pole. I found similar small 3 or 4-foot whips from AliExpress for cheap and these would probably work fine.

I then soldered a long length of speaker wire that was wound into two coils: the top for 17 meters (24 turns) and the bottom for 20 meters (25 turns plus the former 24-turn coil). The speaker wire was the soldered to the center of a BNC connector which I hot glued and taped to the pole. I soldered a short piece of wire from the shield of the BNC for the counterpoise and added an automotive spade connector to attach to a 17-foot length of wire. I also included a switch between the coils and the BNC connector to select either just the top coil (17 meters) or both coils (20 meters) using solder, hot glue, and tape.  I then covered my shame in silicone tape.

The most time-consuming aspect of the project was tuning the antenna. It required trial and error to first tune the number of turns on the 17-meter coil and then the 20-meters coil. I extended the counterpoise (for me it’s best when slightly elevated) and the telescoping whip. I performed the tuning with the whip not fully extended to give room to tune in the field. Using a nanoVNA was useful here, as was soldering a pin to the wire to poke through the wire at various parts of the coil to find the best SWR.

In use, the antenna can be affected by both body capacitance and how the counterpoise is situated, so I found that an in-line SWR meter was helpful in making sure all was well. Once set up it is easy to fine tune by just adjusting the whip length. 1.5:1 SWR is about how well I can tune on average. Obviously if you have a tuner you would just have to get it close.

There are a million variation on a small base-loaded vertical antenna, and you can definitely improve upon this design. And, besides the super well-built and elegant AX-1, QRP Guys sells an interesting looking kit, and there are some good 3D printed designs I might want to try out. But regardless how you go about it, it might be worth giving a tiny antenna a shot.

72, Sam WN5C

16 thoughts on “Sam builds a tiny tabletop HF antenna”

  1. Good article. Just what I was looking for to be able to operate HF from inside of my office at home.
    I just ordered the QRPGuys DS1 Antenna and the DS1 40m Loading Coil.
    Timely article, too, with winter sneaking in on padded feet here in Michigan. Deploying an EFHW wire and counterpoise across the living room and up the stairs wasn’t going to fly for very long (once my XYL found out, that is). Hi hi.

  2. Excellent build Sam. It’s good to read accounts from other DIY enthusiasts about their projects. I recently built a Wolf River Coils clone that cost me less than $10 in materials. I have used that coil on multiple activations this summer so it works FB. I am tempted to give your idea a try; I like the tabletop idea.

  3. Does elevating the counterpoise wire really help? I noticed the buddy pole antenna requires an elevated counterpoise. I’ve always thought the purpose was to connect the antenna to the ground and 3 radials are better than one.

    1. Max, my DIY portable vertical antenna works with either one elevated counterpoise or a set of radials on the ground. The advantage of the ground-mounted radials is they don’t need tuning when changing bands. A raised counterpoise has to be adjusted for resonance on each band for best SWR. I use 4 radials each 10ft long.

      1. ” I then covered my shame in silicone tape.”
        Haha
        There are two kinds of hams. Those that admit to doing this and liars.
        ROFL!
        Great build, great write up.

    2. I’ve had very good luck with the qrp guys ds1, which is the spiritual brother to this. Now you got me wondering if I could design something with a common base and then a series of different loaded coils I could swap in for different bands. That would be a cool project. As someone else said winter is coming, so that’s a good project.

  4. This kind of article is greatly appreciated because it inspires experimentation and creativity, and that.. on a budget.

    I remember back in the 90s when I used Hamstick antennas on my vehicle while traveling around Central Mexico. At one point my wife and I built a NorCal 40a and I tried it on that mobile set up. It amazed me how well it worked!

    72,

    George KG8DA

    1. Thanks George. I found that a budget is the best teaching tool. That’s super cool about pairing a hamstick with a QRP rig. CW mobile (safely of course) would be a blast, and my rig has SSB receive so I could at least keep myself entertained on a road trip.

      1. Back in the 90’s I did a fair amount of mobile CW, while living in Mexico. Traffic wasn’t too heavy in the more remote areas I visited. Additionally, I converted a computer mouse to a straight key. Though it wasn’t a good as a real straight key, it was excellent for mobile use. I could hold it in my lap while driving and send, dropping it instantly if two hands were required on the wheel or if I wasn’t sending.

        Before making my first mobile (moving) QSO in CW I spent hours and hours, while driving, listening in on others’ QSOs and working on my head copy. Eventually I got to where I was copying all the pertinent details without a sheet of paper and, one dark night, when I had to drive 3 hours over a mountain range, I took the plunge. It was a blast!

        When we built the NorCal 40a it added to the enjoyment, though I only logged a couple mobile QSOs with it. I did more portable with the NorCal.

        Now, living in the USA, I either don’t drive far enough to do mobile CW and/or can’t afford to divide my attention. If I do a mobile installation I will probably only listen while driving alone, no more. I wouldn’t hesitate, however, to use said installation, portable, on lunch breaks, etc.

    1. Thanks! That’s a good idea, I’ll try it. Except I try to get an hour of operating in sometimes between dropping my kid at school and heading to work, and with my luck I’ll fall in. 🙂

  5. Thanks Sam.

    I have the AX1 and I think I’m going to try it again here in the condo. I’m not having much luck with other antenna options so I might as well try it again. It has the advantage of being small enough to sit next to the window so maybe that will help.

    W4MKH
    https://w4mkh.com/

  6. Many years ago I built a small portable whip based somewhat on a Hustler MO-4 (22-inch mast) plus resonator. I used a telescoping whip at the top and I tapped the coil and used a pigtail with an alligator clip to adjust the coil. By changing taps plus adjusting the telescoping whip I was able to tune 20 through 6 meters. A bit of PVC pipe and some hot glue was definitely involved. I put a right angle PL-259 at the bottom so I could connect it directly to the back of my FT-817. I clipped my counterpoise directly to the ‘817’s heat sink. It worked quite well, including indoors.

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