So, here in Wales during March 2025 the local RSGB branch are organising a RSGB Region 6 and 7 (Cymru) 21 MHz Challenge. Participants are encouraged to build a home-brew resonant antenna for the 21MHz / 15M band, and make as many QSO’s as possible during the month… interesting, eh! So, keen to participate, I thought I’d put a little something together on an easy antenna design that could be used for the 15M band. Now… I don’t have anywhere near enough room at my QTH for another dipole/doublet: so in this case I need to stick to a vertical. Here’s an antenna design that’s quick and easy to build, and might help others wanting to participate but who are unsure what’s needed to quickly build a simple, effective 15M antenna.
NB: for this particular challenge, the rules state that the antenna must be home-brew, and resonant at 21MHz. I have checked with the organisers and the use of an ATU is permitte as long as the antenna is resonant 'somewhere on the band'.
This is a T2LT Antenna – which is essentially a vertical dipole – it is extremely easy to make and can be constructed from a single length of coax cable. The top half of the antenna (which is a 1/4 ƛ wave) is trimmed down to the dielectric, and the bottom half (also a 1/4 ƛ wave) left as coax. Use a shrink wrap at the joint to avoid water ingress. Make an air-choke or sometimes known as a dirty-choke with 7 turns of the coax on approx a 110mm Ø pipe, then terminate with a PL259 to connect to your radio. Of course, you should only use these figures as a guide for 15M, and ensure you prepare it maybe 20cm longer than needed to allow trimming for resonance.
I had some nice Ultraflex coax hanging around so had a go at building one of these, and it should be noted that the wire length needed to be extended by 8″ as at the length in the design image, the SWR at 21.320 MHz was 1.7:1, with the best SWR of 1:04:1 at 22.800 MHz. I didn’t make any corresponding adjustment to the coax. Here’s the SWR plot from 20 MHz through to 22 MHz using a Nano VNA which shows a pretty good SWR of 1:1.06 at 21.320 MHz. Not half bad for a length of coax cable as an antenna, eh!
I’ve read several reports on these antennas – which incidentally can be made for any band if you have the vertical real estate, which suggest very good performance for such a simple design and a pretty impressive cost-to-effort ratio to boot, I’d say. My rather decent SWR illustrates this also – more by luck than by judgement I guess.
So, onto some of my thoughts about this challenge.
- Personally, though overall I think this is a super little challenge and can only serve to encourage amateur operators to get on the air and make use of what is possibly a slightly underutilised band, I do have some thoughts on how this could be refined and improved, and if the organisers happen to stumble across my comments, that they take my critique in the manner intended.
- The fact that one can only participate; ‘from home QTH only’ precludes two rather large subsets of operators,/ namely those who enjoy getting out or portable /p, and to me, quite unbelievably, amateur radio clubs.
- Whilst appreciating it is a challenge and not a contest, I am rather baffled at the club scoring process! One club member could get 10 SSB QSO’s in the month, and another club member get 5,000 FT8 QSO’s. The current process to determine the points awarded to a club is to average all the QSO’s i.e. to tally the QSO’s and divide the total by the number of club participants. Why not simply add all the QSO’s and multiply by the number of club participants, which would seem a much fairer way of determining club placings. Presently it’s such a shame but there is little motivation for clubs to submit any entries with such a skewed method of determining overall scores.
- Why is there no separate category for data modes.
Regardless, I plan on having a go at trying to get a few QSO’s, why not have a go yourself. Oh and give yourself a few brownie points and a gold star if you noticed that the image I used in my screenshot was of the 80M band and not the 15M band… :).
Enjoy and have fun. 73.