My Lockdown Project – a trap vertical
by Mark, G3YFO
A 6 band 10 metre to 30 metre HF trap Vertical for FT8 (and low power).
First of all after getting interested in FT8 on HF I wanted to try a vertical aerial due to the low angle of radiation for dx working. I use low power on FT8 so it didn’t need high power handling.
Hence I decided to build a trap vertical covering a few bands. You can make traps from coax but I decided to use traps based on toroids as they are neater and easier to make. So first of all I ordered a toroid trap kit from SOTA BEAMS, the 20w low power version costing £11.25p for two traps. Having received the traps I followed the instructions for making them and setting them up (which requires an antenna analyser). Of course with a vertical you don’t need pairs of traps for each band. So the SOTA BEAMS traps were tuned differently, one for 10 meters and one for 12 meters.
They were easy to construct and tune to the frequency, and I decided to make the vertical a 6 band vertical. Rather than buy more SOTA BEAMS traps I decided to make my own, which meant I needed some more toroids, so I went off to e-Bay. I found some T80-6 MICROMETALS TOROIDS rated at 80 watts from 1 MHz to 54 MHz. These were very cheap to buy – 10 for £11.45p. I also needed some 100pf, 68pf and 220pf capacitors all at 3kv. These again I got from e-Bay, costing only £2.27p for 10 of each value. Using these parts I made up 3 more traps, this time for 15 metres, 17 metres and 20 metres. Again these were easy to make and tune. Here are the number of turns and capacitor value I used for each trap:
- 10 meters, using the SOTA traps, is 10 turns with a 50pf in parallel, spacing the turns to acquire the correct frequency then sealing with hot glue.
- The next one is for 12 meters, using the other SOTA trap, is 12 turns with a 50pf in parallel, again spacing the turns to acquire the correct frequency then sealing with hot glue.
- 15 meters this is using the T80-6 toroid 15 turns with a 50pf in parallel, spacing the turns to acquire the correct frequency then sealing with hot glue.
- 17meters this is using the T80-6 toroid 18 turns with a 50pf in parallel, spacing the turns to acquire the correct frequency then sealing with hot glue.
- 20 meters this is using the T80-6 toroid 18 turns with a 100pf in parallel, spacing the turns to acquire the correct frequency then sealing with hot glue.
That concluded the trap making, now onto the wire lengths between the traps. The wire I used was electrical earth wire – green and yellow coated PVC.
I used an SO239 socket (which takes a PL259 plug of course) at the base. From this to the first trap (10 meter) the spacing is 8 feet 4 inches (254 cm)
- From the 10 meter trap to the 12 meter trap 1 foot 1 inch (33 cm)
- From the 12 meter trap to the 15 meter trap – initially this was 1 foot 9 inches (53.3 cm) but see below!
- From the 15 meter trap to the 17 meter trap 1 foot 10 inches (55.9 cm)
- From the 17 meter trap to the 20 meter trap – initially 3 feet 9 inches (114.3 cm) but again see below!
- and finally from the 20 meter trap to the top is 16 feet 7 inches (505 cm)
This is what the aerial looks like:
To check out the resonance frequency on each band I strung the aerial out horizontally in a straight line about 15 feet above ground and used my MFJ SWR Analyser. The tuning might shift once vertical but I would get the antenna tuned more easily this way. I started at 10 metres first. The frequency was set at about 28.100 MHz and I got an swr of 1.9 to 1 so I needed to move the turns on the trap’s toroid closer together to get a better swr at my target frequency of 28.080 MHz. With a bit of playing I got it to 1.3 to 1, so I resealed the trap and moved on to the 12 metre trap. The 12m SWR was very good at 1.4 to1 but I did adjust the turns to get it to 1.2 to 1 before resealing the trap.
The match at 15 metres was way out at 2.3 to 1 and that was at the top end of the band, so I had to increase the wire by 3 inches to get it into the bottom end of the band. This made the wire length from the 12 metre trap to the 15 metre trap 2 feet (61 cm). Now on to 17 metres. This one was good with a swr of 1.2 to 1 so I did not change it. Moving on to 20 metres this was tuned very low at 13.8 MHz. Even by opening the toroid wire spacing up I could not get it resonance, so I had to shorten the wire between the 17 metre trap and the 20 metre trap a lot – to 2 feet 9 inches (83.8 cm) – to get a good match. The final band was 30 metres and this was a very good match at 1.2 to 1.
Now for the crunch. I taped the wire and traps to my 10 metre fiberglass pole and elevated it 10 feet (3m) off the ground, connected 4 wires as radials just to try it as a vertical and it worked on most bands, except 10 metres where the swr was high again at the bottom of the band (2.3 to 1) so I had to play with the turns again on the 10 metre trap, closing them up, and got a 1.2 to 1. Happily the rest of the bands were ok.
To sum up the antenna works very well on all bands 10 metres to 30 metres with a good SWR but limited bandwidth of only 70 to 100 kHz but this is all I need, seeing as most FT8 is at the bottom of the bands.
Please see my drawing for more details of traps and lengths of wire between each trap, this is only a guide as your circumstances may be different to mine, with different mounting height and ground losses.
The original radials I made were for 10 meters and 12 meters and were 11 feet 4 inches (345 cm). These worked ok for 10,12,and 15 meters. I tried to see if I could further improve results on 10,12,15,17,and 20 meters by making 4 trapped radials using more toroids and cap’s to tune them. Each radial has 2 traps in it, one each for 10 metres and 20 metres, both of the same type as those in the main aerial.
From the pole to the 10m radial trap was 8 feet 4 inches (254 cm) and from the 10 meter trap to the 20 meter trap it was 8 feet (244 cm), making a total length of 16 feet 4 inches (498 cm). From the 20 meter trap to get the best on 30 meters it was 5 feet 6 inches (168 cm), so the total for one radial is 20 feet 10 inches (612 cm). By the way either types of radial work fine I was just interested in getting the best results I could on all bands.
Editor’s note: Mark’s antenna looks like great fun to put together and tune up. Although the design is based on 20w-rated Sotabeams traps you could of course build a higher power version using the Sotabeams 100w traps, which are based on T94-6 toroids and 1kv rated 100 pF capacitors. Similarly you could opt to ground mount the antenna and use wire radials rather than trapped ones. Let us know how you get on!