Based on my own experience, you could probably also use some aluminum / aluminium foil (metal) 'duct' tape and don't bother stripping the conductor. Just fold it over the element and make a flag. The increased surface area means that you won't need two feet, but mere inches. My experience was trimming a 15m dipole and overshooting the mark, leaving the dipole a few inches too short. Immediately (instinctively) I added a few square inches of metal duct tape to the ends as folded-over little 'flags' (capacitive hats), and I was very surprised that I had to trim them to barely a couple of square cm to get back on track. Surprisingly tiny area. Cheers.
Hi Peter, found it a very interesting video. Actually i have a Dipol for 40m which I am using for 15 meters too. At 40m it’s lowest swr is at 7100 just in de middle of the band. At 15m the lowest swr is at 20930 so it is to low. Not to high. Parallel (about 5cm) to that wire i have one second wire for 20m. Would be great if I could use that for 10m too. So I made a inverted V dipole for 20m in mmana. With a swr of 1,45. Using that one for 10m I get a swr of 110. Adding a wire in the middle of both legs with 45cm length didn’t change anything. So that seems not to be working for a Dipol. After that I made a efhw for 20 on a hight of 5m, and added a wire in the middle. Without the wire I had a swr of 1,13 at 14,15 and 1,41 at 30 mhz (only calculated until 30) with a wire in the middle of 4 meters it was 1,67 at 29,4 mhz. The 1,13 at 14,5 mhz didn’t change. So it seems that the necessary wire length in real can be a lot shorter then in the simulation.
Hi Peter, I truly believe the "shift" in the harmonics has to do with the non-ideal inductance of the transformer. In the 40m band, the inductance makes it requires a shorter radiator. When you go to 10m, the really high inductance makes the transformer closer to ideal, but as the radiator is shorter, we are way up in the band (or even outside)...
There's also the end effect. It effectively lengthens the antenna at the fundamental by more than on subsequent harmonics (due to more "ends" of the half waves but the wire only physically having two), so if the antenna is tuned to the fundamental, it'll be too short on the next harmonic and get shorter for each subsequent harmonic, though by not as much. Two other ways of getting round this are a capacitor in the centre of the EFHW, which on the fundamental, electrically shortens it at that frequency but has little effect on the harmonics (the wire has to be a little longer overall of course than the otherwise resonant length on the fundamental), or by introducing a small inductor/loading coil which has less effect as you go up in frequency.
Peter! Thank you for this wisdom, BTW I still like using "old money" when building -its easier in my mind to visualize. So I put up a 40 meter inverted v in my postage stamp size yard , it's close to the house, so I had common mode current on my feed line, plus it was noisy QRN I choked my feed at my coax switch, that reduced the noise. as for 15MHz seems the VSWR is workable, but it doesn't hear as well, wondering if adding the capacity hangers would help with RX ? 73 TU DE N3TGY ..
Unlikely that the hats will improve receive, as even with a modest VSWR, receiver side tends to cope OK. You could rig up a temporary 15m dipole and see how that compzres.
