An update on my latest HWEF, probably 10 years after I built my first HWEF. I always thought and still think actually until somebody proves me wrong, my HWEF is and was a true half wave endfed on all bands it was designed to operate on. I’m a bit in doubt lately because I experienced a strange phenomenon on 20m. I’m using two L/C traps in my antenna; one resonant at 14 MHz, one a little under 7 MHz. I’m using Russian doorknob capacitors and the coils are wound on Fritzel isolators. Here’s a picture of a previous version with ceramic caps.
I’m still using my first HWEF design with great succes, of course I refined it over the years. So what does my current HWEF look like. For starters, I swapped my 3:21 double FT240-43 transformer for a commercial 450W version by communicationworld.nl. Great materials and craftmanship for a fair price. This transformer has a single FT140-43 inside an FT240-43. Inducance will be very close to the stacked FT240-43 design I reckon, optimal for 7 MHz and very good on a band up and below. For some reason I did not trust the transformers I built myself and I’m not that good with the enclosure side of things so I thought why not buy one and see if it’s better than my own transformers.
My own transformers have a 3:21 and a 2:14 ratio, both wound on two stacked FT240-43’s. Back in the days and also recently, I experimented with just a wire connected to the transformer, no other traps / coils. While my backyard is about 90 ft deep, there was no way I could put up 40 meters of wire. I’ve experimented with loading coils and made the 24m long short 80-10 version, but that coil limits the bandwidth on 80m drastically and the results on the bands where the wire is a multiple of half wave lengths is kind of unpredictable. I know there are some tricks to overcome this; a 100 – 150 pF cap on the input of the transformer (better SWR on 10m), a 5 turn coil on a 1″ former about 6-7 ft from the transformer’s feedpoint (also better SWR on higher frequencies) or a 220 pF capacitor in the middle of the wire (higher resonance on 80m) but I didn’t want to go that way. This antenna was only 24m and while I can span away around 30m, it also seemed like a waste not to span away what I could span away. I’ve been told that the radiation pattern on multiple-halve-wavelength bands is odd / not optimal anyway.
So, my main goal was to design a true HALFwave endfed for 80m, 40m and 20m using two traps (parallel resonance L/C circuits) for 20m and one for 40m making it electrically a halve wavelength antenna on all three bands. Afther the transformer, there’s a little less than 10m of wire. Then there’s the 20m trap. This trap is supposed to “disconnects” what’s behind it, because of the high impedance on 20m, it acts as an isolator. The 20m trap is a Russian 22 pF 15 kV door knob capacitor from eBay (Ukraine seller) in parallel with a coil wound with .6 mm enameled wire on a Fritzel isolator, around 6.8 uH. The trap resonates around 13.9 MHz.
On lower frequency, the inductance of the 20m trap dominates the capacitance so on 40m and 80m it acts like a loading coil. After the 20m trap, there’s 6 – 7 meters of wire and then a 40m trap (see picture above, ceramic caps in series/parallel are replace by a single Russion 68 pF 15 kV door knob capacitor from same eBay seller). The inductance measures around 7.6 uH and it resonates around 6.9 MHz. This trap is supposed to disconnect the tail wire on 40m so also on 40m, there’s and electrical halfwave with a 6.8 uH coil. After the 40m trap, there’s a tail of 12 – 13 meters of wire of which 4m is going vertically down a tree from 6m agl and then 3m horizontally on top of the kid’s wooden swing. This tail only works on 80m, again my antenne electrically acts like half a wavelength, physically it’s around 30m of wire loaded with two moderate coils 10m and 17m from the feedpoint. On 80m, these inductors are somewhere in the middle of the halfwave where current is at max so they very effiently reduce the physical length sacrificying a little effiency and bandwidth but I take that for granted, on 80m it’s a compromise. Bandwidths are pretty good; on 20m it’s 400 kHz with SWR below 1:2 (so full band), on 40m 200 kHz with SWR below 1:2 (full band again), on 80m it’s 300 kHz with SWR below 1:3 (full band tunable), 140 kHz with SWR below 1:2. While the antenna is kind of a random long wire on 10m, I didn’t have high expectations but for some reason, the X is very close to 0 on 10m as well, probably acting like a multiple of halfwaves on 10m and… on 6m the SWR is flat as a pancake as well but the radiation pattern will be very messy (read: terrible) and I know the transformer is not suitable for 50 MHz so I’m not going to use it there, I have a Diamond V-2000 for VHF/UHF that works fine on 6m.
Now here is my though / worry. While hanging out the wire I found that the tail, that’s only supposed to work on 80m, has a drastic effect on the SWR (not resonance frequency!) on 20m! I was surprised to see that, because the 20m trap should “isolate” everything behind it on 14 MHz. In practice: it doesn’t seem to do so. Then I started to think: normally a trap is used to get a quarter wavelength (e.g. in a 3-band rotary dipole). At the end of 1/4 wave, the impedance is low, introducing a resonant L/C parallel circuit will be like putting a frequency dependent isolator so that will work fine. I never thought about the fact that the end of half a wavelength also has a very high impedance. Does the trap really isolate at the end of half a wavelenth? Or would it resonate and couples what’s behind it, a bit like colinear loading that’s being used in 5/8 or half wave VHF/UHF antennas?! The impedance on 20m went down when the wire was closer (horizontally) to (very wet) ground. When I sloped it from the tree to my fence, the SWR went way up. Strange thing is that on 40m, there was no difference at all. The tailwire did not seemed to do anything on 40m, but it did on 20m. Is the 20m trap “leaking” RF into the wire, coil and wire that’s behind it? Having it closer to the ground seemed to improve SWR on 20m. This is way too complicated for me, but I’m hoping somebody can explain this phenomenon to me.
73 de PA3HHO (Pleun)