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End Fed Half Wave Experiment – Part 4

EFHW Experiment Part 1
EFHW Experiment Part 2
EFHW Experiment Part 3
UPDATED 2021-04-08

On a whim I decided to wind a transformer using two FT-140-43 cores.  I went with the same 2:1 ratio as the first transformer.  SWR testing using this second version showed that all transformers are not equal.  The SWR readings taken using the new transformer with the antenna wire as it was trimmed with the larger transformer did not have similar results.  It turns out the new transformer would require re-trimming the wire lengths to bring the 40m band into the same SWR curve.  Since I had the 80m coil&tail attached using .250 quick connects, it was easy to add wire.  Doing that allowed bringing 15m and 10m to good matches.  20m also found a sweet spot but only on the 6.5:1 tap.

New Version?

Without re-tuning the wire no good matches were obtainable on 40m.  Instead of making any permanent changes to the working 40m loaded antenna/transformer combo I am making a different wire based on a 30m wire length.  It will include a quick connect to allow the antenna to cover 20m by detaching the section of wire beyond the ~34 foot 20m wire length.  That will provide an antenna capable of 30m/20m/15m/10m.  This may become a simple way to add a permanent 30m antenna in the w4kaz antenna farm. 

Maybe I get ambitious and add a second link to include 17m, just for grins.  Does an easy tune 30m/20m/17m/15m/10m antenna sound good?  One could just as easily add quick links for each band.  Using the antenna as a full wave on the 2nd harmonic provides radiation patterns that might be more useful, but either choice provides a method to cover each band.  The initial lost opportunity cost is the need for the transformer.  For quick and dirty field construction a simple wire dipole or vertical would still be easiest – but mostly monoband, as fan dipoles can sometimes be difficult to tune.

Future ideas

Thinking about trying a different twist on using the FT-140-43 size cores by pasting three of them together for a transformer, and trying the transformers out with 3:1 turns ratios.  I have enough of the FT-140 size on hand to do this but used my last two of the FT-240 size.  My first test antenna seems to be a success, so it just need to be put on the air some to get an idea of how it fits in.  It is always good to have the option available.

20m/30m Constructed and Trimmed

Using the transformer built from a pair of ft-140-43 toroids sandwiched together, an antenna for 20m/10m is now built.  It includes a simple wire tail that can be attached as a link to extend for use on 30m/15m.  It should be a simple matter to prepare a wire for adding 40m use.  With the 40m wire 15m should be use-able with two configurations, either 30m or 40m.  If 15m is open that might add some interesting changes in radiation pattern when switching from a full wave to a 3/2 wave.  May as well add a jumper for 17m too(later-much later). 

TAP Cheat Sheet, 30m/20m/15m/10m

Taps 20m/30m linked EFHW (jumper to attach 30m/15m)

BAND TAP Best SWR  SWR range to expect
30m(j) 8.5:1 10.25(1.6:1) Entire band, 1.6:1 with wire drooping
30m(j) 8.5:1 10.1(1.3) Entire band, 1.3:1 with wire pulled taut
30m(j) 7.5:1 10.3(1.4) Alternate tap,1.4:1 with wire pulled taut
20m 8.5:1 14.025(1.4) Entire band favoring CW section
15m(j) 4.5:1 21.010(1.4) Entire band under 2:1(wire drooping)
15m(j) 4.5:1 21.010(1.0) Entire band flat with wire pulled taut
10m 6.5:1 28.000-29.25 28.00 thru 28.800 under 2:1(swr 1.2:1 on CW)
10m 8.5:1 28.00(1.4) 28.00 thru 28.75 < 2:1(swr 1.4:1 on CW)

*(j)= jumper attached for 30m/15m

Other ideas and observations

LINKING – Thinking about creating a “linked EFHW” out of curiosity.  If 10m were reliably open I would pursue the idea seriously.  The different patterns of radiation going from 1/2, fullwave, 3/2, 2xfullwave would be fun to work through.

Using taps on the transformer —-After testing with two different tapped transformers, using the taps makes it simple to obtain a good match.  More on the air testing is needed, but at this point trying to obtain good matches using a single transformer ratio seems futile.

Different cores = different wire  With both transformers I found that the length of wire needed was unique to each configuration.  The lesson is to build the transformer and tune the wire to the actual transformer being used.  The ft-240-43 transformer pair would not match the wire cut for the ft-140-43 transformer pair.  Need to build more transformers to see if a pair of identically built transformers(size and number) will both match the same wire.  Best guess is that will work.  But don’t expect transformers with different toroid sizes, or different number of toroids sandwiched together to all work with a set size of wire.  Be prepared to test or trim.

Wire droop not ideal   Pulling the wire as taut as possible not only shifted resonance slightly, but improved the SWR match on every band.



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