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End Fed Half Wave-Experiment du’Jour : Part 1

Just about the only wire antenna in common use I had not experimented with is the End Fed Half Wave(EFHW).  So WTF.  May as well give it a go. 

2021 planning had me booking a stay on Cape Lookout National Seashore(CALO) for the end of June rather than the end of July.  I tried unsuccessfully to make the reservation in 2020, but could not.  Success for 2021.  So my FD 2021 will be from Cape Lookout. 

Given the layout of the cabin I prefer at CALO campground I decided to tinker with the EFHW as a possible solution for allowing multi bands with least effort.  The “least effort” factor is growing more important with each passing year.  But the EFHW itself piqued my curiosity as well, and it seems it my provide a solution to the “least effort” vs. “works well”  conundrum.   

So, here lies the experimental portion.   NUMEROUS versions of the classic 49:1 transformer are documented on WWW and in the now ubiquitous scrootoobe video.  Version guidelines exist for either 80m or 40m multi band versions.  Quite typically I chucked some of that and decided to re-invent the wheel – because what is the fun of experimenting if you are just going to follow the cookbook?  Wellll…..not quite that either.

Where to start

Looking for a 40m size and decided to try modeling out a 40m EFHW fit out with a loading coil and tag end to allow 80m or 75m.  The idea is to have an antenna that the floppy fiberglass masts would be able to support easily in the normal 20 knot winds typical on CALO.  The feed point will be at about 4 feet high as I expect to deploy it.  The mast will support the wire vertically to about 30 or 35 feet, depending on which mast is used(Spiderpole or Jackite or K4TMC).  The rest of the wire will be stretched out horizontally with the coil supported by a second mast.

This deployment will have 80m or 40m basically functioning as half wave in an L configuration.  20m will probably have a larger horizontal component than the vertical section, and if it works there 15m/10m will provide who-knows-what.  i.e., “PERFECT!” – for values of “perfect” where whatever happens is perfect.

Modeling showed best result with the wire between the feed and the load at a longer length than I expected at 70+ feet.   Using insulated wire for actual construction, I expect that to be shorter in real life.   I plan to test with a .05wave counterpoise, and use a 6 foot coax jumper at feedpoint.  The real feedpoint will be a current choke at the end of the coax jumper.  It seems likely the shield of the coax jumper will act as another counterpoise.  Maybe another choke at the radio end.

Transformer Conundrums

The item I had more questions about was the step-up transformer.  A lot of conventional wizzdom surrounding the 7:1 turns ratio versions.  I chose to give myself more options and provide multiple taps, and use the larger ft-240-43 size toroid to allow 100w.   Hopefully this will give less core heating.  Then two turns vs. three turns on the primary was considered.  Initially I favored using three turns on the primary.   I expect 80m to be more useful than 10m going forward.   Physical reality – two turns seemed more practical with the 16ga wire I used. 

The other wildcard I threw into the construction detail was about sticking to the “norms” of putting the taps on nice-and-easy turns ratios.     While waiting on my toroid order to arrive, I stumbled across N8NK’s videos.  I found the playlist on EFHW and UNUN’s interesting viewing.  The main idea I fortified was to tap the toroid in several different places.  That would have been more versatile using 3:1 windings.  Even with 2:1 windings I still liked the range of impedances available by placing “irregular” taps, i.e not on the even multiple windings.  With 3:1 I had expected to place the taps on every 4th turn.  With 2:1 windings I thought maybe every 3rd turn but with 6 taps every 2nd turn was “good enough”.

The only genuine benefit that conventional “even numbered taps” provides is ease in predicting  the transformation factor(i.e., 7:1 turns gives 49:1 step up).  To switch the flip I decided to tap the transformer at 4.5:1, 5.5:1, 6.5:1, 7.5:1, 8.5:1 and 10:1.  (FWIW, that should be stepping 50 ohms up to about: 1013, 1513, 2113, 2813, 3613 or 5000 ohms.)  Just for grins, it is easy to test with the ordinary antenna analyzers and a resistor test box(or well stocked junk box).  

If it is important to your thought process, measure the damn unun to be sure.  To simplify, the secondary is actually tapped on turn #9,#11,#13,#15,#17, and #20.  With 2 primary turns and just 5 taps on secondary, tapping every 3 turns at #8, #11, #14, #17 and #20 should allow matching impedances of 750, 1513, 2450, 3616, or 5000 ohms.  That includes the obligatory 7:1 ratio for purists.  I only chose to use the odd taps as an appeasement to my increasingly contrarian curmudgeonly nature.

Precision Optional

 I figure to just use the antenna with the tap that provides the best SWR. I can then produce a cheat sheet for each band.  I would like to have a best choice for each band.  If it turns out the same tap works best for all bands – so much the better.   If they are NOT all the same tap I also want compromise choices that will allow one tap to allow operation on several bands without moving the tap.  Another tradeoff option – sometimes moving the tap will be easy, sometimes inconvenient.

Functionally we can call it good if some doing RBN compares to permanent dipoles prove it to perform acceptably.  The truth is I don’t care what the actual step up ratio might be.  I just want the antenna to function, and experimentally finding a good tap is “good enough”.  We are hoping that the taps provide enough options for multi-band operation with a good match.  It will be nice if a single tap is good on all bands.  It is not a deal breaker if changing bands requires moving a tap that can be reached at ground level.

Engineer the possible. Mind the trade-offs, because “best” can be the enemy of “good enough”.

Related Links(updated 2021/03)

End Fed Half Wave Experiment – Part 2
End Fed Half Wave Experiment – Part 3 Multi-Band End Fed Antenna

G0KYA, Steve Nichols.


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