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 EFHW - revisited

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PostSubject: EFHW - revisited   EFHW   -  revisited Icon_minitimeSat Apr 13, 2024 8:33 pm

One of the issues regarding EFHW transformers (Bought or Built)  is that most designs traditionally focus on covering multiple (harmonically related) bands for Ham use on 3.5MHz and upwards. 

By 28 MHz the SWR and coverage pattern of these antenna systems are usually going astray in a big way, yet they are still set up for very long wires and counterpoises /grounds

It isn't possible to do much better while covering several octaves of ham bands, but if the goal is changed to an EFHW  for 'just 27 - 30 MHz' there are other approaches you can take.

  • You only need 0.8m of counterpoise or a choke 0.8m down the coax run.
  • You only need about 5 metres of antenna wire and retain the simplicity / invisibility of a single insulated wire in the air 
  • For use up to say 15W on FM or 60 on SSB you can get away with a single high frequency optimised toroid core and customised winding patterns.

  • Like all EFHW transformers it's lossy (about 1.1dB to be exact) but will give a 1.5:1 bandwidth of about 2MHz and really low SWR (1.15 :1 ) at the minimum 'tune' point
  • Adding a real world length of Coax (and/or an SWR meter etc) means another 1 dB of loss but the 1.5:1 bandwidth the transmitter NOW sees is increased to around 2.5MHz (still 1.15:1 at minimum).
  • If your transmitter will put up with  2.0 :1 then 26 - 30 MHz are all in there without retuning.
  • No retuning, NO ATU, no Capacitors to try to drag the 30 MHz SWR down . . 

To be fair the double bazooka designs are also (almost as ) wideband, equally lossy but while also easy to use they are heavy, visible/ugly and need a coax running to centre feed box.

The design is similar to 'normal' EFHW transformers so if you could build that you could build one for High Bands only.   
A VNA is helpful for test /setup but tweaking with a SWR meter would probably get there in the end 

(Reminder:  This design simply won't work below about 16MHz so if you want that you're stuck with the traditional designs)

My 2 day old prototype has been feeding a 5m wire 'poked  'out of the window, sloping down  4m to 3m above the garden, in a wooded hollow / low point in the area.. 
10m is a bit dead at the moment so using a 0.2W (yes 200mW) WSPR propagation beacon I see it's regularly being received in both America's, the Canary Isles and Turkey and Cyprus both days on 10m

Despite the Transformer and Coax losses the ERP of the End fed half wave will still be close to the Transmitter output, and you're losing less than 1 s-point on receive. 

Happy to post more details if this sounds interesting to anyone else but me    EFHW   -  revisited 1f600

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PostSubject: Re: EFHW - revisited   EFHW   -  revisited Icon_minitimeSun Apr 14, 2024 1:39 am

Excellent post Bruce,

I myself use a tri-band end fed and would echo much of your thoughts. Mine is most effective on 20 meters not bad on 40 but pretty poor on 10.

Of course one of the attractions of an end fed antenna is it's ease of deployment and a relatively low profile. I've recently moved to a new QTH so my HF antenna system is a blank canvas. I'll be giving your thoughts some consideration before I get around to putting something up.

Best 73 Gary.

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PostSubject: Re: EFHW - revisited   EFHW   -  revisited Icon_minitimeSun Apr 14, 2024 7:35 am

I am going to be an echo of Gary in saying.. excellent post Bruce.

With all the antenna experimenting I have done I have never used a multi-band EFHW, but have used an EF "random" wire antenna with a 9:1 transformer. A 10m ish piece of wire and short counterpoise worked well on 40 and 20m, but dedicated antennas I have made worked better on 10/11m.

EFs seem to be very popular. A dedicated 10/11m one would offer an alternative to a T2LT for for those who want something for quick deployment when going portable. I'd love to see further postings on it and I am sure others would too.

If you haven't seen it before, take a look at Kevin Loughin's An end fed half wave cage antenna video, it's an interesting watch. Using the construction style I am sure the bandwidth could be expanded down to 24Mhz as I have a centre fed cage in such arrangement currently set up that does under 2:1 across 10/11/12m. You would of course need a lot more wire and invisibility would be lost with an EF cage.

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PostSubject: Re: EFHW - revisited   EFHW   -  revisited Icon_minitimeSun Apr 14, 2024 9:15 am

I know a good 49:1 makes a single band cut EFHW work on 40m band without any problem using FT240-43

Get a cheap 10m pole and run your antenna vertical for 10m/11m and it might do better, if you clear your roof line, other than noise which will be worse vertical. It will work the same as a Sirio 1/2 wave.

There are few antennas that do not have a very small matching loss, use thick wire to wind, more difficult on the hands but that is the price you pay, if you are a winder that is and not a soft handed "I don't make traps and baluns myself" person. Not really DIY if you did not wind them yourself.

And a large toroid to minimize this FT240-43. Some 49:1 you buy online use the mini ferrite and thin gauge wire and overheat with 100W SSB. And some you may buy ready made you will never see lower than 1.8:1 irrelevant of wire length adjustment.

A 1/4 wave does not have a matching loss of course, other than the ground itself unless you have 16 + radials. However as a downside you have a 1/4 wave, and probably on the ground, which is the very worst place to have an antenna, very easily beaten, they work but do you just want.. "it works" ? Fine in great conditions but again do you want 'it works' in not great conditions ?
" losing less than 1 s-point on receive"

I do not understand that, all antennas need to be matched and fed and that would be TX and RX.

It is easy to think a higher band is dead with a lesser antenna when in fact it is marginally open and you might make an astounding contact if you don't use a compromise antenna.

Ultimately comes down to how much effort you want to put in like everything.

The bottom line for radio, I am  torn between 2 factors...

Personal effort and the occasional satisfaction of bothering to go the extra mile to get real performance vs knowing that conditions and luck ultimately rule everything radio.

We may easily hide our effort shortcomings with "band is dead" that's a comforting guess. In fact, a guess that might offer a dishonest consolation to yourself. However, if you had a really good antenna, that band may have been open. The difference between a good antenna and a not so good antenna (not beams I mean like for like wires) can easily be more than 1 S point on TX and RX. So 'band dead' turns to 'band open' depending on what wire you personally chose to put up.

This should not be forgotten IMO.

You need to find where your individual/personal comfort lies and that is your own little journey with the hobby to find out. Coupled with safety and practicality for your exact situation. No right or wrong, only different ways of approaching.

If we as hams and CB people understood that we would probably argue less about practical operational details. Ultimately you choose yourself what you want to do.

I only take issue with it when something is presented to the public at large that is something great when in fact it is mediocre at best. That's going beyond your own personal choice. It's incorrect advice born from ego that may lead to disappointment in others when DX dreams are never realized.

Above all that, if it is fun and makes you smile, then it is right for that reason alone. Not much of a hobby without fun is it ?
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PostSubject: Re: EFHW - revisited   EFHW   -  revisited Icon_minitimeMon Apr 15, 2024 5:06 pm

So here's where I'm up to. (see pdf hopefully linked)
LINK > G8MCD CB/10m OCFD system
LINK > Coverage reports

An important note is that there's clearly a bit (!) of variation in FT240-52 cores and after some feedback from others I've now built and extensively tested 3 transformers. 
A technical appendix sets out what turned out different but I don't claim to know why  Very Happy .  The design uses the concept of a damped resonant circuit and transformer core losses to broaden the bandwidth and together use fewer secondary turns (giving fewer output capacitance problems?).  Maybe some PhD wants to take over from here ! 

But built for 27 - 30 MHz : They all work and deliver what I hoped and what I am calling a very convenient antenna

There are easier antennas and there are better antennas but for something discrete, broadband and in the middle on performance that you can throw out of a window this might be worth a look
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