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Author Topic: Taming the End-Fed Wire, Article by G3CCB, (SK)  (Read 2439 times)
VA3DDN
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Posts: 3




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« on: November 20, 2012, 03:27:53 PM »

I would like to get a copy of Alan Chesters article  "Taming the End-Fed Wire", that was published by RSGB and in the book "the Antenna File."
The book is no longer available from either ARRL or RSGB.
If anyone can get me a copy of the article it will be much appreciated.
Don
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ZL1BBW
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Posts: 371




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« Reply #1 on: November 21, 2012, 11:41:25 PM »

I did a google and came up with this site.....

http://rapid4me.com/?q=rsgb%20the%20antenna%20file


I offer no warranty that the site ok, but if it is please let us know.  Thanks
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
VE7TIT
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Posts: 62




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« Reply #2 on: November 24, 2012, 10:37:06 AM »

AA5TB's web page on end-fed wires is very good (as is everything at his site): http://www.aa5tb.com/efha.html . I think he cites the coverage in Moxon's book, but his site is really all you need to get going.

I have used end-fed half waves for years, mostly in the field, but at home too. In a nutshell, just make the wire a little longer than 1/2 wave, tune it with an L network that has a series coil and a shunt capacitor that can go to very low capacitance (you'll need very little C), and attach a little piece of wire as a "counterpoise," and you're ready to go. I have about five 8-inch turns of coax at the feedpoint as a choke and a homebrew "artificial ground" at the transmitter end.

Here is a great online tool for figuring out the L-network values: http://www.daycounter.com/Calculators/L-Matching-Network-Calculator.phtml In my experience, though, with multiples of a half wave, you just need 5-50 pF of shunt capacitance and you can hunt around for resonance on the series coil tap--very easy to tune. The AA5TB article explains it. (I have had surprisingly good luck tuning non-half-wavelengh bands with this setup as well--a pleasant surprise). The "tuner" at my window is just a coil of #12 wire with an alligator clip and a small air variable. I can switch bands in seconds by moving the clip and twisting the knob on the cap.

I think end-fed antennas are often kind of underrated, probably because it seems magical to feed an antenna at the end and have no radials (and because it brings the high-voltage end down to the shack--not a big deal for QRPers), but they are simple, forgiving, and can really work. 67 feet of #12 wire strung low over the roofline from a second story window makes an adequate stealth antenna for 7-28 MHz. 10 meters of hookup wire flung onto a tree branch makes a great 20 meter field day antenna (especially when there's a picnic table right under the tree Wink).

I hope that helps. Have fun.
« Last Edit: November 24, 2012, 11:01:35 AM by VE7TIT » Logged
WB6BYU
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Posts: 13147




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« Reply #3 on: November 24, 2012, 11:11:35 AM »

Quote from: VE7TIT

... tune it with an L network that has a series coil and a shunt capacitor that can go to very low capacitance (you'll need very little C) ...



A good estimate for the capacitance in picofarads required to match a 1800 ohm load would be
the wavelength in meters.  So 80pf for 80m, 20pf for 20m, etc.  Doesn't hurt to go a bit larger,
but a variable with 100pf maximum should cover the HF bands.

In practice most wires will have a higher impedance, so probably requires less capacitance,
perhaps 2/3 of that value.  To give some room for adjustment the minimum value should be
no more than half of the original value, which also should take into account the circuit
strays.  That would be 5pf on 10m, at which point the circuit strays need to be minimized.

But that gives a convenient (and easy to remember) rough estimate of the capacitor size
required.



Quote

I think end-fed antennas are often kind of underrated, probably because it seems magical to feed an antenna at the end...



Others think they are black magic because, no matter what they do, they have RF in the
shack and their audio sounds garbled or their keyer sticks on or the display blinks.  (I've had
all these symptoms running QRP.)  That's not to say that they can't work well - I have two
end-fed wires up as my primary antennas right now, and have used several different ones
over the years.  But I wouldn't usually recommend them for a beginner unless they have
a convenient Elmer to help them work through the potential problems.  That's one of the
reasons why they aren't rated as highly as they might be otherwise.
« Last Edit: November 24, 2012, 11:37:02 AM by WB6BYU » Logged
VE7TIT
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Posts: 62




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« Reply #4 on: November 24, 2012, 11:27:23 AM »

...that gives a convenient (and easy to remember) rough estimate of the capacitor size
required.

That's a great rule of thumb.  Thumbs up!  Wink

(I specifically mentioned the low minimum C requirement because an old homebrew L network I have doesn't work with end-feed because the inductance of the rats nest of wires going to the coil taps and switching is too great. With the very low C values needed to match using an L-network and end feed, you need very short, direct wiring, hence my super-simple windowsill tuner.)

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VE7TIT
Member

Posts: 62




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« Reply #5 on: November 24, 2012, 12:42:34 PM »

...they have RF in the shack and their audio sounds garbled or their keyer sticks on or the display blinks.  (I've had
all these symptoms running QRP.)

My window with the tuner is about five meters from my radio, fed by some RG-8. I think the distance, the choke, and the "artificial ground" I mentioned all help to keep the RF where it belongs and at an acceptable level. Yeah, even at QRP, you've got a pretty intense field at the antenna end.
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W2ANZ
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Posts: 9




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« Reply #6 on: November 24, 2012, 01:32:07 PM »

I too have used an end-fed wire as my primary antenna.  As others have said, in order to control the RF in the shack you need to provide a counterpoise at the tuner and then choke the coaxial cable really well between the tuner and operating position. RF is going to find the 'other side' of the antenna.  If you don't provide a counterpoise then the equipment in your shack and the AC wiring of your house will become the other side.  I have had lots of RF burns to prove it (the worst was when I get one on the lip from my microphone). 

I had good results up to 100 watts.  Above that the RFI issues became obnoxious.  I got by with a simple L network - a variable capacitor and an air coil tapped with an alligator clip.
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KB1GMX
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Posts: 761




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« Reply #7 on: November 24, 2012, 07:03:14 PM »

There are two antennas being discussed, End Fed Wire and End Fed Half Wave.  Huge difference.

End fed random length wire will absolutely need a ground/counterpoise at the feed location.
It's a reactive and likely low impedance at the feed point.  I found these to be troublesome
especially if the feed point is at the shack tuner.  Generally there is significant shield currents
unless great pains are taken to suppress them.    I run an inverted L (3/8ths lamda) that is
about 6ft from the  shack at the feed point but the matching is done there and it has a good
earth ground under it and multiple radials to help that not a problem.  I tried it in the shak and it
was impossible to tame, too much RF close by and despite a bus bar and ribbon to ground that
6ft of copper to ground was enough to work like a very poor ground and every thing had
large amounts of RF on it.

End fed half wave (resonant as in R=2000 to 4000ohms and x is near zero) will be less problematic
but it is only usable at half wave, and multiples of an even half wave if the matching system can adjust
for that.   There will be small shield currents but they should be low enough to not be troublesome
unless the antenna is not resonant (if not resonant see random end fed wire).  Again having the
feed point near the radio (less than 3ft) has proven to be a problem for me as the radiator is
less than a few feet away from the radiator.  I have not had  problems with remote feed points
(match at end of coax).

Generally resonant antennas regardless of feed impedance is more easily tamed than some
random length with high reactance and possibly low feed point resistance. 


Allison
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W2ANZ
Member

Posts: 9




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« Reply #8 on: November 24, 2012, 07:46:19 PM »

The concept of a one-sided antenna causes me great consternation.  I have lost sleep trying to understand the end-fed Zepp.   Where do the electric field lines at the feed point of an end-fed 1/2 wave terminate?   Huh
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VE7TIT
Member

Posts: 62




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« Reply #9 on: November 24, 2012, 08:38:08 PM »

The concept of a one-sided antenna causes me great consternation.  I have lost sleep trying to understand the end-fed Zepp.   Where do the electric field lines at the feed point of an end-fed 1/2 wave terminate?   Huh
I'd suggest trying the AA5TB website in post #3, or the discussion of end feed in Moxon or the ARRL Antenna Book if you have them.
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K3ANG
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Posts: 177




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« Reply #10 on: November 24, 2012, 08:44:44 PM »

Found a very short summation of the G3CCB article at
http://www.qsl.net/v/ve3mcf//elecraft_reflect/End_Feed_Antennas.txt
Do a search on the callsign.

73
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