The counterpoise for an end fed half wave (or multiple) really doesn't
have to be anywhere near a quarter wave long. The feedpoint impedance
is something over 2000 Ohms. I use a single wire about 8 feet long,
which seems to work fine. NEC agrees. My simple test is to get it
perfectly matched, then put my hand on the tuner box. If the SWR
doesn't change, the counterpoise is adequate.
I did recently try the 40 M half wave on 80, with two quarter wave
radials. There was, of course, some "rf in the shack", but it didn't
cause any problems, and the antenna seemed to work fairly well,
especially considering that it was in the bottom of Death Valley.
73,
Scott K9MA
On 12/28/2016 18:14, Fred Jensen wrote:
Ron is correct ... however my experience is that the rig, headphone
cable, power cable from the battery, and me [holding the KX1] is more
than sufficient. For mine at home, it is fed with about 25 ft of
coax, the shield of which serves that purpose.
If you attempt to model it with NEC-2, and all you care about are the
radiation patterns, you don't need a counterpoise in the model. While
NEC-2 *really* doesn't like a source connected to the end of a wire,
it only affects the source impedance values. If you want those to be
reasonably accurate and real, a short wire is necessary ... anything
over a few feet seems to work just fine and is in no way critical.
Fred K6DGW
Sparks NV DM09dn
Washoe County
On 12/28/2016 3:43 PM, Ron D'Eau Claire wrote:
While the efficiency as a radiator of a half wave end fed wire is
relatively
unaffected by the "ground" return, some sort of "ground" is needed to
keep
the whole rig from floating up to the RF potential at the end of the
antenna. Often a hunk of wire roughly 1/4 wave long thrown on the
ground is
adequate.
I too have had excellent results with them.
73 Ron AC7AC
-----Original Message-----
From: Elecraft [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of
Fred
Jensen
Sent: Wednesday, December 28, 2016 12:57 PM
To: elecraft@mailman.qth.net
Subject: Re: [Elecraft] Small QRP antenna
The Summits On The Air crowd, at least one of whom "runs" up
mountains, has
had very good luck with end-fed half-waves. The transformer weighs
next to
nothing, and the rest is just wire. Requires no counterpoise, very
ground
insensitive since it's fed at a voltage node. Also because of that, you
need no coax, just an adapter between rig and transformer.
The higher you can get the middle [current node] the better, but it will
work very well with just about any elevation on the far end.
For 40, it's about 67 ft of wire, and is a full-wave on 20 and will work
well there. My KX1, when I still had it, had no problem finding a
match.
My HOA-Stealth here at home is an 80-10 EFHW strung about 6 ft high
along
the top of a wooden fence. Works surprisingly well.
Fred K6DGW
Sparks NV DM09dn
Washoe County
--Northern California Contest Club
--CU in the Cal QSO Party
--7-8 Oct 2017
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Scott K9MA
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