[Antennas] End-fed half-wave religion

[alexander eban]

......and also the j-pole!!! The quarter wavelength stub at the lower end
does exactly what the tuned circuit does! It tansforms the low impedance of
the coax to the 10 kiloohms or so of the end fed impedance. Remember you can
always replace a tuned circuit by a stub, at the expense of bandwidth.

-----Original Message-----
From: psykey@okcforum.org [mailto:psykey@okcforum.org] 
Sent: Sunday, July 14, 2002 10:07 PM
To: k6mhe@mendolink.com
Cc: antennas@mailman.qth.net
Subject: Re: [Antennas] End-fed half-wave religion


Danny--

The classical matching scheme for an end-fed half-wave is a resonant circuit
consisting of a coil and a variable capacitor, tuned to the operating
frequency, with either another coil coupled to the tuned circuit, or a tap
about 20% up the tuned circuit's coil.  For an example, see:

http://www.g3ycc.karoo.net/endfed.htm

Standard web searching techniques can produce many pages with variations on
the same theme.  You might try google with search parameters such as:

"end fed" "half wave" coil

Some suggest connecting a counterpoise at the low-voltage end of the tuned
circuit.  Others do not.

If it's going to be a ground-mounted vertical, then a ground plane might be
provided, to prevent ground losses.  According to analysis I've read, which
makes sense to me, the situation is no different than if one were to mount a
center-fed half-wave (a dipole) in the same vertical configuration, with one
end near the ground.  The proximity of the ground to the center-fed
half-wave would cause the same sorts of losses that would result in a
ground-mounted end-fed half-wave.  Those losses, and the resulting need for
ground radials to prevent them, are, however, peculiar to the situation
where the end of the half-wave is near the lossy earth ground.  There is no
direct parallel to an end-fed half-wave in free space (or high above ground,
measured in wavelengths).

If, however, one chooses to mount an end-fed half-wave in a vertical
orientation near the ground, and not provide a counter-poise or ground plane
of any sort, one can do so, with a loss of around 3 dB or so.  One certainly
could not choose to connect a quarter-wave vertical (regardless of proximity
to ground, or not) in a similar fashion and expect it to work at all (except
in the same manner in which a dummy load "works").  In some situations (for
example, portable operation, or where radials are not feasible) it might be
considered a pretty good deal to be able to put up a ground-mounted end-fed
vertical which can operate without radials, at a penalty of a mere 3 dB
loss.

There are other options for matching/connecting an end-fed half-wave. For
example, there is the J-Pole... it has a matching mechanism, and at the end
of that matching mechanism (normally the "top" of it, but that is not a
requirement) one side is attached to the end-fed half-wave portion of the
J-Pole, and the other is attached to... "nothing".  Or, ARRL publications
such as the handbook and the antenna handbook show horizontal end-fed
half-waves fed with open line balanced feedline. 
Again, one side is attached to the end of the half-wave, and the other side
of the feedline simply terminates.

One may argue that an antenna feedline must in all cases be attached to
"something" on both sides, using the "complete circuit" argument. That
argument starts off a bit shaky, however, since in many types of antennas,
there's no clear path from the antenna element(s) 
connected to one side of the feedline, back to the element(s) connected to
the other side of the feedline.  People do argue that there is still some
kind of path, at least from an RF point of view... but I have never seen any
such argument that convinces me that an actual antenna element must be
present on the "return" side.  Perhaps in the voltage-fed situation, it is
sufficient for the other side of the feedline to just...well... be there.

On some level, it is difficult to successfully argue that the other side of
the feed mechanism *must* be connected to something else, because, hey...
lots and lots of people are using antenna systems in which there is no such
connection, and many of them are well experienced and educated amateurs who
are very happy with the performance of their antenna systems.  It's really
hard to dismiss them all as a bunch of dimwits who little realize that once
in a while, one can even make contacts on a dummy load.

I have seen people point out that there may be some slight skewing of the
pattern of some kinds of end-fed half-wave antennas from the ideal version
of the pattern when the "other" side of the feed mechanism isn't connected
to anything.  Nevertheless, such antenna systems do, by and large, work.
Some of them, I hear, work quite efficiently.

As far as I've been able to gather, there exist quite a few antenna designs
which are based on some sort of end-fed half-wave element, and a
feeding/matching mechanism which involves the "other" side of the feed
mechanism being connected to "nothing."  I gather these antennas (J-Pole,
just to cite one common example) work not merely adequately, but downright
impressively, at least in the clear.  I also gather there are some losses
(though not the huge, dramatic losses other antenna systems would suffer)
operating these end-fed half-waves with one end on or near the ground,
without providing radials to improve the ground's ability to reflect the
signal.  But the evidence seems to suggest that there are schemes for
feeding/matching end-fed half-wave antennas which involve connecting the
"other" side of the feedline to "nothing", and, notwithstanding arguments
that such is impossible, do a fine job anyway.

73,
Jim  WB5UDE
> 
> At 11:46 AM 7/14/2002 -0500, you wrote:
> >There's no need for a counterpoise of any sort to make any of these 
> >function correctly and efficiently, is there?
> >
> >73,
> >Jim  WB5UDE
> 
> Please explain how you go about connecting a transmitter to that?
> 
> If you connect one conductor of your transmission line (or one 
> terminal
> from the transmitter) to the end of your single element where do you 
> connect the other end (or the transmitter's other terminal). And, most 
> importantly, what path does for the current use to get to the end of the 
> transmission line (or the transmitter's other terminal) which is not 
> connected to the antenna element?
> 
> 73
> Danny
> 
> 
> 
> 
> 
> 
> 
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> 
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