[Antennas] End-fed half-wave religion

Sun, 14 Jul 2002 21:19:55 +0100

I'm late in on this one..so don't quite know what the thread is about.. If
it's of any use, I have just added a new page to my website on my hombrew
vertical dipoles for the lowbands. It's not quite finished, but it's at
http://www.iol.ie/~bravo/low_band_antennae.htm  if anyone's interested.

  73
         John EI7BA http://www.iol.ie/~bravo/

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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