[Antennas] End-fed half-wave religion

Sun, 14 Jul 2002 18:21:06 -0500 (CDT)

> I'm sorry but I don't see that you answered my questions.

I'll assume that although you were replying to someone else's email
when you made the comment above, you were (at least in part) directing
the comment at me, since it was me you originally asked, quoted below.

And, since you don't see, let me be more explicit:

> > > Please explain how you go about connecting a transmitter to that?

Typically the transmitter is connected to the low impedance end of
the matching section which transforms from low impedance to high
impedance.

> > > 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).

As I said above, the transmitter's terminals are connected to the low
impedance end of the matching network.

> > > 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?
> 
> I will say that both current and voltage must be present or there is no 
> power. P= I*E

And I will say that as voltage approaches infinity, current approaches
the infinitesimal.  With end-fed half-wave elements driven by an
appropriate high impedance circuit, the current becomes so small that
no actual antenna element is required to provide for its return.  The
mere presence of the end-point of the connection to the other side of
the circuit is sufficient.  Things must be so:  We see antennas working
that are connected in just this way.

I doubt these antennas would work if real-world components strictly
obeyed the properties we ascribe to the models we use to predict their
behavior.  If transmission lines could *only* transfer power, and
nothing else, we'd be out of luck, after all.

If I am not mistaken, the mechanism which allows the magic to occur is
stray capacitance.  The half-wave element is driven by the voltage which
appears at its end.  This voltage can appear because there is stray
capacitance between this end of the antenna element, this side of the
high impedance feed system, and the other side of the feed system, which
isn't connected to anything in particular.

Now, there are purists who argue that transmission lines may only behave
as transmission lines, and that something else must be connected to the
transmission line if it is to perform any other function, and that
therefore, there must be some other something which we connect to the
other side of the high impedance end of our matching network if we are
to have a complete antenna circuit.  These folks conclude that we must
connect a counterpoise of some sort to the other side of an end-fed
half-wave's matching network for it to truly work.  (I suppose these
folks must believe that the many satisfied users of antenna systems
without such a feature would be satisfied with a dummy load with a wire
connected to one side, too.)

Clouding the issue is the question of whether one must have radials
for a ground-mounted end-fed half-wave.  Muddy indeed are these waters,
because it turns out that a ground-mounted end-fed half-wave will in
fact "work" without anything connected to the "other" side of the high
impedance end of the matching network; on the other hand, it will
suffer ground losses in the neighborhood of 3 dB or so in the process.
Purists would argue that an antenna system which is not functioning
at 100% of its theoretical maximum efficiency isn't "working" and that,
therefore, the ground-mounted end-fed half-wave "needs" to have that
other side of the high impedance matching network connected to ground
radials.  One might then extrapolate from this "fact" that all end-fed
half-wave elements must also be given a similar ground plane or
counterpoise to work against, but that would be an error.

More practical, fuzzy-thinking types look at it this way:  If the
thing works pretty well without any sort of formal counterpoise, it
doesn't "need" one after all.  If the stray capacitance between the
end of the un-connected side of the high impedance end of the matching
network and the side which is connected to the antenna allows enough
current to flow to support developing the voltage which will drive
the antenna, then all is well.

The thing I'm remaining curious about is the lack of consensus.  The
above paragraph is founded on the word "if".  *If* end-fed half-waves
work fine without any sort of counterpoise, who needs 'em?  Many hams
report that they do, and some of them are intelligent, educated,
respected hams who ought to know what they're talking about.  So, too,
are some others who insist that such antenna systems will not work
without a counterpoise of some sort.  (To add further to the mess,
those who advocate the counterpoise do not agree whether it's a
random short length, a quarter-wave, or a half-wave counterpoise
that's needed, and some say it doesn't matter.)

James Duffey helped me see this whole thing a little clearer (making
me glad I asked my question when the topic at hand drifted close
enough to justify my bringing this up) by suggesting that bandwidth
may be the limiting factor--these counterpoiseless systems may work
best when the matching network is perfectly matching things up, and
the counterpoise may help it work when the frequency drifts further
from perfectly tuned.  Perhaps that's part of the answer--perhaps
the counterpoise fans (the real-life ones, not the armchair theorists)
like to wander around the band more than the ones who insist you don't 
need the counterpoise.

In any event, I'd love to hear more discussion from any of you who
have an opinion.  Just how necessary is the counterpoise?  Under
what circumstances is it more likely to be necessary, or not?
Which specific antenna systems need it the most, or the least?

73,
Jim  WB5UDE
> 
> Danny
> 
> At 05:50 PM 7/14/2002 -0400, you wrote:
> >Dan, Dan, Dan: m'boy you have to broaden your concept of driving an
> >antenna to include voltage mode, e.g. end fed dipole, as well as current
> >mode, center fed dipole.
> >
> >Hmmmm ....  both are dipoles!! How about that!
> >
> >Deacon Dave, W1MCE
> >
> >Dan Richardson wrote:
> > >
> > > 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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