What exactly do counterpoise wires do for a vertical antenna? I realize
that they allow an impedance match, but what is physically going on with
the current that makes this possible?

What exactly do counterpoise wires do for a vertical antenna? I realize
that they allow an impedance match, but what is physically going on with
the current that makes this possible?

You can think of a counterpoise (somewhat simplistically) as "the
other half of the antenna".

Start by considering a half-wave vertical dipole, fed in the center
with coax. Equal-but-opposite currents flow out of the coax (from the
center conductor, and from the inside of the shield) and into the
radiating elements. Let's assume for the moment that the outside of
the feedline is decoupled/choked (e.g. there's a 1:1 balun at the
feedpoint) , so that current can't flow back down the outside of the
coax. In this situation, equal currents flow from the coax onto the
two halves of the dipole... and since they were opposite in direction
inside the coax, and since the dipole wires go in opposite directions,
the currents on the two halves of the dipole are in phase, and the RF
fields created by them reinforce one another. Power flows efficiently
from the coax to the dipole (reasonably low SWR, depending on coax
impedance and the presence of nearby conductors) and you get a good
signal on the air.

Now, take away half of the dipole... creating a vertical monopole,
with no counterpoise.

You still want current to flow into the half-dipole... but you've got
a problem. Current can flow easily from the center conductor of the
coax to the vertical... but there's no good place for current to flow
out from the inside of the feedline shield. It can't flow onto the
lower half of the dipole (that's gone) and it can't flow back out
along the outside of the feedline shield (the balun prevents that).
Instead, the flow of current just stops... there's a very high
impedance at the end of the coax shield (how high depends in part on
how good a balun you've got). Instead of there being 70 ohms
(nominal) of impedance at the feedpoint, there could be several
thousand ohms (35 ohms from the vertical, and the rest from the abrupt
current-stop at the end of the shield). You now have a terribly high
SWR and can't get a signal out.

Now, fasten one or two wires to the coax shield (right by the
feedpoint, on the antenna side of the balun). Cut 'em to 1/4
wavelength. You have now restored the current balance of the
antenna... current coming out along the inside of the feedline can
flow out along this wire (or wires), balancing the equal flow onto the
vertical radiator.

If the wire(s) droop downwards (directly below the vertical) then you've
completely restored the dipole... you'll see a 70-ohm (nominal)
impedance at the feedpoint.

If, instead, you run the wire or wires out sideways, you'll see a
different (lesser) impedance, and the antenna's radiation pattern will
be somewhat different. Two 1/4-wave wires, stuck out in opposite
directions, at a 90-degree angle to the vertical, form a simple basic
"ground plane" antenna, There will be equal-and-opposite currents in
these two wires, and the net RF radiation from them will be zero.
In the simple case, in free space) you'll see an antenna feedpoint
impedance of around 35 ohms.

Counterpoises can be used with more complex types of antennas, as well
(i.e. with radiators other than 1/4 wavelength). In each case, their
basic purpose is to create a "total antenna length" (radiator plus
counterpoise) which is resonant or nearly so, so that currents flow
easily from the feedpoint (seeing a convenient amount of resistance,
and little or no reactance) and an efficient matching of feedline to
antenna can be achieved.

If, instead of using a counterpoise, you simply don't put a balun at
the feedpoint... then you may or may not end up with antenna which
works acceptably. In this situation, the outside of the coax serves
as a counterpoise-of-sorts. Whether it works well (allows for a
decent match/SWR) is going to depend on the length of the coax and its
grounding arrangement and its path.

This is why you sometimes hear people (particularly in the CB
community) being very concerned about adjusting the length of the coax
from radio to antenna, in order to get a better SWR. Whenever you
hear this, you can be pretty sure that you're dealing with an antenna
which isn't balanced and doesn't have a well-designed counterpoise or
grounding arrangement, and the feedline is actually serving as part of
the antenna (and often radiating RF where it isn't really wanted).

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
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Al Fansome wrote:
What exactly do counterpoise wires do for a vertical antenna? I realize
that they allow an impedance match, but what is physically going on with
the current that makes this possible?

Visualize a 1/2WL center-fed dipole with one element
laying on the ground. The wire on the ground is the
counterpoise. What happens to the radiation when the
wire is laying on the ground?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com