A coil-loaded counterpoise at a low height above ground is used to
provide an artificial ground for an antenna when, for some reason, a
true ground connection is not possible or there is not enough space to
erect a full 1/4-wavelength horizontal wire.

The principal effects of low height are a loss resistance induced in
the wire from the ground, and a reduction in propagation velocity
along the wire.

A counterpoise can be considered to be an artificial ground. Its
input resistance at resonance is equivalent to the input resistance of
a true ground connection but its resistance will never be as low as a
good ground.

To provide a lower input resistance three or four counterpoises can be
used in parallel. Or they can be resonant on different bands.

New program C_POISE estimates the performance of a single coil-loaded
counterpoise. The number of turns on a coil of given length and
diameter needed to resonate a low wire to 1/4-wave resonance is
calculated. The input resistance at resonance is also estimated..

All program output data is approximate due to the very low height of
the wire above ground and uncertainty in the characteristics of the
soil or whatever other materials may be under the wire. The wire may
be allowed to rest on the ground surface but uncertainty will be at a
maximum. It will nearly always be needed to prune the wire length for
resonance. Or when the wire length is fixed the coil turns may need
adjustment.

When wire height exceeds wire length accuracy is much improved. Input
resistance at resonance will be fairly small. Working Q will be high
and the counterpoise will behave as a short antenna. For precise
resonance the wire will still need pruning.

Download program C_POISE in a few seconds from website below and run
immediately.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Reg,

Thanks. I assume the coil is located next to the feed point?

In the case of a single ground radial what can you say about the radiation pattern?

Thanks,

Dan

Reg Edwards wrote:
A coil-loaded counterpoise at a low height above ground is used to
provide an artificial ground for an antenna when, for some reason, a
true ground connection is not possible or there is not enough space to
erect a full 1/4-wavelength horizontal wire.

The principal effects of low height are a loss resistance induced in
the wire from the ground, and a reduction in propagation velocity
along the wire.

A counterpoise can be considered to be an artificial ground. Its
input resistance at resonance is equivalent to the input resistance of
a true ground connection but its resistance will never be as low as a
good ground.

To provide a lower input resistance three or four counterpoises can be
used in parallel. Or they can be resonant on different bands.

New program C_POISE estimates the performance of a single coil-loaded
counterpoise. The number of turns on a coil of given length and
diameter needed to resonate a low wire to 1/4-wave resonance is
calculated. The input resistance at resonance is also estimated..

All program output data is approximate due to the very low height of
the wire above ground and uncertainty in the characteristics of the
soil or whatever other materials may be under the wire. The wire may
be allowed to rest on the ground surface but uncertainty will be at a
maximum. It will nearly always be needed to prune the wire length for
resonance. Or when the wire length is fixed the coil turns may need
adjustment.

When wire height exceeds wire length accuracy is much improved. Input
resistance at resonance will be fairly small. Working Q will be high
and the counterpoise will behave as a short antenna. For precise
resonance the wire will still need pruning.

Download program C_POISE in a few seconds from website below and run
immediately.
----
.................................................. .........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........

Reg,

Thanks. I assume the coil is located next to the feed point?

In the case of a single ground radial what can you say about the
radiation pattern?

Thanks,

Dan
=========================================
Yes, the loading coil is located next to the feedpoint. I forgot to
mention it in the program notes.

The antenna used with a counterpoise will very likely be a simple
vertical or an inverted-L

When the counterpoise is not near the ground the radiation pattern of
the whole system, antenna plus counterpoise plus feedline, will be
somthing like that of a bent, off-centre-fed dipole, which extends
between the top of the vertical antenna and the end of the
counterpoise, and slopes at an angle relative to the vertical.

It will be non-descript and have no interesting properties, not worth
the time and trouble of modelling with Eznec.

When the counterpoise is lying on the ground the radiation pattern
will be similar to that of the antenna alone. But it cannot be
modelled with Eznec.

Counterpoises are useful because they allow end-fed antennas to work
moderately well, at one frequency, when a good ground connection is
not available. With an antenna wire they behave as an off-cenre-fed
bent dipole with one end near the ground.
----
Reg.

Reg , What is your opinion about the effectiveness of (a) counter
poise(s)
of any length in series with an adjustable series resonance circuit
,adjusted for maximum RF current at the operating frequency ?

Frank GM0CSZ / KN6WH
===========================================

Frank, the effectiveness of a counterpoise depends on its input
resistance. The lower this resistance better it is. It is the
'ground' connection for the antenna. (And the transmitter).

But there's no point in making it longer than 1/4-wave. This increases
coupling with the ground, increases loss resistance and also
undesirably increases radiation resistance. The counterpoise then
behaves more like a very low antenna.

When it is 1/2-wavelength long it has a very high input resistance and
is entirely non-effective. It may just as well not be there.

At some lengths the loading coil would have to be replaced with a
loading capacitor.

Remember, the antenna wire plus counterpoise is behaving as an
off-centre fed, bent, dipole with the lower section being very near to
the lossy ground. (Whatever the ground is). We don't want the lower
section to be TOO long. The current into the counterpoise is the same
current as is entering the antenna. If there is an antenna tuning unit
it will tune up the counterpoise simultaneously with the antenna. They
behave together as one thing!

So there is no point in having a separate tuner just for the
counterpoise. Maximum current flows into the counterpoise at the same
time as it flows into the antenna proper.

The purpose of the loading coil in the counterpoise is to provide a
low impedance connection to the braid of the coaxial feedline. If
there is no feedline then it provides an artificial 'ground' for the
transmitter plus tuner.

And all the foregoing arises because it is difficult or impossible for
the station owner to provide a set of buried ground radials.
----
Reg, G4FGQ