How much does a counterpoise radiate?
 

Antenna restrictions here in HB9-land are severe, but I want to get on
the air from my 3rd-floor apartment. I have draped a horizontal
longwire outside along the wall of the building. This wire is now just
short of 20m.

Before I transmit, I want to make sure to minimize any RFI that
bothers the neighbors' stereo systems and other electronic equipment.
First, I will put on a lowpass filter at the tx.

Past experience at other QTHs tell me that I will also need a
counterpoise. But here's the question: does the counterpoise radiate,
even if it's cut to 1/4 wavelength? Then, if I put the counterpoise on
the floor in my shack I haven't really solved the problem because the
CP radiates into my shack and my neighbors' apartments?

I realize that a counterpoise is the "second half" of an antenna. That
being the case, what's the difference between a traditional dipole and
a longwire/counterpoise configuration? Both legs radiate, correct?

Thanks for any clarification anyone can shed on this mystery!

73,
Paul HB9DST
An expat-American cliffdweller in Switzerland

How much does a counterpoise radiate?
 

On 7 Apr 2007 16:35:31 -0700, "HB9DST"
wrote:

does the counterpoise radiate

Hi Paul,

Yes, absolutely. How much the contribute to radiation is a matter of
phase contributions. The many elements (or spokes) of a ground plane
will tend to cancel their radiations in the far field, but absolutely
every inch of everyone of them radiates. For "the other half" you
will not achieve this cancellation, except in some directions (given
the classic dipole's figure 8 pattern).

73's
Richard Clark, KB7QHC

How much does a counterpoise radiate?
 

HB9DST wrote:

Past experience at other QTHs tell me that I will also need a
counterpoise. But here's the question: does the counterpoise radiate,
even if it's cut to 1/4 wavelength?

Yes, it will radiate - *especially* if it's cut to a quarter-wavelength.

Then, if I put the counterpoise on the floor in my shack I haven't
really solved the problem because the CP radiates into my shack and my
neighbors' apartments?

I realize that a counterpoise is the "second half" of an antenna. That
being the case, what's the difference between a traditional dipole and
a longwire/counterpoise configuration? Both legs radiate, correct?

Correct. If you call it a "counterpoise" instead of "the second half of
the antenna", it changes nothing but the name. It is still the same
wire, and will radiate exactly the same in both cases.

However much RF current you manage to push into the part of the antenna
that you do want to radiate, you can be sure that an equal current is
flowing into the other part... or maybe into the mains wiring.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

How much does a counterpoise radiate?
 

Ian White GM3SEK wrote in
:

However much RF current you manage to push into the part of the antenna
that you do want to radiate, you can be sure that an equal current is
flowing into the other part... or maybe into the mains wiring.

.... or the dirt, as the case may be.

Owen

How much does a counterpoise radiate?
 

On 8 abr, 01:35, "HB9DST" wrote:
Antenna restrictions here in HB9-land are severe, but I want to get on
the air from my 3rd-floor apartment. I have draped a horizontal
longwire outside along the wall of the building. This wire is now just
short of 20m.

Before I transmit, I want to make sure to minimize any RFI that
bothers the neighbors' stereo systems and other electronic equipment.
First, I will put on a lowpass filter at the tx.

Past experience at other QTHs tell me that I will also need a
counterpoise. But here's the question: does the counterpoise radiate,
even if it's cut to 1/4 wavelength? Then, if I put the counterpoise on
the floor in my shack I haven't really solved the problem because the
CP radiates into my shack and my neighbors' apartments?

I realize that a counterpoise is the "second half" of an antenna. That
being the case, what's the difference between a traditional dipole and
a longwire/counterpoise configuration? Both legs radiate, correct?

Thanks for any clarification anyone can shed on this mystery!

73,
Paul HB9DST
An expat-American cliffdweller in Switzerland

Hello Paul,

You are fully right, the current that goes into your CP wire does
radiate. With regards to nearby interference, you also have to
incorporate the near fields.

First you can reduce the current into the CP by using a halve wave
radiator. That has kOhms of input impedance, hence reducing the
current into your CP by about 8. So the 20m wire is not a good choice
for 80m.

Other option is to use more wires to emulate a star configuration, the
current in the wires reduce and the resistive part of the CP (reducing
losses). But there is more. When you use for example 4 wires, the
currents in the opposite CP wires, flow in opposite directions,
reducing total radiated field. Because of lower current, near field
does reduce also.

I think the best option is to connect as much as metal (fences,
concrete wire grid, etc) to each other. The goal is to spread the CP
current as much as possible over the apartment building (in both
horizontal and vertical direction).

How far is the antenna wire from the wall? If it runs parallel, the
radiation will go into the building again (if there is no reinforcing
wire mesh present). When it runs too close to the wall, the wall will
absorb significant part of the power and/or unintended radiation from
domestic electronic equipment will enter your antenna.

Best Regards,

Wim
PA3DJS

How much does a counterpoise radiate?
 

On 7 Apr 2007 16:35:31 -0700, "HB9DST"
wrote:

Antenna restrictions here in HB9-land are severe, but I want to get on
the air from my 3rd-floor apartment. I have draped a horizontal
longwire outside along the wall of the building. This wire is now just
short of 20m.

Before I transmit, I want to make sure to minimize any RFI that
bothers the neighbors' stereo systems and other electronic equipment.
First, I will put on a lowpass filter at the tx.

Past experience at other QTHs tell me that I will also need a
counterpoise. But here's the question: does the counterpoise radiate,
even if it's cut to 1/4 wavelength? Then, if I put the counterpoise on
the floor in my shack I haven't really solved the problem because the
CP radiates into my shack and my neighbors' apartments?

I realize that a counterpoise is the "second half" of an antenna. That
being the case, what's the difference between a traditional dipole and
a longwire/counterpoise configuration? Both legs radiate, correct?

Thanks for any clarification anyone can shed on this mystery!

73,
Paul HB9DST
An expat-American cliffdweller in Switzerland

Another way to look at it, without the counterpoise, you may have
trouble getting the swr down to a reasonable level, and the rig may be
very rf-hot. I recently played with an end-fed wire on 20 meters, I
believe it was 3/4 wavelength long, and without a 1/4 wave
counterpoise on the tuner's ground lug, it was untuneable. With the
counterpoise, it worked.

bob
k5qwg