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