RadioBanter - Antenna polarity?

On 1/28/2014 3:19 AM, Gone Fishin' wrote:
My brother is broadcasting from a little low-mW FM transmitter he's using to
broadcast a web-streamed station to a receiver in his small home.

The receiver is using a dipole and the transmitter a telescoping.

Should the telescoping antenna be vertical?

Thanks.

This is a more complicated question than it first appears.

In general, the two should be parallel - that is, if the transmitting
antenna is vertical, the receiving antenna should also be vertical. And
if the transmitting antenna is horizontal, the receiving antenna should
be horizontal. There can be a large loss when one is vertical and the
other horizontal.

However, this is also only true in free space. In your brother's home,
there will be reflections which can change the polarity of the signal.
So the polarity of the signal can change around the house so that the
signal is not perfectly horizontally or vertically polarized.

Another concern is the radiation pattern of the antennas. Simple wire
antennas such as the telescoping whip and dipole have maximum radiation
perpendicular to the direction of the wire, with very little off the
ends of the wire. This means that for a vertically polarized whip or
dipole, maximum radiation will be in a horizontal direction. So if the
antenna is in the attic and he's trying to receive in the basement, he
may not get much signal. But again, reflections will affect the signal.

There might even be places the reflections merge to cancel the signal
out - in which case moving the transmitting antenna a few inches can
make a difference.

Theoretically it would be possible to calculate all of the effects of
the above, but he'd have model the entire house in formulae, including
anything which will reflect or attenuate the signal (i.e. metal,
concrete, etc.). This would take a large amount of effort, and will
only be as accurate as the input data.

If it were my house, I would start with the two having the same
polarization and experiment in the places he wants to hear the signal.
Change the polarization on the receiver (i.e. hold the receiver in
various positions) so see what effect it has. Then change the
polarization of the transmitting antenna and repeat the experiment. See
what works the best in his situation.

OTOH, if he's happy with the results he has now, I would suggest he not
touch it.

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Jerry, AI0K

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