.. .
Yuri Blanarovich wrote:

There is another potential problem if feeding two amps to two antennas
that are reasonably close: RF from one antenna is induced into the
other
antenna, gets fed into the amp and causes some interference (mixing ?)
that shows up as a raspy signal.
Tried it, and heard it on another station attempting the same setup.
It seems that it would be easier to control the phasing at the input of
the PAs, but the above effect messes it up, unless antennas are widely
separated and they do not "feed" each other setup.
So, it appears that it is better to use one bigger amp and use proper
phasing to feed the two or more antennas.


I hope I did not cut incorrectly in the complex post...

The effect is that the *impedance* looking into each antenna will change due
to the mutual coupling of the two antennas. You can also think of it as
each antenna is not just staring into open space, but you can consider the
field of the other antenna to be changing the space around the first antenna
(and visa versa). It is also not umlike simply putting a reflecting plane or
other paracitic elements neat a driven element - the feed point impedance
changes. This power, being from the same transmitter, is coherent (phase
locked) and therefore changes the current that flows which manifests itself
as an impedance change. Given the current from the feed line voltage and
then the superposition of the current induced from the other antenna, you
get a new current that flows (aparently from the feed line voltage) and
therefore it appears as a different impedance (from the feed line's
perspective). I hope one of these analogies works for you.
This can indeed cause amplifiers to do funny things if they can't handle the
resulting SWR. If you correctly match to the new impedances, there should
be no problems, just the new antenna radiation pattern.

Side bar:
For some situations, the resistive part of this new impedance can even go
negative. This means that this antenna is absorbing power, not radiating
it. This can happen in phased, directive arrays used in broadcast. A
friend of mine described such a situation that vexed him for a while until
he improved his calculations to be more general to be applicable to this
situation (to get the correct result).
Don't ask me how to match to a negative resistance. All I know is that
given some certain directional pattern that is desired, this happens and
with the proper math you can design a matching network that works and
restores order to the universe and keeps the big bang happy (:-)

73, Steve, K9DCI