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Old 16-Jun-2013, 4:57 PM   #33
Senior Member
Join Date: Jan 2010
Location: Dallas, TX
Posts: 139
Tradeoff Gain as Example

Originally Posted by Stereocraig View Post
Very interesting, Tripelo.

Would the isolation provided by one preamp per antenna, also allow for dissimilar antennae and/ or different feed lengths to the combiner?
Yes, but

There are tradeoffs. Some mentioned earlier, but not every detail.

Depends on objectives.

An example:

If the two antennas (either alike or different) are receiving signals of similar power level from different stations, then probably the preamps should be well matched in gain and noise figure. In general, transmission line length doesn't matter (but in specific cases, see below, it can matter).

After combination, if amplifiers are not gain matched, the noise from amplifier with the high gain will dominate the noise floor for both signals. Then the signal from the amplifier with low gain may suffer a noise floor that is higher that it may have had before combination (thus lowering S/N ratio for that particular signal).

As with most things, this (preamp gain mismatch) can either be an disadvantage or an advantage.

If one had a really strong signal, then that particular preamp gain could be chosen to be lower than the preamp for the weaker signal. This could help the weaker signal after combination because the noise floor would not be quite as high as otherwise. This is at cost of some reduction in S/N for the strong signal.

Signals received from more than one antenna

If both the antennas (different or not) are receiving appreciable signal power from a desired station, when combined there can be phase interference, somewhat like short delay multipath. Since this uncontrolled phase relationship between signals is somewhat random, odds are that it may not be severe.

For consistent results, each desired station's signal power should be pretty well eliminated from all but one antenna.

Conceivably, one could adjust transmission line length to help phase align the signal from one station that was being received by both antennas.

Last edited by tripelo; 16-Jun-2013 at 5:00 PM.
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