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Old 10-Apr-2020, 5:49 PM   #6
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 173
Quote:
Originally Posted by bobsgarage

Here is a scan comparing my one operational 30 - 2470 versus the two paired up and extended:

...So, why is the signal just mediocre?
The blue trace (antenna pair) shows a dip in middle of RF 8 spectrum.
That is indicative of two signals interfering with each other (out-of-phase).

An out-of-phase condition can result from:

* Multipath - the signal from the station arriving from two different paths,
one path delayed or advance relative to the other.

* Phase unbalance in Antenna system

One antenna providing a signal that when combined with phase opposing
the signal from the other antenna.

There are about five places in your setup (shown in initial post) where
phase misalignment could occur:

1. The balun at the antennas
2. Unequal coax lengths from antenna to preamps
3. RCA Preamps, Internal filters aligned different yielding phase differences
4. Combiner - Internal circuitry unbalanced
5. Unequal coax lengths from preamps to combiner

Seems, most likely suspect is the preamps (#3 above).

There are several Inductors (L) and capacitors (C) at input and output
of the RCA preamps. The L & C have to be hand tweaked for best signal
amplification, but there is no attempt to make one preamp identical to another.

If the preamps could be bypassed with F-81 connectors (barrels).
The signal would be lower, but if the spectrum was flatter, that might add insight.
May need to turn off power to avoid damage to power source.

If spectrum was flatter, then compare dual antennas without preamp
to single antenna without preamp.

------------------

If it appeared that cancellation was still occurring, check #1 above:

Might try switching baluns from from spare Driven elements (Folded dipoles) ,
to determine if the baluns were source of phase difference.

Quote:
It's possible the sagging in the antennas is causing the directors
to be out of alignment with the dipole?
Doesn't seem that would have much affect on phasing. Very tiny
(almost not measurable) affect on gain.

Quote:
The funny thing is some UHF channels come in perfectly. Even from the opposite direction;
Have seen similar performance with another different set of long Yagis.

.
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