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From Decibel's description DB-228...
>A high-gain, broadband antenna for 138-174 MHz, the 8-dipole DB228
combines
two DB224E offset-pattern antennas to provide a 9 dBd gain omni or a 12 dBd gain offset horizontal pattern. Both DB224E antennas are mounted with the ends together, and the phasing harness is terminated at the center. A special bracket positions the antenna at a pre-determined distance from the tower.< you wrote...
>Antenna system consists of two Decibel DB228 8-dipole arrays,
mounted
on a 90ft self supporting tower. RX antenna is at the top, TX is mounted from about mid-tower down. Vertical separation between antennas is not much due to tower height limitations, and each antenna's length, but each DB228 is mounted on a separate adjacent tower leg.< The DB-228 is about 41 feet long. Top antenna cannot be fully top
mounted due to it's size so that means that about 20 ft is side mounted.
That puts the bottom of the TX antenna at 70 ft. >TX is mounted from
about mid-tower down.< OK I am interperating this to mean that the top
of the bottom antenna is at say 50 ft, which puts the bottom at 10
ft. Total antenna seperation 20 ft. or less. Big
problem.
When you purchased the system, was it engineered for this spacing or did
the antenna system come later?
You need more isolation between the antennas.
I would strongly suggest that you replace the antennas with DB-224's (20 ft
long). Aim for a minimum of 60 ft of space between the bottom of the
top antenna and the top of the side mount antenna. Since the description
of the DB-228 indicates that it is made of 2 DB-224E's it may be possible to
simply split one of the antennas apart and test that
way.
If you must stay with these high gain antennas then
your TX combining and RX distribution systems need to be reengineered taking
into account the short spacing (low isolation) between the TX and RX
antennas.
Ditch the RG-58 and the 9913 and use RG-214 or Superflex for jumpers. Verify the tuning of the cavities. Things
change in shipment, especially in rough terrain.
Milt
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