[Premium-Rx] Harris RF-551A Preselector

[Blair Batty]

Hi All:

You might be familiar with the Harris RF-551A Preselector,
which is a common companion to the Harris RF-590 Receiver,
Harris RF-1310 Exciter (as a Postselector) and much other
Harris gear. Some background:

   - Stand-alone, 19" rackmount, 5" high,
   - Front tuning controls, to the nearest kHz
   - will auto-tune to RF-590 w/ cable & interface
   - "N" connector RF in,  BNC RF out
   - Amplification: High +8 dB, Low -24 dB,
   - Frequency range 2MHz - 29.999 MHZ
   - Tuning accuracy typically 0.5%
   - Overload protection: no damage with 100 Watts of RF power

The RF-590 receiver already has an excellent built-in preselector,
and is very selective, so why did Harris think this gear necessary?
Well I found the answer in some Harris literature (w/ some editing):

"...Presume a condition with two 35 foot whip antennas located
12 feet apart, one being used for transmit and the other for receive.
If each antenna is tuned with an antenna coupler, and the frequencies
are at least 10% removed, a 1000 watt transmitter will couple only
39.8 watts into the receive antenna. (Tuned to the same
frequency will result in 500 watts coupled!).

The Preselector input protection circuitry activates at 100 watts,
so will not be activated by the 39.8 watt signal. Further, the preselector
should be able to provide  60 dB of Attenuation to a signal
whose frequency is 10% removed; reducing the 39.8 watt signal
to -14 dBm (44.6 mv)."

The second improvement is improving the sensitivity of the receiver,
by the Preselector removing the transmitter's broad band noise.

Imagine a battleship with half a dozen 1000 watt transmitters
and receivers with a tower bristling with antennas, and you can
see how Preselectors are necessary.

You can download a (20 mb) PDF of the RF-551A Preselector manual here:
http://www.ontariorocks.ca/temp/RF-551A.pdf

Should you run out and buy a RF-551? If you live in a RF quiet area,
using a premium receiver w/ builtin preselector you'll gain little.
But if you're in a noisy area, with multiple transmitters and
antennas a RF-551 can help.

I see no difference in reception using my RF-590, with or without
the preselector. But I do see a definite reduction in intermod, etc with
lesser radios. I use the Preselector all the time with my RF-590 anyway;
I like the extra layer overload protection, the extra filtering can't
hurt, and I like the satisfying whirr of the motors, as the Preselector
tracks the receiver...

Sincerely
Blair