[Premium-Rx] Harris RF-551A Preselector

[w3jn]

These were also used in full-duplex synchronous data circuits.  In this 
circuit, both stations are simultaneously xmitting and receiving; the 
"receving station" transmittng handshaking to the "transmitting station" so 
as to correct or re-xmit errors.  Obviously you need a very good preselector 
for this type of operation.  No, the xmitter and receiver didn't share 
antennas.

73 John

----- Original Message ----- 
From: "Blair Batty" <Blair at OntarioRocks.ca>
To: <premium-rx at ml.skirrow.org>
Sent: Saturday, April 16, 2005 12:24 PM
Subject: [Premium-Rx] Harris RF-551A Preselector


> 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
>
>
>
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