[Premium-Rx] Preselectors

[w3jn]

The RF-551 was used in fixed full-duplex HF data circuits.  I'm not sure 
what the frequency separation was, but they were used with RF-590s, the 
matching exciter, and the Harris 2KW amp.  The RX used a RF-551 as a pre and 
the exciter was used as a post selector.  Another key to the success of this 
was the very low IMD rating of the Harris amplifier.

I'm of the school of thought that looks with extreme suspicion on any 
preselector circuit using varactors or elements switched using PIN diodes - 
or any solid state elements for that matter.  A well-designed mixer using 
the 7360 sheet-beam tube will handle tens of volts of RF before crumbling.


----- Original Message ----- 
From: "Gary Geissinger" <ggeissinger at digitalglobe.com>
To: <premium-rx at mailman.qth.net>
Sent: Friday, November 30, 2007 5:10 PM
Subject: RE: [Premium-Rx] Preselectors



If you are looking for a possible preselector to try, here is a
suggestion.  Our amateur radio club evaluated a number of different
preselectors for use during field day.  The problem is that there are
multiple operating positions in close proximity.

We settled on Harris RF-551 preselectors after we did a test.  We
transmitted 1500 watts on 14 MHz from an antenna about 25 feet or so
from a 7 MHz receiving antenna.  Using the RF-551 we observed
essentially no interference in the receiver.

While they are not good enough to help with interference that is close
in frequency (7.1 rx and 7.25 tx, for example), they are pretty amazing
with wide enough frequency spacing.

If you decide to get one check the continuity through the rivets on the
circuit board.  Over time and humidity they tend to open up the soldered
connections due to different metals in contact.  One of the 4 units we
own was modified by someone; all the rivets were jumpered with wires.
It has never had a problem. Others periodically need to be opened up and
resoldered; maybe someday we will modify them as well.

We have two of them running by remote control; they are slaved to our
transceivers.  I have one at home I slaved to my HF-1000.  Another
plus...they can take significant accidental RF power input without
damage.  Don't try putting 100W into your receiver antenna input;
according to the manual, the RF-551 handles it just fine. (It's not like
I'm going to test it!)

Not much help for BCB Dxing though, just SWL/ham.

Gary WA0SPM



-----Original Message-----
From: premium-rx-bounces at mailman.qth.net
[mailto:premium-rx-bounces at mailman.qth.net] On Behalf Of Tim Shoppa
Sent: Friday, November 30, 2007 2:53 PM
To: premium-rx at mailman.qth.net; michaelob666 at ntlworld.com
Subject: Re: [Premium-Rx] Preselectors

"Michael O'Beirne" <michaelob666 at ntlworld.com> wrote:
> I'll add my sixpence worth of wisdom to the debate.
> If you want to see "real" preselectors with proper switches and
variable capacitors you basically have to look at non-PR radios -
typically the oldish Racal RA1772 (mid 70s).
> The only two PR radios I know of with motorised preselectors are the
Redifon R500 (a work of much beautyand expense) designed for warships
and all the RF nasties that you find on board, and the Eddystone 1650
(but it was an expensive option).
> The fact is that motorised preselectors are uneconomical nowadays
unless you are in a very hostile situation with masses of RF next door.

On my 135 foot doublet up about 90 feet, I get about 70V P-P from a
local BCB blowtorch literally a few blocks away. I can hook up a neon
lamp and watch it light on modulation peaks. Is that at least moderately
hostile, if not warship hostile? :-).

I suspect a lot of places in Europe have similar RF energy.

*All* consumer SW radios have nothing but images up and down the
spectrum from the local BCB stations, and that's often with their little
whip antennas. Most of my older equipment with tuned circuits in the
front end does just fine.

Funny, I just looked at the specs of my WJ-8716, and it says you're only
supposed to have 1.5V of RF coming into the antenna jack. Hah!

Obviously for many purposes I have a 630kc trap hooked up to take out
the majority (but not all) of the local blowtorch.

Tim.
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