[Premium-Rx] Preselectors

[Michael O'Beirne]

Evening All,

Most of us fortunately don't live next door to a big Tx and have no need for 
preselectors but when you do they are essential.  I have three examples for 
you.

1.    When I did military things we usually had to run full duplex HF FSK 
nets from tactical locations with a frequency separation of about 300kHz and 
an antenna separation of 20 to 100 feet and with 300 watts pumping out 
continuously from the Tx.  At very close antenna separations it was 
essential to use the external Marconi passive preselector to back up the 
internal tuned circuits in the Rx.

2.    Some stations in LandRovers (using tactical SSB transceivers) ran two 
HF nets with about 30 - 40W PEP output using whip antennas mounted on each 
side of the vehicle and separated by 6 - 7 feet.  This was only possible if 
both sets used external passive Rx preselectors, which also acted 
simultaneously as Tx postselectors, and a frequency separation of 20% or 
more.  This unit - the Clansman "SURF" - is now being sold off surplus in 
the UK as the Army is migrating to the new Bowman system and they are 
absolutely excellent.  I measured the tuned loss in my one at under 1dB and 
the ultimate selectivity at over 70dB.  They are fully sealed, have 
silver-plated inductors and gold-plated piston trimmers.  Original cost? I 
dare not think.

2.    A pro told me a couple of years back that he worked at a large Air 
Traffic Control centre near London where his building was stuffed full of 
QRM-producing computers.  They had to monitor a particular HF channel for 
which they used an old Eddystone 958 (which has 3 tuned circuits before the 
1st mixer).  After 20 years or so they needed a replacement.  None of the 
new "PR" type receivers trialled were any good at that location until he 
tested a JRC NRD-535, which (to his considerable surprise) worked very well 
and was a lot cheaper.  He reckons the internal preselector was doing its 
stuff in reducing the unusually high level of wideband QRM to a level 
acceptable to the 1st mixer.

73s
Michael
G8MOB