[R-390] preselectors

[Bernie Nicholson]

The only case I could see for the use of a preselector with a 390A RX is if you wanted to hear in duplex mode close to your transmit frequency . Both Racal and Collins have produced boxes for this purpose, I had a Pair of URC32 trancievers from one of our Oberon class Submarines  that were fitted with Tracking Preselectors and I was able to have Full Duplex QSO s with in the 80 meter band with with 200Khz separation  and running 1Kw input .I have also seen similar boxes for the 618T system to permit full duplex telephone calls on 747 aeroplanes ,I still own a Racal box called a preselector and protection unit that I use in conjunction with my Ra17L ,it was desighned so that you could opperate the reciever in a transmitter hall and not burn out the rf coils when you tuned across the transmit freq. and so you dont hear birdies associated with mixer overload This box also has the facility to instataniously switch it out of circuit so you can see what it is doing and I have never experienced a change in signal to noise ratio and all these boxes do is Increase the dynamic range of the system, further the dynamic range is increased much more on the lower frequencies than the higher frequencies due to the attainable Q of the tuned circuits dropping off as the frequency goes up there are diminishing returns.But in a contest on 80 or 40 meters and someone running a kilowatt just down the road one can opperate with impunity , or if you lived next door to a 50 Kw broadcast station you would probablly gain from having one of these boxes, the dynamic Range of the 390A is in the order of 80Db which is pretty amazing for a general coverage reciever of this age. HOW GOOD is it  well looking on the TENTEC site the published figures on the Kenwood TF2000 is 69Db the FT1000Mp is 73Db and so on, so even after all this time the 390A is still a FORCE to be reckoned with amongst recievers, I hope I have explained the use of PRESELECTORS , regards to all.