Friday, 31 October 2014

144MHz masthead preamp

 
Most of today was spent building a144MHz masthead preamplifier. I have not used a masthead preamp on 144MHz previously, but for MS and especially EME, a preamp is essential and I really wanted to see how much difference it would make.

The active preamp stage uses a PGA 103+ PHEMT on a WA5VJB PCB. 

G4SWX and I have previously developed a 'better' 130MHz High Pass Filter that has a deep notch in Band2 to suppress FM strong signal effects and a 150MHz Low Pass Filter at the output to suppress high frequency signals reaching the following receiver.
A preamplifier in a box is of little use without a method of powering it at masthead and switching the antenna between transmit and receive.
I chose ( eventually) to use one of the larger Schubert ABS enclosures. These are now available from G3NYK in the UK.
I use a Relcomm SDL 12V coax relay with three N connectors. The relay is rated to 1500W at 150MHz
Power for the preamp and relay is fed over the receive coax and a homebrew bias tee. The bias tee can be seen on the right in this photo. The preamp is in the tinplate box In the centre.




Following my usual masthead practice, the relay is mounted with the three N connectors through the box lid.
N connectors are used to connect to the preamplifier input and output. A short low loss coax cable connects the relay normally open port to the preamplifier input.

The numbers?
Noise figure 0.70dB
Gain 23dB
These numbers are with the relay and two short lengths of coax in front of the 0.5dB preamp stage.

Sam





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