Feb 21, 2008

50MHz preamp by SSB Electronics

This is wat you see when opening the metal box from the SSB 6m preamp.

It has an MGF1302 low noise input followed by a BFQ34 gain stage. The output has a 2 pole bandfilter. There is some vox/ptt control, which allows 200W/750W in SSB.

Why do you want to use a preamp?
The only right answer would be; you want to hear more.
Translate that rightfull wish into technical mumbo:

a. You want a better total system noise figure
b. The extra gain offers flexibility in your receiver chain
c. You need to concur any (feeder) losses

Noise figure
The spec shows 0.8 dB nf, which is totally ridiculous at 6mtr as the terrestrial skynoise is already >6dB. However my TS570S has a nf of about 8-9dB so there is something to gain here. From 2dB onwards a difference can be heard by the human ear. Forget about the 0.3dB preamps and 'every tenth of a db is important', that is total crap!! At least on 50MHz terrestrial use it is.

Some extra gain up front can be helpfull to challenge the less than optimal gain in your various receiver IF stages. I prefer using a home made 3+6+6 dB attenuator to try finding the best s/n result.
Next to that, your internal transceiver preamp is probeabely a low quality wideband type. So leave it switched off and use the better quality external preamp.
I have found this to be the biggest advantage during the last couple of years.

Feeder losses
Not a big deal at home, where you can spare a dB or two on the coax without being able to hear the difference. The preamp can be in the shack, there is no need to put it up in the tower.
This summer we will have a fieldday style operation where the total coax adds a loss of >4dB, plus there I want to split the signal to 2 different receivers via a 2-port combiner, which kills another 3 dB.

Yesterday evening I had the opportunity to test and realign it with the help of a network analyzer. The total amplification turned out to be 27.2 dB at max setting. The internal potmeter was set to 20dB, which will cover all experiments on local or fieldday criteria in the future.
The output bandfilter might be helpfull to supress unwanted high levels of TV carriers in the 47-49MHz range during Es popagation. After carefull trying different alignment settings of both the input and output coils we noticed that it was already perfect. Almost, because we managed to get another 5 dB attenuation on 48.5Mhz by aligning the left slope slightly steeper. Mind you; watch the measured final diagram; with +20dB total amplification there is still +5dB left at 48.5....

Future wishes?
  • smaller bandfilter with better shape factor
  • custom made preamp with 2x BFR 96
  • handling higher output
Let's first try to catch some multi-hop Es-F2 dx during this summer !

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