Norton Wideband HF pre-amp

The 20m vertical antenna looks good, VSWR < 1.3 : 1 but RX might be a bit deaf. RX details: 1dB antenna cable loss: + 14MHz to 144MHz SBL-1 mixer (straight 6dB loss) :+ IC 202 144MHz receiver (8dB NF). Hence total receive noise figure is at least 15dB. Built a Norton HF preamp (2n5109) to try and improve situation. (Is this necessary given the noise level at 14MHz? see following. Photo below shows the circuit and the measured cbe voltages resulting from a 13.9v supply.

First audible results were however not particularly impressive. The dominant noise is the external noise? and this is in excess of any receiver contribution - even at 15dB noise fugure? But I will look into this and quantify the position.

OK, this is a simple circuit and the 50 ohm output load is transformed by the broadband auto transformer to the collector load. The actual turns ratio used was 3 to the tap and then 11 to the collector. The turns ratio is then 14/3 or 4.6 which is the voltage transformation. This was checked by injecting a 100mV peak signal from the test oscillator at the input to the amp and this was measured with a scope at the emitter. The resulting voltage at the collector was 1.6v peak and the voltage at the tap with a 50ohm load was 350mv peak. So the auto transformer voltage ratio is 1.6/0.35 or 4.6. So everything ok there. The amp gain, i/p and o/p in 50 ohms was therefore 20log(0.35/0.1) or 10dB. When the amp was reinserted into the rx and the 50 ohm output load replaced with the mixer the gain dropped to 9.5dB. So the amp is fine - but should build a second one and have two in series to overcome the front end losses or is even one required?.

If the amp can have a 10v (5v peak) collector swing, the input will be around 500mV peak or 2.5mW or +4dBm before compression.

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G4AKW

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