Few words about Low noise preamplifiers protection during VHF Contests.

Well known situation: after many hours of assembling the antenna towers are finally erected and the contest is running. Suddenly one preamp is deaf. Well / what is your choice now? Put the tower during the night, rain and windy weather down and try to fix it, or write the contest off...

Present high level contest traffic request use of multiple antenna and PAs. Although these PAs are controlled by sequencers, time to time may happen and the operator turn both antennas face to face. And then just minor failure, like delay of one antenna relais is enough for  tear off of LNA transistor arrow.

Well, during work on antenna, while rain became stronger and stronger you probably go round and round in your head - is there some protection, which can protect us from such mistake?

I am quite sure, that you remind antiparallel diodes across the LNA input and immediatelly refused that configuration because of intermodulation from other bands electromagnetic field.

But - exists some protection of LNAs?  Yes. In our Contest club we are using for long years this one:

The above classic LNA input circuit transform low input impedance to the high one for FET gate by use of parallel resonance. However this transformation exist only for operational frequency. When the microwave diode is placed from the Gate of FET to GND, it has more-less no influence out of band as well as for low input signals in the band (however except small capacity). In case of "big" input signal in the band the protection diode starts switching on, input circuit is very attenuated and detuned and the FET is by this way very well protected. But of course, this protection can not secured LNA against few Watts on the input...

What is the influence to the LNA?  Compare these measurments:

  LNA 144 MHz  BF910 LNA 144 MHz  MGF1302 LNA 432 MHz  CF300 LNA 432 MHz  MGF1302
without protective diode 21 0.9 + 5 21 0.3 - 1 17 0.65 - 4 15.5 0.4 -1
with protective diode 21 0.9 - 1 21 0.3 - 3 17 0.65 - 5 15.5 0.4 -3


Note: in the columns are this values: LNA gain [dB], noise factor [dB], input IP [dBm]
The IP value has measured by the  1 dB compression point method. See here. Noise factor has been measured on HP8970A + HP346C system.

Interesting, is not it?... Supposed degradation of Nf is so low, that precision of such measurment is not relevant . Maybe it could be ~ 0.02 dB, if you will test more amplifiers. Experimentally based experience says, that the best diode for such protection would be low capacity microwave detection Schottky diode. Maybe the PIN diodes could be even better, but not measured yet.

We are using older TESLA Schottky detection diode KAS31 similar to Siemens BAR28 detection diode. In any case the diode should to have as low as possible capacity (at least below 1,5 pF). Practical experience since last 15 years are positive - no damaged LNA yet.
 

For final few additionl construction tips:

- as blocking capacitors in the source of LNA transistor (C3 a C4) should be used classic low inductance chip capacitors (see. here), usage of SMD capacitors (if the design fo not need some inductance). As well in the drain (C6) use rather ceramic path through capacitors rather than parallel conection of two capacitors.  

- LNA noise can be degraded by noise of voltage stabilizer, which need to be filtrated.

- input capacity trimmer (C1), as well as C2 need to be   HI Q type, such a Johanson, AirTronic, Tronser, etc. Transition resistance between rotor and trimmer statoris probably the most important factor for ability to get as low noise factor. If you have non - hi Q trimmer, you can test rather fixed capacity capacitors, than low quality trimmers.

- keep connectors on mind - only the best connectors are good enough for low NF. Chinese "no name" for example measured adapter increased the noise by 0.5dB.

Good results in the 2007 season by OK1VPZ
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