[Premium-Rx] Radios and High RF powers

[James C. Garland]

>Hello list.
>
>The problem of making receivers work in strong RF fields is not a new one.
>I would suggest that one of the prime criteria for the design of a 
>'military (or
>commercial) application' receiver - our premium receivers - is that it should
>work satisfactorily in very high RF fields (we are talking of Volts not
>milliVolts).
>
>***************************************************************
>Richard Reich
>Principal Engineer Hardware
>SAAT Technology Ltd
>Web: http://www.saatt.co.uk

Hi Gang,
Richard highlights a problem which has plagued me recently with my homebrew 
wideband h.f. receiver preamplifier, and that is how to protect the 
delicate input RF stage from potentially damaging RF voltages. My preamp 
uses a Minicircuits broadband microchip amplifier, rated at DC - 3 GHz, 
with 12 db gain and a noise figure of 4.5 db.  The chip is rated at +13dbm 
(1 Vrms) maximum voltage at the input. I've lost several of the chips, 
despite using back-to-back diodes across the antenna input (which I 
generally despise, because they cause IMD problems).

In trying to understand where the damaging signals were coming from, I 
connected my antenna to the input of a Tektronix TDS-420A digital 
oscilloscope. I set the trigger to capture events exceeding 1V of 
RF.  Interestingly, the test showed that my problem wasn't ambient RF (even 
from my 1KW AM transmitter), but rather static pulses caused by 
thunderstorms -- sometimes so far away that I couldn't hear the 
thunder.  These lightning-generated impulses are very high amplitude -- 
many volts -- with frequency components that go from DC up to hundreds of 
MHz.  My diode clippers (a pair of back-to-back 1N914a diodes) clamped the 
low-frequency components of the pulses okay, but not the VHF and UHF 
components.  Because the Minicircuits device has an intrinsic frequency 
response that extends up to the GHz range, it was quite susceptible to VHF 
and UHF overload.

I wonder how high-end commercial receivers solve this problem? Diode 
clipping is generally not desirable, and spark-gap devices aren't sensitive 
enough. Obviously, tuned fulters at the input (as is done in the optional 
preamps for, e.g., the WJ HF-1000A) will solve the problem for a particular 
frequency band, but don't work for the full HF spectrum.  Any comments 
would be most welcome.
Thanks,

Jim Garland W8ZR