[Premium-Rx] Radios and High RF powers

[John Perlick]

Jim:

the problem is that your amp is wideband....and those pulses have wideband
energy that will get in.  You could fix the amp, and make it perform even
better, by making it either narrow band or semi-wideband.  For example, drop
in some octave filters...like a 5-10 MHz one, then a 10-20 MHz one.  There
will be a much smaller amount of energy spread across the much smaller
window.  It's an old problem and most of the wideband radios have at least
sub-octave filters in the inputs, if not good preselecters.

John
----- Original Message -----
From: "James C. Garland" <4cx250b at muohio.edu>
To: <premium-rx at ml.skirrow.org>
Sent: Friday, May 30, 2003 5:41 AM
Subject: Re: [Premium-Rx] Radios and High RF powers


>
> >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
>
>
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