[Premium-Rx] Radios and High RF powers

[George Georgevits]

Has anyone thought about using a balanced antenna and balanced feed for such
situations? To my mind, the balanced antenna/feed system could be fitted
with a longitudinal ferrite choke, preceded by a 90 volt 3 electrode
arrester, like the one used on telephone circuits. Such an arrangement would
not affect the balanced mode HF signals in the feeder unduly, but it should
stop all those nasty longitudinal surges caused by lightning and shunt them
safely to ground, and it should not cause IMD in the receiver either. Gas
arresters are very tame devices until they fire. The ferrite choke should
block the fast pulses and help the arrester fire quicker, due to its high
longitudinal impedance.

Now I am going to duck!

Regards,
George Georgevits
VK2KGG


-----Original Message-----
From: premium-rx-bounces at ml.skirrow.org
[mailto:premium-rx-bounces at ml.skirrow.org]On Behalf Of James C. Garland
Sent: Friday, 30 May 2003 8:42 PM
To: premium-rx at ml.skirrow.org
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


_______________________________________________

Premium-Rx Mailing List
To Post: premium-rx at ml.skirrow.org
For Info: http://ml.islandnet.com/mailman/listinfo/premium-rx