[Premium-Rx] Radios and High RF powers

Mon Jun 2 15:16:27 EDT 2003

Frank,

Yes, a spark gap will work fine in the event of a direct strike to your
antenna. They used to use spark gaps as arresters in very early protector
blocks in telephone exchanges and at subscribers' premises where the phone
line enters the house. Then they moved to carbon blocks. These had lower
breakdown voltages and were more predictable in performance. These days they
use gas arresters because they have lower still breakdown voltages and the
breakdown point can be more accurately controlled. The spark gap is OK if
you only want to save your own neck, but if you want to save the gear as
well, the gas arrester will do a much better job. It also helps that they
put telephone cables underground these days, rather than having them up on
poles!

Also, I have seen an installation where hundreds of gas arresters were
fitted to a small country telephone exchange main distribution frame. During
a thunderstorm, the frame lit up like a Christmas tree! This was caused by
induced pulses from what were often cloud to cloud strokes. I am told that
only a small fraction of lightning strokes make it to ground. The others can
still do damage to your installation through induction. Given that the
arresters were 230V breakdown voltage, (they have to withstand the ring
voltage of 75V rms without firing) one can guess that the induced lightning
pulses had a bit of oomph behind them!

Of course a good earth is also required for any such scheme to work.

Regards,
GG

-----Original Message-----
From: Carcia, Frank A. HS [mailto:francis.carcia at hs.utc.com]
Sent: Monday, 2 June 2003 10:11 PM
To: 'George Georgevits'; premium-rx at ml.skirrow.org
Subject: RE: [Premium-Rx] Radios and High RF powers

George,
I run a 250 foot dipole fed with balanced line. The tuner is a pair of L
networks fed out of phase with a broad band transformer
that has a grounded center tap. The 1/4 inch
spaced Cardwells will arc over with a close  lightning strike.   fc

-----Original Message-----
From: George Georgevits [mailto:georgg at bigpond.net.au]
Sent: Friday, May 30, 2003 6:29 PM
To: premium-rx at ml.skirrow.org
Subject: RE: [Premium-Rx] Radios and High RF powers

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

_______________________________________________

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