[Premium-Rx] Radios and High RF powers

Mon Jun 2 19:19:46 EDT 2003

Yes in the days of open feed lines we had horn-gaps at the point where
the down feed meet the feed line going to the station and another horn
gap near the entry point to the building.
RICH  WA6KNW
--- George Georgevits <georgg at bigpond.net.au> wrote:
> 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
> 
> 
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