[Premium-Rx] AW: Premium-Rx Digest, Vol 6, Issue 3 RE: Radios and High RF powers (Andreé Knott)
AKnott at WTD71.de
AKnott at WTD71.de
Tue Jun 3 04:49:38 EDT 2003
If the ignition voltage of a gas discharge arrestor or neon lamp is too high
for your purpose, it is possible to bias the gas discharge device by a high
voltage (low) power supply (which should be "noise free") with fine tuneable
voltage fed through a good rf-choke (no series resonances throughout
operational frequency range) and a resistor which limits the current if the
gas discharge device "fires" to protect the gas discharge device and the
power supply as well. Possibly decoupling capacitors may be necessary to
keep the bias voltage away from the rf-frontend or parts which can be
touched.
The burning voltage of a gas discharge device is lower than the ignition
voltage. Therefore the bias voltage has to be lower than this burning
voltage, otherwise it will burn for ever. If the gas discharge device
"fires" there is still this burning voltage across the gas discharge device.
The following circuits have to cope with this voltage.
I saw gas discharge devices integrated into preselectors at certain points
of relatively high impedance. So the burning volage is lowered by the
step-down transformer the preselector includes. To explain this here is a
simple example. Think of a preselector of two inductively coupled parallel
LC-circuits (C is used for tuning). The input is coupled by a small coil
(few turns) inside the first parallel LC-circuit. The output is coupled by a
small coil inside the first parallel LC-circuit as well. So you see inside
the preselector there is first a step-up and then a step-down transforming
done. The "hot" end of each parallel LC-circuit is such a point of high
impedance. I saw gas discharge devices connected between such a point and
ground. Avoid long wires, take the next point (case of the preselector) to
ground.
If the light of the gas discharge device or the current of the gas discharge
device or the current of the bias power supply is supervised, it can be used
to switch in a power attenuator automatically if the period of activation of
the gas discharge device is longer than a certain time. This is done at
certain preselectors used in shipborn application. A military ship really is
the worst application you can think of. I saw such preselectors used on
ships which have several (parallel used) high power transmitters in the
range of HF up to SHF. Due to the small distance decoupling between antennas
is VERY low...
Yours
Andreé DD3LY
> -----Ursprüngliche Nachricht-----
> Von: premium-rx-request at ml.skirrow.org
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> Gesendet am: Montag, 2. Juni 2003 21:48
> An: premium-rx at ml.skirrow.org
> Betreff: Premium-Rx Digest, Vol 6, Issue 3
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> Today's Topics:
>
> 1. RE: Radios and High RF powers (George Georgevits)
> 2. RE: Radios and High RF powers (Carcia, Frank A. HS)
> 3. RE: Radios and High RF powers (Gary Geissinger)
>
>
> ----------------------------------------------------------------------
>
> Date: Tue, 3 Jun 2003 05:16:27 +1000
> From: "George Georgevits" <georgg at bigpond.net.au>
> To: "Premium Rx" <premium-rx at ml.skirrow.org>
> Subject: RE: [Premium-Rx] Radios and High RF powers
> Message-ID: <NFBBJOGIILKFHDOFAKBDOEPACMAA.georgg at bigpond.net.au>
> In-Reply-To:
> <F8D87025893CD311B93600A0C9A3C9D00C703D34 at hsmx17nt.hsd.utc.com>
> Content-Type: text/plain;
> charset="Windows-1252"
> MIME-Version: 1.0
> Content-Transfer-Encoding: 7bit
> Precedence: list
> Message: 1
>
> 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
>
>
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>
> Premium-Rx Mailing List
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>
>
> ------------------------------
>
> Date: Mon, 2 Jun 2003 15:22:07 -0400
> From: "Carcia, Frank A. HS" <francis.carcia at hs.utc.com>
> To: "'George Georgevits'" <georgg at bigpond.net.au>,
> Premium Rx
> <premium-rx at ml.skirrow.org>
> Subject: RE: [Premium-Rx] Radios and High RF powers
> Message-ID:
> <F8D87025893CD311B93600A0C9A3C9D00C703D41 at hsmx17nt.hsd.utc.com>
> Content-Type: text/plain;
> charset="windows-1252"
> MIME-Version: 1.0
> Precedence: list
> Message: 2
>
> George,
> My point was the center tap of the transformer is at ground so the whole
> antenna is at DC ground but the rise time
> is fast enough to generate a voltage greater than 10 KV across the
> network.
> I do have gas tubes at the RX. My best protection is to disconnect the
> antenna while not in use.
> My LPDA is also at DC ground but you still can generate high voltage
> across
> the coax.
> fc
>
> -----Original Message-----
> From: George Georgevits [mailto:georgg at bigpond.net.au]
> Sent: Monday, June 02, 2003 3:16 PM
> To: Premium Rx
> Subject: RE: [Premium-Rx] Radios and High RF powers
>
>
> 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
>
>
> _______________________________________________
>
> Premium-Rx Mailing List
> To Post: premium-rx at ml.skirrow.org
> For Info: http://ml.islandnet.com/mailman/listinfo/premium-rx
>
> ------------------------------
>
> Date: Mon, 2 Jun 2003 13:47:32 -0600
> From: Gary Geissinger <ggeissinger at digitalglobe.com>
> To: Premium Rx <premium-rx at ml.skirrow.org>
> Subject: RE: [Premium-Rx] Radios and High RF powers
> Message-ID:
> <782C91C1988A404980FDAEA2E0C65CC01831D6 at comail.digitalglobe.com>
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>
> George, Frank,
>
> You may want to disconnect the radio from AC power during a storm as well.
>
> In addition to the gas filled tubes, I have the coaxial RF fuses in
> antenna
> input lines into the receivers. They came from Fair Radio. The original
> application was to protect mil-spec RF generators when testing
> transceivers.
> I am using 1/16th amp 250 V fast blow instrument fuses. I have had to
> replace the fuses twice in the last couple years...and I keep my radios
> disconnnected when I am not using them!
>
> Good Luck,
>
> Gary WA0SPM
>
>
> -----Original Message-----
> From: Carcia, Frank A. HS [mailto:francis.carcia at hs.utc.com]
> Sent: Monday, June 02, 2003 1:22 PM
> To: 'George Georgevits'; Premium Rx
> Subject: RE: [Premium-Rx] Radios and High RF powers
>
>
> George,
> My point was the center tap of the transformer is at ground so the whole
> antenna is at DC ground but the rise time
> is fast enough to generate a voltage greater than 10 KV across the
> network.
> I do have gas tubes at the RX. My best protection is to disconnect the
> antenna while not in use.
> My LPDA is also at DC ground but you still can generate high voltage
> across
> the coax.
> fc
>
> -----Original Message-----
> From: George Georgevits [mailto:georgg at bigpond.net.au]
> Sent: Monday, June 02, 2003 3:16 PM
> To: Premium Rx
> Subject: RE: [Premium-Rx] Radios and High RF powers
>
>
> 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
>
>
> _______________________________________________
>
> Premium-Rx Mailing List
> To Post: premium-rx at ml.skirrow.org
> For Info: http://ml.islandnet.com/mailman/listinfo/premium-rx
>
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>
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> To Post: premium-rx at ml.skirrow.org
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> <TITLE>RE: [Premium-Rx] Radios and High RF powers</TITLE>
> </HEAD>
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>
> <P><FONT SIZE=3D2>George, Frank,</FONT>
> </P>
>
> <P><FONT SIZE=3D2>You may want to disconnect the radio from AC power =
> during a storm as well.</FONT>
> </P>
>
> <P><FONT SIZE=3D2>In addition to the gas filled tubes, I have the =
> coaxial RF fuses in antenna input lines into the receivers. =
> They came from Fair Radio. The original application was to =
> protect mil-spec RF generators when testing transceivers. I am =
> using 1/16th amp 250 V fast blow instrument fuses. I have had to =
> replace the fuses twice in the last couple years...and I keep my radios =
> disconnnected when I am not using them!</FONT></P>
>
> <P><FONT SIZE=3D2>Good Luck,</FONT>
> </P>
>
> <P><FONT SIZE=3D2>Gary WA0SPM</FONT>
> </P>
> <BR>
>
> <P><FONT SIZE=3D2>-----Original Message-----</FONT>
> <BR><FONT SIZE=3D2>From: Carcia, Frank A. HS [<A =
> HREF=3D"mailto:francis.carcia at hs.utc.com">mailto:francis.carcia at hs.utc.c=
> om</A>]</FONT>
> <BR><FONT SIZE=3D2>Sent: Monday, June 02, 2003 1:22 PM</FONT>
> <BR><FONT SIZE=3D2>To: 'George Georgevits'; Premium Rx</FONT>
> <BR><FONT SIZE=3D2>Subject: RE: [Premium-Rx] Radios and High RF =
> powers</FONT>
> </P>
> <BR>
>
> <P><FONT SIZE=3D2>George,</FONT>
> <BR><FONT SIZE=3D2>My point was the center tap of the transformer is at =
> ground so the whole</FONT>
> <BR><FONT SIZE=3D2>antenna is at DC ground but the rise time </FONT>
> <BR><FONT SIZE=3D2>is fast enough to generate a voltage greater than 10 =
> KV across the network.</FONT>
> <BR><FONT SIZE=3D2>I do have gas tubes at the RX. My best protection is =
> to disconnect the</FONT>
> <BR><FONT SIZE=3D2>antenna while not in use.</FONT>
> <BR><FONT SIZE=3D2>My LPDA is also at DC ground but you still can =
> generate high voltage across</FONT>
> <BR><FONT SIZE=3D2>the coax. </FONT>
> <BR><FONT =
> SIZE=3D2> &nb=
> sp; &nb=
> sp; fc</FONT>
> </P>
>
> <P><FONT SIZE=3D2>-----Original Message-----</FONT>
> <BR><FONT SIZE=3D2>From: George Georgevits [<A =
> HREF=3D"mailto:georgg at bigpond.net.au">mailto:georgg at bigpond.net.au</A>]<=
> /FONT>
> <BR><FONT SIZE=3D2>Sent: Monday, June 02, 2003 3:16 PM</FONT>
> <BR><FONT SIZE=3D2>To: Premium Rx</FONT>
> <BR><FONT SIZE=3D2>Subject: RE: [Premium-Rx] Radios and High RF =
> powers</FONT>
> </P>
> <BR>
>
> <P><FONT SIZE=3D2>Frank,</FONT>
> </P>
>
> <P><FONT SIZE=3D2>Yes, a spark gap will work fine in the event of a =
> direct strike to your</FONT>
> <BR><FONT SIZE=3D2>antenna. They used to use spark gaps as arresters in =
> very early protector</FONT>
> <BR><FONT SIZE=3D2>blocks in telephone exchanges and at subscribers' =
> premises where the phone</FONT>
> <BR><FONT SIZE=3D2>line enters the house. Then they moved to carbon =
> blocks. These had lower</FONT>
> <BR><FONT SIZE=3D2>breakdown voltages and were more predictable in =
> performance. These days they</FONT>
> <BR><FONT SIZE=3D2>use gas arresters because they have lower still =
> breakdown voltages and the</FONT>
> <BR><FONT SIZE=3D2>breakdown point can be more accurately controlled. =
> The spark gap is OK if</FONT>
> <BR><FONT SIZE=3D2>you only want to save your own neck, but if you want =
> to save the gear as</FONT>
> <BR><FONT SIZE=3D2>well, the gas arrester will do a much better job. It =
> also helps that they</FONT>
> <BR><FONT SIZE=3D2>put telephone cables underground these days, rather =
> than having them up on</FONT>
> <BR><FONT SIZE=3D2>poles!</FONT>
> </P>
>
> <P><FONT SIZE=3D2>Also, I have seen an installation where hundreds of =
> gas arresters were</FONT>
> <BR><FONT SIZE=3D2>fitted to a small country telephone exchange main =
> distribution frame. During</FONT>
> <BR><FONT SIZE=3D2>a thunderstorm, the frame lit up like a Christmas =
> tree! This was caused by</FONT>
> <BR><FONT SIZE=3D2>induced pulses from what were often cloud to cloud =
> strokes. I am told that</FONT>
> <BR><FONT SIZE=3D2>only a small fraction of lightning strokes make it =
> to ground. The others can</FONT>
> <BR><FONT SIZE=3D2>still do damage to your installation through =
> induction. Given that the</FONT>
> <BR><FONT SIZE=3D2>arresters were 230V breakdown voltage, (they have to =
> withstand the ring</FONT>
> <BR><FONT SIZE=3D2>voltage of 75V rms without firing) one can guess =
> that the induced lightning</FONT>
> <BR><FONT SIZE=3D2>pulses had a bit of oomph behind them!</FONT>
> </P>
>
> <P><FONT SIZE=3D2>Of course a good earth is also required for any such =
> scheme to work.</FONT>
> </P>
>
> <P><FONT SIZE=3D2>Regards,</FONT>
> <BR><FONT SIZE=3D2>GG</FONT>
> </P>
> <BR>
>
> <P><FONT SIZE=3D2>-----Original Message-----</FONT>
> <BR><FONT SIZE=3D2>From: Carcia, Frank A. HS [<A =
> HREF=3D"mailto:francis.carcia at hs.utc.com">mailto:francis.carcia at hs.utc.c=
> om</A>]</FONT>
> <BR><FONT SIZE=3D2>Sent: Monday, 2 June 2003 10:11 PM</FONT>
> <BR><FONT SIZE=3D2>To: 'George Georgevits'; =
> premium-rx at ml.skirrow.org</FONT>
> <BR><FONT SIZE=3D2>Subject: RE: [Premium-Rx] Radios and High RF =
> powers</FONT>
> </P>
> <BR>
>
> <P><FONT SIZE=3D2>George,</FONT>
> <BR><FONT SIZE=3D2>I run a 250 foot dipole fed with balanced line. The =
> tuner is a pair of L</FONT>
> <BR><FONT SIZE=3D2>networks fed out of phase with a broad band =
> transformer</FONT>
> <BR><FONT SIZE=3D2>that has a grounded center tap. The 1/4 inch</FONT>
> <BR><FONT SIZE=3D2>spaced Cardwells will arc over with a close =
> lightning strike. fc</FONT>
> </P>
>
> <P><FONT SIZE=3D2>-----Original Message-----</FONT>
> <BR><FONT SIZE=3D2>From: George Georgevits [<A =
> HREF=3D"mailto:georgg at bigpond.net.au">mailto:georgg at bigpond.net.au</A>]<=
> /FONT>
> <BR><FONT SIZE=3D2>Sent: Friday, May 30, 2003 6:29 PM</FONT>
> <BR><FONT SIZE=3D2>To: premium-rx at ml.skirrow.org</FONT>
> <BR><FONT SIZE=3D2>Subject: RE: [Premium-Rx] Radios and High RF =
> powers</FONT>
> </P>
> <BR>
>
> <P><FONT SIZE=3D2>Has anyone thought about using a balanced antenna and =
> balanced feed for such</FONT>
> <BR><FONT SIZE=3D2>situations? To my mind, the balanced antenna/feed =
> system could be fitted</FONT>
> <BR><FONT SIZE=3D2>with a longitudinal ferrite choke, preceded by a 90 =
> volt 3 electrode</FONT>
> <BR><FONT SIZE=3D2>arrester, like the one used on telephone circuits. =
> Such an arrangement would</FONT>
> <BR><FONT SIZE=3D2>not affect the balanced mode HF signals in the =
> feeder unduly, but it should</FONT>
> <BR><FONT SIZE=3D2>stop all those nasty longitudinal surges caused by =
> lightning and shunt them</FONT>
> <BR><FONT SIZE=3D2>safely to ground, and it should not cause IMD in the =
> receiver either. Gas</FONT>
> <BR><FONT SIZE=3D2>arresters are very tame devices until they fire. The =
> ferrite choke should</FONT>
> <BR><FONT SIZE=3D2>block the fast pulses and help the arrester fire =
> quicker, due to its high</FONT>
> <BR><FONT SIZE=3D2>longitudinal impedance.</FONT>
> </P>
>
> <P><FONT SIZE=3D2>Now I am going to duck!</FONT>
> </P>
>
> <P><FONT SIZE=3D2>Regards,</FONT>
> <BR><FONT SIZE=3D2>George Georgevits</FONT>
> <BR><FONT SIZE=3D2>VK2KGG</FONT>
> </P>
> <BR>
>
> <P><FONT SIZE=3D2>-----Original Message-----</FONT>
> <BR><FONT SIZE=3D2>From: premium-rx-bounces at ml.skirrow.org</FONT>
> <BR><FONT SIZE=3D2>[<A =
> HREF=3D"mailto:premium-rx-bounces at ml.skirrow.org">mailto:premium-rx-boun=
> ces at ml.skirrow.org</A>]On Behalf Of James C. Garland</FONT>
> <BR><FONT SIZE=3D2>Sent: Friday, 30 May 2003 8:42 PM</FONT>
> <BR><FONT SIZE=3D2>To: premium-rx at ml.skirrow.org</FONT>
> <BR><FONT SIZE=3D2>Subject: Re: [Premium-Rx] Radios and High RF =
> powers</FONT>
> </P>
> <BR>
> <BR>
>
> <P><FONT SIZE=3D2>>Hello list.</FONT>
> <BR><FONT SIZE=3D2>></FONT>
> <BR><FONT SIZE=3D2>>The problem of making receivers work in strong =
> RF fields is not a new one.</FONT>
> <BR><FONT SIZE=3D2>>I would suggest that one of the prime criteria =
> for the design of a</FONT>
> <BR><FONT SIZE=3D2>>'military (or</FONT>
> <BR><FONT SIZE=3D2>>commercial) application' receiver - our premium =
> receivers - is that it</FONT>
> <BR><FONT SIZE=3D2>should</FONT>
> <BR><FONT SIZE=3D2>>work satisfactorily in very high RF fields (we =
> are talking of Volts not</FONT>
> <BR><FONT SIZE=3D2>>milliVolts).</FONT>
> <BR><FONT SIZE=3D2>></FONT>
> <BR><FONT =
> SIZE=3D2>>***********************************************************=
> ****</FONT>
> <BR><FONT SIZE=3D2>>Richard Reich</FONT>
> <BR><FONT SIZE=3D2>>Principal Engineer Hardware</FONT>
> <BR><FONT SIZE=3D2>>SAAT Technology Ltd</FONT>
> <BR><FONT SIZE=3D2>>Web: <A HREF=3D"http://www.saatt.co.uk" =
> TARGET=3D"_blank">http://www.saatt.co.uk</A></FONT>
> </P>
>
> <P><FONT SIZE=3D2>Hi Gang,</FONT>
> <BR><FONT SIZE=3D2>Richard highlights a problem which has plagued me =
> recently with my homebrew</FONT>
> <BR><FONT SIZE=3D2>wideband h.f. receiver preamplifier, and that is how =
> to protect the</FONT>
> <BR><FONT SIZE=3D2>delicate input RF stage from potentially damaging RF =
> voltages. My preamp</FONT>
> <BR><FONT SIZE=3D2>uses a Minicircuits broadband microchip amplifier, =
> rated at DC - 3 GHz,</FONT>
> <BR><FONT SIZE=3D2>with 12 db gain and a noise figure of 4.5 db. =
> The chip is rated at +13dbm</FONT>
> <BR><FONT SIZE=3D2>(1 Vrms) maximum voltage at the input. I've lost =
> several of the chips,</FONT>
> <BR><FONT SIZE=3D2>despite using back-to-back diodes across the antenna =
> input (which I</FONT>
> <BR><FONT SIZE=3D2>generally despise, because they cause IMD =
> problems).</FONT>
> </P>
>
> <P><FONT SIZE=3D2>In trying to understand where the damaging signals =
> were coming from, I</FONT>
> <BR><FONT SIZE=3D2>connected my antenna to the input of a Tektronix =
> TDS-420A digital</FONT>
> <BR><FONT SIZE=3D2>oscilloscope. I set the trigger to capture events =
> exceeding 1V of</FONT>
> <BR><FONT SIZE=3D2>RF. Interestingly, the test showed that my =
> problem wasn't ambient RF (even</FONT>
> <BR><FONT SIZE=3D2>from my 1KW AM transmitter), but rather static =
> pulses caused by</FONT>
> <BR><FONT SIZE=3D2>thunderstorms -- sometimes so far away that I =
> couldn't hear the</FONT>
> <BR><FONT SIZE=3D2>thunder. These lightning-generated impulses =
> are very high amplitude --</FONT>
> <BR><FONT SIZE=3D2>many volts -- with frequency components that go from =
> DC up to hundreds of</FONT>
> <BR><FONT SIZE=3D2>MHz. My diode clippers (a pair of back-to-back =
> 1N914a diodes) clamped the</FONT>
> <BR><FONT SIZE=3D2>low-frequency components of the pulses okay, but not =
> the VHF and UHF</FONT>
> <BR><FONT SIZE=3D2>components. Because the Minicircuits device =
> has an intrinsic frequency</FONT>
> <BR><FONT SIZE=3D2>response that extends up to the GHz range, it was =
> quite susceptible to VHF</FONT>
> <BR><FONT SIZE=3D2>and UHF overload.</FONT>
> </P>
>
> <P><FONT SIZE=3D2>I wonder how high-end commercial receivers solve this =
> problem? Diode</FONT>
> <BR><FONT SIZE=3D2>clipping is generally not desirable, and spark-gap =
> devices aren't sensitive</FONT>
> <BR><FONT SIZE=3D2>enough. Obviously, tuned fulters at the input (as is =
> done in the optional</FONT>
> <BR><FONT SIZE=3D2>preamps for, e.g., the WJ HF-1000A) will solve the =
> problem for a particular</FONT>
> <BR><FONT SIZE=3D2>frequency band, but don't work for the full HF =
> spectrum. Any comments</FONT>
> <BR><FONT SIZE=3D2>would be most welcome.</FONT>
> <BR><FONT SIZE=3D2>Thanks,</FONT>
> </P>
>
> <P><FONT SIZE=3D2>Jim Garland W8ZR</FONT>
> </P>
> <BR>
>
> <P><FONT =
> SIZE=3D2>_______________________________________________</FONT>
> </P>
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