CB SWR meter on 122Mhz?

Jay Maynard wrote:
On 2008-02-17, Jumpin Jahosaphat wrote:
Also you might try putting a 20-30 db attenuator on your radio connector.
If the signal remains then probably originating (ie mixing products)
outside the radio. If the interference goes away it is most liking
occuring inside the radio.

Good idea. One note: DO NOT TRANSMIT WITH THE ATTENUATOR IN THE LINE. You
WILL destroy it, and likely your transmitter's final amplifier as well if
you transmit for too long after destroying the attenuator.

While it may cook the attenuator, provided he's move on from vacuum
tubes, it's unlikely that a 12 or 24v driven transistor final is going
to burn up even if it's a complete mismatch.

On 2008-02-26, Ron Natalie wrote:
Jay Maynard wrote:
Good idea. One note: DO NOT TRANSMIT WITH THE ATTENUATOR IN THE LINE. You
WILL destroy it, and likely your transmitter's final amplifier as well if
you transmit for too long after destroying the attenuator.
While it may cook the attenuator, provided he's move on from vacuum
tubes, it's unlikely that a 12 or 24v driven transistor final is going
to burn up even if it's a complete mismatch.

Transmitting at full power into an open circuit - which is what you'd get
after burning up the attenuator - is about the worst thing you can do to a
final, especially a transistor one. Solid state finals are less forgiving
than tube finals, and abuse that would just make the tubes run hotter than
normal will burn up transistors quickly.
--
Jay Maynard, K5ZC http://www.conmicro.com
http://jmaynard.livejournal.com http://www.tronguy.net
http://www.hercules-390.org (Yes, that's me!)
Buy Hercules stuff at http://www.cafepress.com/hercules-390

Excuse me, sir, but most of us who design moderate power solid state
transmitters wouldn't THINK of a circuit that didn't have full VSWR
protection, either by overdesign of the output devices or a VSWR loop to
shut the finals down in the event of a situation that you describe.

And no, they don't run hotter. The normal failure mode is secondary
breakdown due to voltage spikes in the output matching circuitry.

And, given a random length of coax between the transmitter and the open
circuit, all you can guarantee is that you will be on the outer ring of the
Smith chart somewhere between an open and a short ... a pure open is a very
low probability.

Jim




Transmitting at full power into an open circuit - which is what you'd get
after burning up the attenuator - is about the worst thing you can do to a
final, especially a transistor one. Solid state finals are less forgiving
than tube finals, and abuse that would just make the tubes run hotter than
normal will burn up transistors quickly.

On 2008-02-26, RST Engineering wrote:
Excuse me, sir, but most of us who design moderate power solid state
transmitters wouldn't THINK of a circuit that didn't have full VSWR
protection, either by overdesign of the output devices or a VSWR loop to
shut the finals down in the event of a situation that you describe.

You or I wouldn't do that. I've fried more than one solid state radio's
finals doing that, however, including one (a Kenwood TR-7800) that used a
module that supposedly had all that built in. (In 35 years as a ham, I've
made all of the classic mistakes, and a few not so classic.)

Bendix King and Garmin probably do the right thing. Would you bet
significant money that Narco or ARC did?

And, given a random length of coax between the transmitter and the open
circuit, all you can guarantee is that you will be on the outer ring of the
Smith chart somewhere between an open and a short ... a pure open is a very
low probability.

That one I'll give you, as long as the attenuator is placed down the
feedline instead of at the back of the radio.
--
Jay Maynard, K5ZC http://www.conmicro.com
http://jmaynard.livejournal.com http://www.tronguy.net
http://www.hercules-390.org (Yes, that's me!)
Buy Hercules stuff at http://www.cafepress.com/hercules-390