CB SWR meter on 122Mhz?

On 2008-02-13, RST Engineering wrote:
"noman" wrote in message
...
Otherwise, range and clarity are fine. I just got to wondering if my
homemade antenna or cable are a problem. To do the original install,
I used an rf field strength meter and simply adjusted the antenna
length for highest reading.
That probably wasn't the cleverest way of doing it.

Maybe not the cleverest way, but it's probably good enough that it's working
fine.

Describe your installation a little more, but I think I can guarantee you
that it isn't the antenna.

I agree. This sounds like a front end overload problem. No antenna in the
world will fix that. (Well, at least not and still work for the intended
purpose.)
--
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

Jay Maynard wrote:

On 2008-02-13, RST Engineering wrote:
"noman" wrote in message
...
Otherwise, range and clarity are fine. I just got to wondering if my
homemade antenna or cable are a problem. To do the original install,
I used an rf field strength meter and simply adjusted the antenna
length for highest reading.
That probably wasn't the cleverest way of doing it.

Maybe not the cleverest way, but it's probably good enough that it's working
fine.

I've had variable luck using that approach on other types of antennas.
It's probably a better approach than the typical SWAG.

Describe your installation a little more, but I think I can guarantee you
that it isn't the antenna.

I agree. This sounds like a front end overload problem. No antenna in the
world will fix that. (Well, at least not and still work for the intended
purpose.)

I agree - the only way his antenna is likely to fix it is by removing
it altogether (which, as you point out will not help the operation of
the radio much).

No doubt the problem is that (like Jim pointed out) the FM signal is
mixing with some other signal. The question is whether that's
happening inside the portable radio, or outside it.

If it's inside the radio, reducing the amount of broadcast FM signal
getting into the receiver's front end would help. Personally, I'd do
this by putting a T connector in line with the antenna, and plugg in a
shorted quarter-wavelength (at the offending FM frequency) chunk of
shorted coax on the extra connector. The length of the stub depends
on the coax you use, and the type of connector.

If this doesn't help it's likely that the mixing is occurring
somewhere else. If it's outside the aircraft, there's not a lot you
can do (though that would mean that almost everyone would have the
problem). If the mixing is happening in the aircraft, it'll be "more
fun" to find. Someone suggested unplugging your ELT (a possibility,
and easy enough to do - just don't schedule any crashes when doing
this). ;-) I've seen all kinds of metal structures become effective
RF mixers - so the problem could be almost anywhere in a typical
aircraft (note: I don't recommend removing metal in the process of
looking for the mixing source - you may really NEED that ELT if you do
that). ;-)

Mark Hickey

"Mark Hickey" wrote in message
...

-----------------preceding posts snipped-----------

If it's inside the radio, reducing the amount of broadcast FM signal
getting into the receiver's front end would help. Personally, I'd do
this by putting a T connector in line with the antenna, and plugg in a
shorted quarter-wavelength (at the offending FM frequency) chunk of
shorted coax on the extra connector. The length of the stub depends
on the coax you use, and the type of connector.

----------------remainder snipped-----------------

Mark Hickey

This is the method which I have seen work most frequently, although that
mainly involved listeners radios in their homes. It also has the advantage
of being cheap.

Peter

On Thu, 14 Feb 2008 19:16:09 -0700, Mark Hickey wrote:

Jay Maynard wrote:

snip
If this doesn't help it's likely that the mixing is occurring
somewhere else. If it's outside the aircraft, there's not a lot you
can do (though that would mean that almost everyone would have the
problem). If the mixing is happening in the aircraft, it'll be "more
fun" to find.

snip
Mark Hickey

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.
John

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.
--
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

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.