CB SWR meter on 122Mhz?

On Feb 13, 9:45*am, "Wayne Paul" wrote:

I played around with this a couple years ago. *I trimmed an antenna using a
CB SWR to as close to 1:1 as I could get. *I then measured it with my
RST-721. *The '721 read 3.5:1. *That is quiet a difference.

I did the same test using my quality HF SWR unit and '721 with the same
results.

Thanks for that info. I guess its time for me to get a better SWR
meter.

Bob K.

On Feb 13, 8:28 am, "RST Engineering"
Not an unusual situation with inexpensive radios with poor front end

The radio is an Icom A5. Definitely on the low end of the radio cost
spectrum.

That probably wasn't the cleverest way of doing it. Describe your
installation a little more, but I think I can guarantee you that it isn't
the antenna.

The A5 is connected to a DRE portable intercom and then to my
headphones. The
antenna is a simple wire about 22 inches long soldered to an rf
connector. The
connector is mated to a bulkhead rf fitting that has the cable
soldered to it. I forget the
cable designator, but it has fairly low attenuation at 100mhz, and is
the same impedance
as the output of the radio. I ran the power, headphone, and push to
talk wiring through
torroids near the radio, but that didn't do much improvement. The
only time this shows
up is near the more powerful transmitters. I suspected the front end
of the radio, but I
don't know how to prove or disprove that. I figured that if power was
clean and antenna was
clean, then I'm pretty much left with the radio. Oh yeah, taking the
intercom out of the
mix makes no difference.

Then why in heaven's name would you suspect YOUR antenna if the same thing
happens to OTHER aircraft and their radios? Their antennas couldn't have
ALL failed in the same way.

Agreed, and I misspoke. There was only one other plane that I know
has this problem.
But, again, I'm trying to eliminate what I can with the equipment I
have at my disposal. As
you can see from my choice of radio gear, I'm on a budget. 8^)

tom

noman wrote:
My radio seems to be overloaded by strong local commercial stations.
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. It all seems to work fine, I can talk to
aircraft 40 miles away when we are both at 1000agl, but when I get
near some commercial antennas the station bleeds in and breaks the
squelch, even with it turned all the way up. I have seen this on
other rental planes, too.
tom

What are you trying to find out? Unless something is corroded or broken,
whatever problem you have is probably not your antenna.
--
Jay Maynard, K5ZC

I had similar symptoms in an earlier airplane, the problem turned out to be a problem with my ELT. I was led to that by info gained from this group. Try temporarily disconnecting the antenna from your ELT and see if your comm radio problems go away. Mine did. I bought and installed another ELT, no more problem. I have helped several other local people who were tearing their hair out trying to solve similar problems by giving them this information.

Jim Austin

--
"If you think you can, or think you can't, you're right."
--Henry Ford

"Bob Kuykendall" wrote in message
...
On Feb 13, 8:11 am, Jay Maynard
wrote:

Not worth a damn....

My experience runs the other way. I've tested and troubleshooted
several VHF radio antenna installations using a cheap SWR meter, and
system performance has seemed pretty uniformly inversely proportional
to the SWR reading. Of course, that's far from scientific, but it
suggests that inaccurate measurements are better than no measurements
at all.

There is a vast difference between a "cheap" SWR meter and one optimized and
sold specifically as a "CB SWR Meter". I've seen "cheap" SWR meters
designed for HF/VHF (and the one in the mentioned RST-721 is about as cheap
as you can get) give nearly identical results to the Bird and high end
meters. The difference is in the number of decimal places of accuracy. For
aircraft antennas that have to cover a 15% instantaneous bandwidth, you can
generally measure with a micrometer, mark it with chalk, and cut it with an
axe and come close enough.




What are you trying to find out? Unless something is corroded or broken,
whatever problem you have is probably not your antenna.

That also runs counter to my experience. Yeah, where we're talking
about classic quarter-wave whips or commercially available blade
antennas, I certainly agree. But when it comes to custom antennas,
internal antennas, copper-tape dipoles, oddly-shaped dipoles, dipole
antennas in fins and rudders, dipoles near metal parts, and carbon
fiber ground planes (all of which are common in sailplanes), radio
problems are way too often caused by the antenna or its installation.

Yep, but this guy mentions a 22" length of wire (? brazing rod perhaps ?)
soldered into a coax connector and then that connector run into a "bulkhead"
connector with the coax soldered onto the back end. I think he meant "panel
mount" rather than "bulkhead" because a bulkhead connector by definition is
a coax connector with both ends terminated in an identical connector (like a
back-to-back female BNC with a mounting thread in the middle). Be that as
it may, I'd take a magnifying glass and a little wiggle action to see if the
"soldered" joints really are. Other than that, unless he's got that panel
mount connector on some sort of insulator other than a metal airframe skin,
the odds of it being the culprit are damned near zero.

Also, the comment about the ELT is well founded. Rather than just
disconnecting the antenna, I'd take the batteries out. THere is generally
enough leakage through the ELT plastic case to cause a bit of trouble also.
(Who'da ever thunk that the CB junction of the output transistor in the off
mode would be such a great varactor multiplier???)

Jim

I have a Radio Shack SWR bridge/wattmeter designed for CB radio. It uses
standard 1N914 diodes for the detectors, and it seems to work fine at air
band frequencies. I've never bothered to check it's calibration, but it's
plenty good for a rough check.

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 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

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

I have a used DAIWA CN-101L, 1.8-150MHZ SWR/PowerMeter for sale. $85
with BNC adapters.
Contact me at http://www.abri.net/sq2000/emailpers.html

On Feb 13, 10:04 am, noman wrote:
How well do you think a CB SWR meter will work for testing the antenna
installation in an airplane?

thanks,
tom