CB SWR meter on 122Mhz?

[noman]

How well do you think a CB SWR meter will work for testing the antenna
installation in an airplane?

thanks,
tom

[Jay Maynard]

On 2008-02-13, noman > wrote:
> How well do you think a CB SWR meter will work for testing the antenna
> installation in an airplane?

Not worth a damn. The sensing circuitry is almost certainly not going to work
properly at VHF. A meter designed for the 2-meter ham radio band would work,
though.

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 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

[noman]

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

[RST Engineering]

As stated, the odds of a 27 MHz. SWR meter giving useable results on the VHF
aircraft band are slim to zero.

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

"noman" > wrote in message
...

> My radio seems to be overloaded by strong local commercial stations.

Not an unusual situation with inexpensive radios with poor front end
filtering, especially for high power FM stations at the top of the band and
at the image frequency of the radio (plus the various harmonic, crossmod,
and intermod combinations of the FM stations and your radio's
superheterodyne configuration).



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


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.

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.

[Jay Maynard]

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

[Wayne Paul]

Tom,

I concur with Jay. You need a unit designed for the frequency being tested.

MFJ Enterprises has as series of suitable VHF SWR/Watt Meters starting at
about $40.
http://www.mfjenterprises.com/Catergories.php?sec=13&PHPSESSID=3de2d1c7b5867eeea645fc51f462651f
or
http://tinyurl.com/yo26k6

I personally use the SWR in my old RST-721 test set.

Wayne
W7ADK
http://www.soaridaho.com/



"Jay Maynard" > wrote in message
...
> On 2008-02-13, noman > wrote:
>> How well do you think a CB SWR meter will work for testing the antenna
>> installation in an airplane?
>
> Not worth a damn. The sensing circuitry is almost certainly not going to
> work
> properly at VHF. A meter designed for the 2-meter ham radio band would
> work,
> though.
>
> 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 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

[RST Engineering]

{;-)

Jim


> I personally use the SWR in my old RST-721 test set.
>
> Wayne
> W7ADK
> http://www.soaridaho.com/

[Wayne Paul]

Jim,

The RST-721 is a great little unit. In addition to tuning the antennas on
the local experimental sailplanes, I use it for my 2 meter equipment.

I didn't build one of your transceivers. I inherited the unit from a friend
who retired from soaring in his 80s.
(http://www.soaridaho.com/photogallery/valley/Frank_is_Ready.jpg)

I wish aircraft tranceiver kits were still available. I enjoy building
electronics as much as working on my sailplane.
(http://www.soaridaho.com/Schreder/HP-14/N990/N990.html) I built all my
amateur radion HF equipment. (Heathkit SB-310, SB-200, SB-101 and SB640)
These old tube-based units are still running great.

Wayne
W7ADK
http://www.soaridaho.com/Schreder



"RST Engineering" > wrote in message
...
>
>
>
> {;-)
>
> Jim
>
>
>> I personally use the SWR in my old RST-721 test set.
>>
>> Wayne
>> W7ADK
>> http://www.soaridaho.com/
>
>

[Bob Kuykendall]

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.

> 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.

Thanks, Bob K.
http://www.hpaircraft.com

[Wayne Paul]

Bob,

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.

Wayne
HP-14 "6F"
http://www.soaridaho.com/


"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.

> 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.

Thanks, Bob K.
http://www.hpaircraft.com

[Bob Kuykendall]

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.

[noman]

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

[Jim Austin]

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

[RST Engineering]

--
"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

[Ron Webb]

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.

[Mark Hickey]

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

[Peter Dohm]

"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

[Jay Maynard]

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

[Jumpin Jahosaphat]

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

[[email protected]]

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

[Ron Natalie]

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.

[Jay Maynard]

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

[RST Engineering]

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.

[Jay Maynard]

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