SWR Meter Not Swinging All The Way Right... Where To Look?

[ludeykrus]

Hey guys,

Recently I got into radios and picked up a CB and antenna. I purchased a Midland 75-822 with a Cobra CBRHGA1500 antenna (36" whip, magnet mount, 10' coax) here:

http://www.amazon.com/gp/product/B00005N5X2/ref=oh_details_o01_s02_i00?ie=UTF8&psc=1


The CB sounds pretty good with the rubber ducky antenna, but has a very limited range. I hooked up the 36" antenna, mounted in the center of the top of my F250's roof, and I get a much better range bu a LOT of baseline static. I picked up an SWR meter, an Astatic PDC1, from a truck stop along with a 12' coax patch cable. I hooked it up, and could not get it to calibrate. When I was supposed to turn the cal knob so the gauge arm aligned with the "SET" mark to the far right, the arm will only move 75-90% to the SET mark even with the cal knob fully cranked.

Where do I need to start looking for problems? I don't think it's a problem with the SWR meter. All connections are solid. I read something about possible a grounding/shorting problem with the antenna; maybe a missing insulating washer. Problem is where exactly would I look? Anything else that could cause similar behavior with the SWR meter?

 

[4x4junkie]

Sounds like it's possible your SWR might be off the charts. Did you try the "REF" or "SWR" (whichever it's labeled) setting anyway? Was the needle just as high or higher than where it was calibrated to? Or did it still drop down to somewhere near the 1.5:1 or lower marks?

Are you using something like paper or napkins under the antenna to protect your roof? (anything thicker than a single sheet of say, like copy paper can cause bad SWR to occur on a mag mount antenna. A single thin piece of cellophane (Saran Wrap, etc.) would be better).

 

[DaveInDenver]

Is this the meter?

• Make sure the right switch is set to SWR
• Turn the CAL knob all the way to the left, counter clockwise
• Verify that the radio and antenna are connected to the right places in the back (TRANS means transceiver, e.g. the radio)
• Set the left switch to FWD
• Key the mic and hold it down*
• Turn the CAL knob so that the needle swings all the way to the right of the meter, it should point exactly to SET on the scale with the mic key held down*
• Unkey the mic
• Set the left switch to REF without touching the calibration knob
• Key the mic*
• The needle will point to the amount of reflected power indicated by an SWR reading*
• Unkey the mic

*Do these steps quickly, you are transmitting, which is bad for the radio if the load is poor. Not to mention anyone listening will hear you testing.

If you follow these steps and the needle still swings wildly or indicates any reading higher than 3:1 then you have an issue as junkie mentions. Ideally it barely moves when you key on the REF step, which indicates very little reflected power. It could be the meter is giving you false information but you can't say that for certain without checking with a dummy load or a known good antenna.

If you place the right switch to PWR and select the left switch to 10W you should see the needle swing to 4 on the meter. If it does this then the radio is dissipating and meter is measuring the 4 watts forward correctly. If you switch the left to 100W the needle should barely move. I don't believe the knob should have any affect in this mode but can't say for sure. But if that works then the meter is probably OK.

 

[ludeykrus]

Quick update while I'm at lunch, grabbed this video showing my radio's behavior today on the SWR meter:

https://www.youtube.com/watch?v=zoxcYdU2570&feature=youtu.be


It's acting MUCH better, which makes me think the rain from yesterday was causing issues with my antenna/mount, possibly they're shorting out somehow.

 

[DaveInDenver]

A flat 1:1 across the whole band is pretty incredible bandwidth. Is that typical of these antennas?

 

[digitaldelay]

https://www.youtube.com/watch?v=zoxcYdU2570&feature=youtu.be

Something doesn't seem right here. If you have 1:1 match, why are you only pushing 0.3W?

I have the same radio and meter. I might play around with it later to see what results I get.

Jason


Sent from my HTC One using Tapatalk

 

[4x4junkie]

^^

Agreed, the meter should swing up to somewhere around 3-4 watts if both switches are up ("10W" and "PWR").

Check that the patch cable you're using between the radio and the meter isn't bad.

 

[ludeykrus]

Something doesn't seem right here. If you have 1:1 match, why are you only pushing 0.3W?

I have the same radio and meter. I might play around with it later to see what results I get.

Jason

Please do, I'd love to see your results. I don't know what's 'normal' aside from the generic "how to tune your antenna" articles. Mine's obviously not fitting what they say...

^^

Agreed, the meter should swing up to somewhere around 3-4 watts if both switches are up ("10W" and "PWR").

Check that the patch cable you're using between the radio and the meter isn't bad.

Will do. I'm very savvy with electronics, but not radios in particular. Could I check the patch cable with a multimeter?

 

[ludeykrus]

Is this the meter?

• Make sure the right switch is set to SWR
• Turn the CAL knob all the way to the left, counter clockwise
• Verify that the radio and antenna are connected to the right places in the back (TRANS means transceiver, e.g. the radio)
• Set the left switch to FWD
• Key the mic and hold it down*
• Turn the CAL knob so that the needle swings all the way to the right of the meter, it should point exactly to SET on the scale with the mic key held down*
• Unkey the mic
• Set the left switch to REF without touching the calibration knob
• Key the mic*
• The needle will point to the amount of reflected power indicated by an SWR reading*
• Unkey the mic

*Do these steps quickly, you are transmitting, which is bad for the radio if the load is poor. Not to mention anyone listening will hear you testing.

If you follow these steps and the needle still swings wildly or indicates any reading higher than 3:1 then you have an issue as junkie mentions. Ideally it barely moves when you key on the REF step, which indicates very little reflected power. It could be the meter is giving you false information but you can't say that for certain without checking with a dummy load or a known good antenna.

If you place the right switch to PWR and select the left switch to 10W you should see the needle swing to 4 on the meter. If it does this then the radio is dissipating and meter is measuring the 4 watts forward correctly. If you switch the left to 100W the needle should barely move. I don't believe the knob should have any affect in this mode but can't say for sure. But if that works then the meter is probably OK.

Thanks for the input! Good to see what it should be doing.

 

[ludeykrus]

Are you using something like paper or napkins under the antenna to protect your roof? (anything thicker than a single sheet of say, like copy paper can cause bad SWR to occur on a mag mount antenna. A single thin piece of cellophane (Saran Wrap, etc.) would be better).

Nope, just what the antenna came with. Antenna mount with a super thin layer of plastic. Even with, it scratched the **** out of my roof's paint when moved!

 

[DaveInDenver]

Could I check the patch cable with a multimeter?

Ideally you check coax cables with RF equipment, but there are sanity checks you can do with a DMM. I assume you did not make up the cable so it's worth verifying that it's supposed to be a 50Ω cable (type is something like RG-58, RG-8, which should be printed on the outer jacket). Also make sure the connectors seem crimped or soldered solidly and look clean, no stray strands of wire, kinks or dents in the dielectric foam or nicked jacket, etc.

With meter set to a high range or auto:
Check center pin on connector 1 to outside of connector 1 - should be open*
Work the coax, don't try to break it but flex and bend it along its length and where it transitions to the connectors to verify that there are no places where it shorts with movement

With the meter set to it's lowest range (if yours has a continuity beep setting that would work):
Check center pin on connector 1 to center pin on connector 2 - should be zero*
Check outside of connector 1 to outside of connector 2 - should be zero

Short connector 2 with a jumper cable# and check center pin on connector 1 to outside of connector 1 - should be zero
Repeat the flex test, make sure that the cables doesn't jump up to an open suddenly with movement

If it passes all these then it's probably fine.

* Open and zero are practically speaking, zero could be readings like 0.5Ω and open might read 1MΩ
# Short with an alligator cable, you can get these at Radio Shack (I think still)

 

[DaveInDenver]

You can also check the antenna with a DMM and it might give you some indications. It's tougher to know for sure with a mag mount because the mount is floating relative to the vehicle (they ideally should couple with RF) so the DC path is not necessarily solid as it would be with a hard mounted antenna. But you might be able to the read the capacitive coupling with a DMM, so measuring between the coax connector outside and vehicle ground might measure something.

In any case, measure the antenna connector and see what it reads. Measure the outside to the vehicle ground, center pin to ground, etc.

I don't know the configuration of this particular antenna but it's possible that you will see what looks like DC conductivity in the antenna. This is not necessarily an indication that it's bad. You might also see something odd like it's a short that opens after a few seconds. That is again not a definite indication that something wrong (that might be a cap charging for example). Depends on the matching network if there's one.

My guess is with this antenna you'd expect that the antenna looks like an open with a DMM, so there would be no conductivity between the center and outside of the connector nor center of the connector to vehicle ground either. If there are either it might cause me to make sure everything is good with it. IOW, check that the cable is not damaged, there are no broken contacts or anything. Kind of depends on the construction of the antenna. The mounts I use have a little tab inside that make up the center pin path and it's not uncommon for them to get bent and short, for example.


BTW, I might at this point also consider that your radio is working against you. Any idea if the radio you're using has a high SWR roll back feature? That is a fairly common feature in ham and business radios, it protects the output should the antenna be missing or bad. When the radio senses a high SWR it cuts power. My Uniden doesn't do this but I'm not sure if it's something that newer CBs do or not. If so you need the antenna to be sorta close to tuned, at least in the ball park, before a simple SWR bridge will work. If yours has this and your antenna has a major issue, that could explain why it's only dissipating 10% of its power.

 

[4x4junkie]

I suspect the SWR at the antenna is probably OK (the meter is dropping to zero or close to it when set to "REF").

That radio doesn't have a "Low" power setting, does it? (maybe it's set to "Low"?) Otherwise, my $$$ is on a bad patch cable, and/or there's an internal problem with the radio (bad output transistor maybe).

 

[digitaldelay]

That radio doesn't have a "Low" power setting, does it?

It does, actually. I think the default is full power, though. I can't tell from the video if "Low" is activated.

Jason

Sent from my HTC One using Tapatalk

 

[digitaldelay]

I think the default is full power

Looks like it boots up with whatever settings were active before, so just make sure "low" is off. It's the H/L button on the right.

Jason


Sent from my HTC One using Tapatalk