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SWR meter gives inaccurate reading

Chris Lawrence

Active Member
Jul 9, 2021
93
43
28
Am I missing something or is process to align the radio’s SWR meter not in the alignment procedures?

I’m not referring to the simple process of checking your SWR’s. I’m referring to making sure the radio was aligned properly internally so that the external meter will give you the proper results when calibrating it.

I have a Cobra 29 that’s gives really bad readings. It’s off by 2-3. Antenna setup, using rig expert says 3.9. Radio shack meter says about that and radio shows 1.2. Also, the radio shows 1.2 but the antenna light comes on so it doesn’t agree with the stock meter.

Can’t find anything on vr52. Don’t know if it’s related. Also, new radio so the cbtricks stuff will be out of date.

Thank you :)
 

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Anyway to adjust this?

Not really. There is a fixed terminating resistor for the forward side and one for the reverse side of the directional coupler coil. Years ago we would scab a trimpot onto that part of the old Black Cat meters. This improved accuracy, but the trimpot's wattage rating just wasn't up to the job and would burn out. Makes me wonder if the same trick would work in that radio's SWR meter. Never had an incentive to try. We just tell Joe Driver to check his SWR as a pre-trip item. So long as the antenna had been checked with a real meter, it was probably okay. Make a note of what the radio's meter shows. If it's always the same, you're good to go. And if the reading changes, that means it's time to put a real SWR meter or analyzer on the antenna to see what changed. When the reading changes it's almost never for the better.

73
 
Not really. There is a fixed terminating resistor for the forward side and one for the reverse side of the directional coupler coil. Years ago we would scab a trimpot onto that part of the old Black Cat meters. This improved accuracy, but the trimpot's wattage rating just wasn't up to the job and would burn out. Makes me wonder if the same trick would work in that radio's SWR meter. Never had an incentive to try. We just tell Joe Driver to check his SWR as a pre-trip item. So long as the antenna had been checked with a real meter, it was probably okay. Make a note of what the radio's meter shows. If it's always the same, you're good to go. And if the reading changes, that means it's time to put a real SWR meter or analyzer on the antenna to see what changed. When the reading changes it's almost never for the better.

73
[edit] Looks like R52 has been replaced by a variable resistor. VR52 needed calibrating. 1k ohm.
 
Whoa! R53 in the schemo is a fixed 560 ohm resistor. If VR53 is connected the same as the R53 in the diagram, it should let you adjust the SWR reading.

Missed that update to this radio.

7

, but I think it’s VR52 if memory serves . I’ve got two in a row where they don’t want to adjust properly. I’m wondering if it goes into “limp
Mode” so to speak if the Swr is too high.

I have a failed 150ohm dummy load and a failed 16.7 ohm to try and get 3:1
Reading to set it. Umtilnthose arrive tomorrow, I’ve had to mid-tune an antenna to give it a 3:1
Reading and I’m getting inconsistent readings on my
Zoom35. So,
I’ll wait til I get a 300 ohm variable rheostat. Hopefully that should work like am antenna timer.
Whoa! R53 in the schemo is a fixed 560 ohm resistor. If VR53 is connected the same as the R53 in the diagram, it should let you adjust the SWR reading.

Missed that update to this radio.

73
Some changes on the board, especially with the newest FM models.
 

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Am I missing something or is process to align the radio’s SWR meter not in the alignment procedures?

I’m not referring to the simple process of checking your SWR’s. I’m referring to making sure the radio was aligned properly internally so that the external meter will give you the proper results when calibrating it.

I have a Cobra 29 that’s gives really bad readings. It’s off by 2-3. Antenna setup, using rig expert says 3.9. Radio shack meter says about that and radio shows 1.2. Also, the radio shows 1.2 but the antenna light comes on so it doesn’t agree with the stock meter.

Can’t find anything on vr52. Don’t know if it’s related. Also, new radio so the cbtricks stuff will be out of date.

Thank you :)
Remember your checking the SWR, not SWR's. Historically that feature on a cb radio has been a ballpark reading at best. By the time you step up to HF rigs things get much better. You can fiddle with it if it bugs you, best bet is always use an external SWR/Watt meter.
 

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