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

[ludeykrus]

It was in the default 'high' position.

Update: Due to work, haven't had a chance to take a multimeter to the cables, but played with it for 15 minutes at lunch. Managed to have it work PERFECTLY on the SWR meter for about 5 minutes before it started malfunctioning again. The only thing I think I was doing was moving the cables around. I think it's a cable/connection problem, but I couldn't get it to work again regardless of moving/bending/unconnecting/reconnecting/crossing eyes/wiggling ears. Ended up having the SWR portion working pretty well, but the wattage was still low. When working correctly, channel 1 was around 1.5 on SWR and channel 40 was about 2. Will play with it again this evening with a meter. Thanks for the help so far guys.

 

[ludeykrus]

Got home and messed around with the truck with a multimeter. Checked the cables and everything checked out. It continued the symptoms at first. I pulled all the wiring out, strung it all out, and started checking for changes when bending the cables. No change. After about 20 minutes of messing with the cables, I hooked everything up with the SWR meter and I managed to get everything acting right. The radio started out at 3.5 watts initially, and after a minute of working it stayed at 4. SWR on channel 1 was around 1.3, and channel 40 was about 1.1.

EDIT - The only thing i did that I can think might have made a change was there was some water inside the magnetic mount that I blew out...

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)

Thanks a LOAD for the info! It definitely helped. Got a question for ya. You mentioned that the cable should be 50-ohm cable. I don't have the number from the cabling jacket handy, but pin to pin it measured near zero ohms (as expected from a straight run of cabling). So what's the 50-ohm 'rating' supposed to mean for me? Seeing as I didn't see 50 ohms anywhere in the line...

 

[4x4junkie]

So what's the 50-ohm 'rating' supposed to mean for me? Seeing as I didn't see 50 ohms anywhere in the line...

It's hard to explain...

It's RF (radio frequency) impedance is 50 ohms, not it's DC resistance (DC resistance is what you would measure with a meter or a continuity tester). It has to do with the ratio of voltage to current at which a wave of RF energy travels (or propagates) along the cable (and needs to match with that of your radio's output and your antenna system).

Some in-depth detail about it here:
http://en.wikipedia.org/wiki/Transmission_line

 

[DaveInDenver]

Got home and messed around with the truck with a multimeter. Checked the cables and everything checked out. It continued the symptoms at first. I pulled all the wiring out, strung it all out, and started checking for changes when bending the cables. No change. After about 20 minutes of messing with the cables, I hooked everything up with the SWR meter and I managed to get everything acting right. The radio started out at 3.5 watts initially, and after a minute of working it stayed at 4. SWR on channel 1 was around 1.3, and channel 40 was about 1.1.

EDIT - The only thing i did that I can think might have made a change was there was some water inside the magnetic mount that I blew out...

Those measurements seem more reasonable. Anything under 1.5:1 is a good match for our purposes. When measuring total loss that works out to about 96% of the power (a return loss of 14dB) generated by the radio being absorbed or dissipated by the load (e.g. the antenna and feed lines). In fact really anything under 2:1 is fine and 1.3:1 would be more than sufficient for me tuning a CB antenna.

Most radios will tolerate as much as 3:1 VSWR, which is around 25% of the incident power being reflected back into the generator. This much power coming back into the radio creates heat that must be handled, so radios that include output protection will reduce output power to avoid damage. Some radios don't and they will happily try to transmit into a terrible load with resultant stress to their outputs. I don't know if your radio rolls back on high SWR, but based on the things you've shown I suspect it does. High SWR protection can be infuriating but mostly is a good thing in mobile radios.

Thanks a LOAD for the info! It definitely helped. Got a question for ya. You mentioned that the cable should be 50-ohm cable. I don't have the number from the cabling jacket handy, but pin to pin it measured near zero ohms (as expected from a straight run of cabling). So what's the 50-ohm 'rating' supposed to mean for me? Seeing as I didn't see 50 ohms anywhere in the line...

As 4x4junkie mentions this one simple question about characteristic impedance took the first 8 or 10 weeks of EM Fields I to develop in engineering school. The only case resistance and impedance happen to match is with purely real loads such as a non-inductive resistor, which is the basis for a dummy load. Most anything that does actual work in an alternating current system will have some complex component that can't be seen with a simple DC measurement, so DMM tests are limited usefulness beyond what are essentially workmanship tests.

This site distills things down pretty well. Certainly not as detailed as it could be but also doesn't hand wave the black magic too much.

http://www.antenna-theory.com/tutorial/txline/transmissionline.php#txline

Suffice to say, the reason we use 50 ohms for transmission lines is that it must match the antenna impedance but there is a trade-off between maximum power and minimum attenuation which are not the going to come at the same impedance. So the historical convention was to settle on 50 ohms.

http://www.microwaves101.com/encyclopedia/why50ohms.cfm

If you see a SWR of 1:1 it's sort of a hint to an engineer or radio amateur that something is off. This is especially true when you sweep an antenna and feed line. To understand why does take some digging into technical background. The length of the transmission line in this case is does cause it to become an impedance transformer to the non-50 ohm (typically) antenna impedance, so you can actually get very low SWR but this only happens well at one frequency. So if you get such a low SWR in multiple places usually means something is wrong, which is kind of not what your intuition says as everyone wants to see low SWR. SWR is just one measurement, so you want to see reasonable numbers, none of which exceed a maximum threshold.

In your case the water under the magnet might have been shorting, which means you were essentially measuring the characteristic impedance of the coax, which would give 50 ohms and a perfect match. That's going to look great but the problem is it's a terrible antenna and your radio apparently knew it, thus it cut power.