Troubleshooting Unusual SWR Readings

[magicmary]

Hello, all. I recently completed an install of a Breedlove 503 mount on the roof of my Tacoma and I'm seeing an SWR reading that I've never encountered before. Using a NanoVNA calibrated for the 144 - 148 MHz range, the SWR starts out at 2.6 and linearly digresses to 1.1. I'm using RG8X coax and the run is less than 25 feet. I've soldered two ring terminals to the 503, one for the center conductor of the cable and one for the shield. I've also soldered a PL-259 connector to the other end. I've tested a Larsen 2/70, two Comet B-10's and a Diamond NR72; all four antennas show the same SWR pattern and roughly the same values. Understanding that a high SWR on the low end of the frequency range and a low SWR on the opposite end means that the antenna is too short, I extended Larsen 2/70 by touching the whip with one of the Comet antennas. As the Comet antenna moved up the whip, the SWR lowered into an acceptable range.

Does anyone have any ideas as to why this is happening and how to resolve it? Thanks in advance.

 

[magicmary]

After being able to punch into a local 2m repeater on 10 watts with the Diamond NR72, I'm starting to think that this may not be an issue. After further research, it seems that most VSWR equipment is calibrated for a 50 ohm transmission line--the VNA included--and it would appear that my particular setup is not 50 ohm. If that's the case, then the VSWR readings are inaccurate.

I failed to mention above that I've tested the install with a multimeter. I've tested the center pin of the PL-259 connector to the NMO mount center pin, the NMO mount threads to the barrel of the PL-259 connector, the mount body to the barrel connector and to chassis ground--all of which returned expected results. To be more specific, the resistance on the chassis ground to the barrel was 0.2 ohms.

I also read that a long length of coax may cause inaccurate VSWR readings. In my previous install, the coax was approximately 6.5 to 7.0 ft. from the right corner of the hood to the rear cabin; this coax install is slightly less at around 5.0 ft. I mention this because the previous location, used a Laird MABVT8 mount with a PL-259 connector and the VSWR readings were acceptable at roughly 1.8. My thinking on this is that the reflected energy in the previous install was dissipated over the extra length and this shorter length would show a more accurate reading if the transmission line matched 50 ohms.

I'm happy to receive any feedback on this thought process. As many of you know, the TM-D710GA is no longer in production and I would like to keep the radio for as long as possible. As a result, I'll be operating at a low power--less than 10 watts--until I've resolved this potential issue.

Thanks!

 

[mmckenna]

Did you check for continuity from the center pin of the NMO mount to ground? Should be no continuity. If there is, you've got a short.

Make sure that the center tab on the antenna is actually making contact with the center pin on the NMO mount.

Coax length can throw SWR off, but that's probably not your issue. A properly tuned antenna will fix that.

The antenna and coax should be 50Ω, is there a reason why you think it is not? Tuning will impact that, but they should be pretty close from the factory.

 

[magicmary]

Did you check for continuity from the center pin of the NMO mount to ground? Should be no continuity. If there is, you've got a short.

Make sure that the center tab on the antenna is actually making contact with the center pin on the NMO mount.

Coax length can throw SWR off, but that's probably not your issue. A properly tuned antenna will fix that.

The antenna and coax should be 50Ω, is there a reason why you think it is not? Tuning will impact that, but they should be pretty close from the factory.

I just checked the continuity from the center pin of the mount to chassis and there is no continuity with the PL-259 disconnected from the radio--with the PL-259 attached to the radio, there is continuity from pin to ground.

The center pin of the antenna is making contact with the NMO mount.

The only reason I have to believe that the transmission line is not 50 Ohm is due to the ring terminal install--this is the first time I've done an install like this.

EDIT: I just double checked, for my own sanity and I found no continuity between the whip and the center pin of the PL-259. I removed the whip of the Larsen 2/70 and checked the center pin of the body of the antenna and the pin of the PL-259, no continuity. I did find continuity between the barrel of the PL-259 and the center pin of the antenna body once the whip was removed. I'm guessing I have a short somewhere in the system.

 

[mmckenna]

I just checked the continuity from the center pin of the mount to chassis and there is no continuity with the PL-259 disconnected from the radio--with the PL-259 attached to the radio, there is continuity from pin to ground.

OK, that is good, that means your coax is good through to the radio and there are no shorts to ground.

The center pin of the antenna is making contact with the NMO mount.

Excellent.

The only reason I have to believe that the transmission line is not 50 Ohm is due to the ring terminal install--this is the first time I've done an install like this.

That would impact things, but PL-259's are not a perfect 50Ω either. I don't think that's enough to throw your SWR off unless those leads are REALLY long.

EDIT: I just double checked, for my own sanity and I found no continuity between the whip and the center pin of the PL-259. I removed the whip of the Larsen 2/70 and checked the center pin of the body of the antenna and the pin of the PL-259, no continuity. I did find continuity between the barrel of the PL-259 and the center pin of the antenna body once the whip was removed. I'm guessing I have a short somewhere in the system.

Remove the coil from the NMO base and check for continuity between the threaded stud on the top of the coil and the center tab on the underside of the NMO, there should be continuity.

I believe these antennas have a "DC grounded coil", so getting continuity between the outer shield and the center pin with the coil installed would be normal.

 

[magicmary]

In testing the center pin of the Larsen, it broke off. Swapped it for the Diamond and re-tested VSWR with the VNA--still in the 2.2 range. To shed some additional light on the install, I have three runs of RG8X, one from each NMO mount, bundled with a run of RG-58 which is coming from a WeBoost Drive Reach Fleet cellular booster. The cellular booster is receive only on 700, 850, 1900, and 2100 MHz--this antenna is 10 inches from the center mounted Breedlove NMO mount. I don't think this would be a cause of my problem as the Breedlove NMO mount is connected to the 2 m/70 cm transceiver. I have two additional Breedlove NMO mounts--one to the left and one to the right of center--mounted at 12 inches apart. The left and right NMO mounts are currently not in use. Is it possible that the WeBoost is contributing to some form of interference?

 

[cavmedic]

Get a nmo to uhf adapter. Put that on the antenna mount and put a dummy load into that. Good diagnostic tool to test your harness and mount.



Another option is to remove the Breedlove , and put in a standard laird NMO kit , and retest results. The laird is half the price of the Breedlove and half the work messing around with the ring terminals. The losses are minimal between feed lines.

 

[John_S]

It won't be unusual to find continuity between ground and center at the coax connector on the radio. So if the coax is connected, this would show at the antenna end of the coax. You should check the quality of the ground connection between the mount and the body of the vehicle. Where the mount "bites" into the sheet metal, the metal should be bare and shiny. Another thing to look at is to widen the frequency range of the VNA to see if the antenna achieves resonance anywhere close to where you expect it. Oops...ok, I see you checked the chassis to mount. I don't think there's anything your doing that's wrong with respect to the ring terminals, as long as they're crimped and soldered and fasteners are tight. Is there some way to maybe borrow a basic mag mount just to test with?

 

[prcguy]

How long is the center conductor and braid where they split and connect to the screw terminals on the mount? I’m talking unshielded center conductor to the ring terminal center. I would make them 1/4” long each if possible and I think the Breedlove mount is intended more for CB use and not VHF/UHF due to the screw terminal connections.

 

[AK9R]

I think the Breedlove mount is intended more for CB use and not VHF/UHF due to the screw terminal connections.

Given the market for most Breedlove products, I would agree that this particular mount is intended for CB or HF radio use. I would not think of using this mount for VHF/UHF.

For a VHF/UHF NMO mount, I'd be more inclined to use a Larsen or Laird mount. The Breedlove mount is $41, doesn't include the coax so you have to do all the work of installing the coax. A Laird mount like this is $24, includes the coax installed on the mount with a pre-installed PL259 connector.

 

[magicmary]

I'll try rerouting some of the cables this weekend--perhaps there is some unexpected interference emanating from the bundle. If that doesn't work, I'll swap one of the mounts to a "traditional" NMO mount. Hopefully one of those two attempts will resolve the issue. Thanks for the help thus far, everyone.

 

[merlin]

The only input I can add is coax length has no effect on SWR at all. Presuming the coax is good.
Loss increases at a given frequency as length increases.

 

[magicmary]

I took measurements on all my mounts with all of my available antennas. All three Breedlove mounts with a Comet B-10 2/70 antenna on 70cm are sub-1.3 VSWR. I re-soldered two of the PL-259 connectors last night, so maybe that fixed any issue I had. Waiting on a Larsen 2/70 coil to come in to replace the one that broke. Once it arrives, I'll test 2m all three mounts and report back. It's possible that the Larsen 2/70 was already damaged during the first set of measurements as it was an older antenna; it's just that bending the tab finally did it in.

 

[magicmary]

Final update on this thread. The antennas I was using were too short for the frequency range I wanted. For example, the Larsen 2/70 at stock length was too short on 2m; I solved that problem by adding a spring and trimming the whip. I also have a 2/70SH for in-town use that is perfect at factory length. The Comet 2/70 antennas are perfect for 70cm and I have a Larsen NMO 27 that I've added a spring to and trimmed for CB operations.

Thanks for all of the suggestions and help. First time I've had to tune antennas to my setup, so that didn't immediately come to mind. For reference, I found this guide from Firestik exceptionally helpful.

 

[John_S]

Chalk it up to experience. If you'll be experimenting with more antennas, you might take a look at getting a nanoVNA to help you. Having info on a graphic display is excellent, although there is a bit of a learning curve to get going. But once past that, it's very helpful.