WTH did I DO???

[WRYT685]

So, I'm setting up my in cab rig for my offroader, MXT275, MXTA26, hood mount for the whip.  It SWRs out to 2.96.  Pulled the whip up as far as I can, 2.46.  Okay, can't add more metal, so did some digging, seems a lot of folks trim it down quite a bit to bring it in...trimmed off a smidgen, 2.35, okay, right direction.  A little bit more...WHOA NELLY!  10.7!!!!  Put my stubby on, which had measured at 1.0 earlier, and it was 10.6...did I cook my coax with a measly 15 watts?

[WRQC527]

I would suggest a couple of things. First, make sure your coax is intact with no kinks, breaks, pinches, etc. Also use a digital multimeter to make sure of three things. That there is not a short between the center conductor and the outer shield, that there is continuity between both ends of the center conductor, and continuity from the shield from end to end. Make sure your antenna mount is grounded as well. All this can be checked with a $5 digital multimeter. Many high-swr problems are due to shorts and bad grounding. It's highly unlikely you damaged the coax with 15 watts. No doubt others will chime in with ideas as well. 

[Sshannon]

11 hours ago, WRYT685 said:

So, I'm setting up my in cab rig for my offroader, MXT275, MXTA26, hood mount for the whip.  It SWRs out to 2.96.  Pulled the whip up as far as I can, 2.46.  Okay, can't add more metal, so did some digging, seems a lot of folks trim it down quite a bit to bring it in...trimmed off a smidgen, 2.35, okay, right direction.  A little bit more...WHOA NELLY!  10.7!!!!  Put my stubby on, which had measured at 1.0 earlier, and it was 10.6...did I cook my coax with a measly 15 watts?

I’ve never heard of coax being cooked at low wattage. I don’t think that’s what happened. But it certainly might have gotten damaged in a way that allows a short or open.

Try another piece of coax.

 

 

[marcspaz]

An MXTA26 doesn't require cutting. It's a tuned antenna. If the SWR was high to start, either the cable was/is bad or you need a better mounting location.  Chances are the antenna is damaged and no longer usable without risk of damaging the radio.

 

I would trace the coax and look for signs of crimping or damage. If there isn't any, the NMO mount may have a partial short or broken wire in the base of the mount. Usually due to over-spinning the connector during install of the mount ot antenna. I had to toss a few before I learned my lesson. 

 

You may need to buy a new coax and antenna. 

[WRYT685]

More data:  pulled antenna and checked for shorts at both ends, because OCD.  No shorts.  Installed a jumper between center pin and outer connector, got good continuity at the NMO pad end, switched ends, same-same.  Resistance measured out 0.9 on 200 Ohm setting.

 

For giggles, I broke out the teeny mag mount antenna and put it on the meter: 1.0 sitting next to the old mount on my hood.

 

*throws papers in the air*

[Sshannon]

Did you test the mount or just the coax?

 
You may need to put an antenna analyzer on the antenna that gives you the high SWR and see where you are on the SWR curve. Without knowing where you are on the curve it’s more difficult to know how to tune an antenna and Marc’s right, the MXTA26 doesn’t need tuning. By shortening the whip, you might have just  increased the SWR.  

[WRQC527]

10 minutes ago, WRYT685 said:

Installed a jumper between center pin and outer connector, got good continuity at the NMO pad end, switched ends, same-same. 

Not sure why "good continuity" is a good thing between the center pin and the outer shell of the connector. Scrap the coax. Start over. Don't cut the antenna. Hopefully you haven’t cut to much of it already.

[Sshannon]

Just now, WRQC527 said:

Not sure why "good continuity" is a good thing between the center pin and the outer shell of the connector. Scrap the coax. Start over. Don't cut the antenna. Hopefully you haven’t cut to much of it already.

No, he was testing both the shield and the center conductor at once by putting a jumper at one end and the ohmmeter at the other end. 

[WRQC527]

24 minutes ago, Sshannon said:

No, he was testing both the shield and the center conductor at once by putting a jumper at one end and the ohmmeter at the other end. 

I suppose that's one way to interpret what he said. I'll wait until he clarifies.

[marcspaz]

7 minutes ago, WRQC527 said:

I suppose that's one way to interpret what he said. I'll wait until he clarifies.

 

It's a continuity test.  It simultaneously confirms the shield and the center conductor do not have an open.  If either were open, there would be a high resistive value.

[WRYT685]

2 hours ago, WRQC527 said:

Not sure why "good continuity" is a good thing between the center pin and the outer shell of the connector. Scrap the coax. Start over. Don't cut the antenna. Hopefully you haven’t cut to much of it already.

I was loop testing to make sure both the center conductor and the braided shielding weren't broken.  It's a common technique in Aviation to jumper a pair of wires/conductors to verify continuity.

[wayoverthere]

Loop testing is great to test no breaks in either conductor in one shot, but it's not a great check to make sure the center and braid aren't shorting together.

Did OP by chance shorten the cable and crimp on a new connector?

[WRYT685]

2 hours ago, wayoverthere said:

Loop testing is great to test no breaks in either conductor in one shot, but it's not a great check to make sure the center and braid aren't shorting together.

Did OP by chance shorten the cable and crimp on a new connector?

Checking for shorts is the first thing I did.  Yes, I shortened the cable and crimped on a new one.

[WRQC527]

13 minutes ago, WRYT685 said:

Checking for shorts is the first thing I did.  Yes, I shortened the cable and crimped on a new one.

The more I hear, the more it sounds like your coax is somehow jacked up and your antenna is now possibly too short. Antennas that are slightly too short or too long, (which is unlikely with an out-of-the-box MXTA26), shouldn't generate a 10:1 SWR reading. Bad coax, bad grounds, even one tiny wire from the braid or center conductor shorting out at the connector will jack up your SWR that much. If you can take the coax completely out so it is not connected to anything, there should be no continuity between the center pin and the outer shell of the connector on either end of the coax. It should be an open circuit.

[WRYT685]

On 9/24/2023 at 8:37 PM, WRQC527 said:

The more I hear, the more it sounds like your coax is somehow jacked up and your antenna is now possibly too short. Antennas that are slightly too short or too long, (which is unlikely with an out-of-the-box MXTA26), shouldn't generate a 10:1 SWR reading. Bad coax, bad grounds, even one tiny wire from the braid or center conductor shorting out at the connector will jack up your SWR that much. If you can take the coax completely out so it is not connected to anything, there should be no continuity between the center pin and the outer shell of the connector on either end of the coax. It should be an open circuit.

You're probably right.  Just for the Halibut, I clipped off the crimp connector and used a solder connector...SWR got better by half, but were still 5+, so that coax is getting round filed this weekend.  I have a new Midland NMO mount and coax that I'll be putting on, along with a new antenna.

Now, for the eternal debate...should I or shouldn't I shorten the coax vs. looping the excess in the engine bay?  (not enough room behind the dash/console for a 12" loop)

*fight's on*

[WRQC527]

8 minutes ago, WRYT685 said:

looping the excess

Looping excess coax may lead to problems. Coiled coax acts as a choke, which is good in some situations if the coil is a specific diameter and number of turns, but it's probably better to have little or no excess. I would suggest getting as close to the length you need and don't try to shorten it or cut off factory-installed connectors. There's nothing wrong with doing that but it opens up opportunities for things to go wrong, like short or open circuits.

[marcspaz]

1 hour ago, WRQC527 said:

Looping excess coax may lead to problems. Coiled coax acts as a choke

 

This isn't a problem in the 400-500 MHz range. Both Randy and I have shared test results for this. Unless you plan on winding the coax to less than an inch in diameter while using some super cheap (unshielded) coax, it won't matter. 

Now, when you start getting down to 50 MHz to about 21 MHz, 8 inch to 25 inch coiled coax just spooled up and tossed aside becomes an issue with SWR.  That's why when we do CB installations, we use either a 9 foot or 18 foot cable and just snake the extra cable around the interior, trying to avoid half or full loops in the cable. The lengths are balanced 1/4 or 1/2 wave.

[Lscott]

5 hours ago, marcspaz said:

 

This isn't a problem in the 400-500 MHz range. Both Randy and I have shared test results for this. Unless you plan on winding the coax to less than an inch in diameter while using some super cheap (unshielded) coax, it won't matter. 

Now, when you start getting down to 50 MHz to about 21 MHz, 8 inch to 25 inch coiled coax just spooled up and tossed aside becomes an issue with SWR.  That's why when we do CB installations, we use either a 9 foot or 18 foot cable and just snake the extra cable around the interior, trying to avoid half or full loops in the cable. The lengths are balanced 1/4 or 1/2 wave.

Magnet mounts on 6M and HF don’t couple very well to the body of the vehicle. The cable ends up being part of the ground plane. On a 1/4 wave 6M antenna I ended up using a ferrite toroid near the magnet mount base to decouple the antenna. Otherwise moving the coax around resulted in significant SWR changes. 
 

I also paid a few bucks, at a flea market, for some magnet mount CB antennas, Little Wilson, with the idea of cutting them down for 10M. Finally gave up since the bandwidth was very narrow and the SWR jumped around every time I moved the coax a bit while adjusting the antenna.

[WRYT685]

12 hours ago, marcspaz said:

 

This isn't a problem in the 400-500 MHz range. Both Randy and I have shared test results for this. Unless you plan on winding the coax to less than an inch in diameter while using some super cheap (unshielded) coax, it won't matter. 

Now, when you start getting down to 50 MHz to about 21 MHz, 8 inch to 25 inch coiled coax just spooled up and tossed aside becomes an issue with SWR.  That's why when we do CB installations, we use either a 9 foot or 18 foot cable and just snake the extra cable around the interior, trying to avoid half or full loops in the cable. The lengths are balanced 1/4 or 1/2 wave.

Not planning on doing any super tight winds; I just spent two hours writing up a Quality Assurance report to Engineering on a landing gear harness with excessive bend radius.  As anal as we have to get with the fiddly bits on jets, I see no reason to not take the same precautions on my rig...and we run loads of coax in stacked runs.

[WRYT685]

UPDATE:

One new coax later, with the excess coiled by the air cleaner, I'm getting 1:1.02, mounted on the roof rack.

[WRQC527]

55 minutes ago, WRYT685 said:

UPDATE:

One new coax later, with the excess coiled by the air cleaner, I'm getting 1:1.02, mounted on the roof rack.

If you haven't done so already, and if you haven't already thrown it away, cut the old coax into a bunch of useless pieces so you will never be tempted to "save it for some other project down the line" and five years later forget why you replaced it in the first place.

[WRYT685]

17 minutes ago, WRQC527 said:

If you haven't done so already, and if you haven't already thrown it away, cut the old coax into a bunch of useless pieces so you will never be tempted to "save it for some other project down the line" and five years later forget why you replaced it in the first place.

It's already round filed.