Mobile SWR High in rain

[E412B]

My SWR goes through the roof in the rain. Let me tell the whole story.

My Connex CX3300HP bit the dust, sending it off to get repaired. In the meantime I was going to use an old uniden 510 and a Texas Star MC250.

Yesterday
So I made a dummy load (50 ohm and 8 watt) and tested the Uniden had 3 watt dead key don't remember the swing but it wasn't much. So I know it's not going to fry the Texas Star. Meter (MFJ-972) showed 1.05:1 on channel 1 and 1.1:1 on channel 40. Took dummy load to antenna feed point and meter showed 1.1:1 on channel 1 and 1.2:1 on channel 40.

I removed all the points of contact and cleaned them with a wire wheel on a drill. Then reassembled and added dielectric grease to prevent rust and such. I removed the old ground straps. And did the same thing. Removed paint with wire brush. I didn't have any old coax shield around, so I used #8 copper wire to make a ground straps from ladder rack brace to the side of the van's rain guard.

I connected the antenna, K40, and tuned so I had 1.2:1 on channel 1 and 1.3:1 on channel 40. Made a couple of test calls and couldn't get a response. Just nobody on the air. I then connected the Texas Star. All I had was a 9ft piece of coax so I used it. I now have the Uniden with a 3ft jumper going to the Texas Star. From Texas Star to MFJ meter with a 9ft jumper. From the MFJ to antenna.

Set the Texas Star to low power and SWR was 5:1 on channel 1 and 7:1 on channel 40. Shouldn't have transmitted but I did. Got a few responses back from a county away. Started getting dark and had to work the next day so I picked up tools and went in the house.

Today

It was raining (flash floods watches) I knew I couldn't use the Texas Star with the SWR the way it was. So I took that and the extra 9ft coax jumper out of picture.

That left me with the Uniden connected with a 3ft jumper to meter to rest of antenna. I knew that was OK cause I just set SWR yesterday and it was 1.2:1 on channel 1 and 1.3:1 on channel 40. I knew it.

Well I don't know jack. On channel 1 the SWR was 3:1 and on channel 40 it was 2.5:1. Now my antenna was short. I don't have time to mess with it. Got a 8hr drive to get to area where I'm working on Friday.

Drive for about 2 hours rain stops. Drive for another hour, some blue skies. I stop for gas and check radio with dummy load at back of meter. Get same test reading as I did yesterday. I connect the antenna and check SWR and have good reading now 1.2:1 on channel 1 and 1.3:1 on channel 40.

Only thing I can figure is the rain and my ground plane are messing with my SWR.

After talking it out with a friend, I think I have to ground plane figured out and a fix for it. The vehicle is a 2018 Chevy Express 2500 van with a ladder rack. I know the ladder rack isn't helping but it is what it is.
When I look at the rain gutter on the van it has sealant where the roof and side panels are welded on. My friend suggested that the roof and panels MAY BE GLUED and not welded. OR the rain gutter may be glued. Don't think so but who knows now a-days. Friend said they glue side panels on truck beds and other stuff.
So I'm going to put a ground straps straight down off antenna mount to the top of roof and seal with silicone. Then replace the ground straps I made on the passenger side and screw straight into that side panel where the doors open.

But that is the only thing I can think of is the that rain gutter is somehow not connected to rest of van. And I not getting a good ground plane because of it. I've got a few pics if I can get them to upload.
If you look close at the pics with the green ground 'strap' you can see the sealant around the rain gutter.



Any thoughts or suggestions?

Thanks in advance for looking.

 

[Big Kahuna]

 

[Big Kahuna]

So a couple of things - changing out coax lengths or removing them OR removing/adding amps can change SWR readings even though the SWR at the antenna isn't actually changing. https://www.ad5gg.com/2017/06/11/coaxial-cable-length-does-not-change-swr/

Next thing - once the other pieces were removed

Your first SWR test - were you moving, were their cars around, buildings, etc.

Second test in rain - were you moving, were their cars around, buildings, etc.

There are so many factors that can cause differences in SWR. If you did your first test while parked next to other cars your SWR might be different than your second test and vice versa.

If you're on the highway and your antenna is bending at an angle your SWR can change, etc.

Now after all that....yes, rain can effect SWR, it's been discussed in many forums CB and Ham. It usually doesn't make systems unusable but there can be changes.

 

[E412B]

Thanks for the reply. I'll read the link you posted a little later. It's kinda late and I should be asleep.

But to answer your questions.

The first test I was parked in corner of yard. Cars and house at least 100ft away. No overhead power lines.

Second test everything was the same. I'm lucky enough to have the room and safe enough neighborhood so I just left van in the same spot.

The only thing that really changed was time of day and the rain.

I even thought that the rain dripping down the stinger into the adjustment mast (?) (Where the set screws are) of the K40 may be doing something weird with the SWR

I've got a 14 hour trip on Monday the 7th down to New Orleans and was wanting to use that Texas Star. I just didn't want to burn it or the radio up.

I've got an older Cobra 25 classic with side mounted mic. Nice little radio I picked up for $5. Meter showing 5watts dead key and 18 swing. Wanted to hook that up to the Texas Star for the trip. But again didn't want to burn anything up.

Should I add the ground straps to the top center of (company) van, put on the Cobra and the Texas Star AND set SWR or just leave antenna as is and put in the new equipment.

Thing is when I had the Texas Star connected and keyed up, the MFJ meter sung backwards instead of forward. That tells me I've got too high SWR with that amp. Right?

Anyway, thanks for the quick reply. I check that link you posted probably Saturday.
I'll figure it out one way or another. If nothing else I'll fix it with my Thor (3lb mini sledge) hammer.

 

[E412B]

So from what I'm getting from the article that Big Kahuna gave me is

1. The extra coax I used did NOT play that much difference in the SWR going up. ( I can test that easy. Add barrel splice and the coax and put dummy load at antenna with meter where it's at.)

2. That adding and transmitting with added POWER will make the SWR go UP if antenna is not tuned properly.

3. That the SWR meter need to be at the antenna feed and not at the radio. Got to get the wife to key mic for me. Might be easier to grow 10ft arms. LOL

4. Putting meter at the antenna feed will always give a truer reading of SWR than meter at radio. Even if the coax is 1000ft long. Due to signal loss when reflected back down the coax from the antenna.

If I read and understood the article wrong let me know. I'm not the sharpest knife in the drawer.

Almost forgot about Milli Vanilli, NOW will have to try and forget them again. Time to kill more brain cells.