Sudden SWR Increase

Status
Not open for further replies.

Harlock

Member
Joined
Aug 2, 2007
Messages
89
I have a dual band 2m / 70cm j-pole antenna on my house that normally has terrific SWR from 5w - 65w.

This past weekend, after a little snow storm, I noticed that the SWR was really high. In fact, the impedance was all over the map too. The antenna was not covered in snow or ice at the time of the readings, and I got two different readings (both terrible) depending on whether I had the antenna connected to the SWR meter via the SO-239 and with a BNC connector.

I refrained from transmitting once I saw the SWR flash on my 857d, but would periodically check it through the course of about an hour. It seemed to come down a little, but not much, so I shut off the rig for the evening.

I'm hoping that this is just a case of a little bit of moisture near the top of the feedpoint that will evaporate, but wanted to see what you all thought too.

There was a good bit of snow on the porch roof below the antenna, but this was several feet down & I don't think that would affect the impedance too much or at all.
 

Rred

Member
Joined
Nov 21, 2014
Messages
830
Moisture in the cable, or moisture becoming ice in the cable? Which is how old?

Certainly might be worth eyeballing the connections to see if moisture might have gotten it, and meanwhile checking SWR and operating cautiously, as you are doing.
 

sloop

Member
Joined
Jul 19, 2008
Messages
249
Location
Millers Creek, NC
You are probably right. Moisture in the connector or in the coax itself. Snow/ice can work its way into places that rain wont. Let it dry the check again. In any case I would check the connector and re-seal the connection between the coax and the antenna.
 

KC4RAF

Member
Joined
Sep 30, 2006
Messages
1,599
Location
Davenport,Fl.- home to me and the gators and the s
As already posted about the moisture possibly inside the cable. Also check to see if the feed points have migrated from their original spot.
A few months back, I noticed that my vswr was on the high side. Checked the hose clamps and found the center feed had slid down about 3/4 inch. Rechecked and all was well.
hth
 

Harlock

Member
Joined
Aug 2, 2007
Messages
89
Thanks for the feedback, gang. The whole installation is less than 2 months old, so I'm a little bummed to have an issue like this already. Will check the SWR again tonight and see how it looks. Unfortunately, the feedpoint is not terribly accessible without a ladder, and getting on the roof, which I'm not really able to do right now (recovering from knee surgery).
 

Harlock

Member
Joined
Aug 2, 2007
Messages
89
The feedpoint was taped, but not weather sealed, so I'll have to remedy that at some point when I can get up there.

As a follow-up to this, the SWR readings last night were back to normal, so it looks like the snow/ice was the culprit.
 

KE5MC

Member
Premium Subscriber
Joined
Dec 19, 2002
Messages
1,074
Location
Lewisville, TX
Was there a wide swing in temperature between good-->bad-->good VSWR? I'm not so sure that once water gets in, its out quickly. A temperature sensitive connection can be a quick in and out depending on temperature swing.

Mike
 

mmckenna

I ♥ Ø
Joined
Jul 27, 2005
Messages
13,855
Location
SNCZCA01DS0
The feedpoint was taped, but not weather sealed, so I'll have to remedy that at some point when I can get up there.

As a follow-up to this, the SWR readings last night were back to normal, so it looks like the snow/ice was the culprit.
Many years ago there was a VHF amateur repeater in my area that was having weird issues like this. After a lot of "experts" hacked away at it, replaced coax, etc. someone finally climbed up the tower to actually look at the antenna.
What they found was that there was water collecting at the base of the antenna on the outside. The design of the antenna base made a nice little reservoir for morning dew, rain, etc. to collect. It would bridge across between the grounded base and the radiating element. The issue would come and go depending on temperature, humidity, time of day, wind, etc. Drove those guys nuts for a while.

But I agree, usually wet coax won't dry out that fast. Once it gets wet, it tends to stay wet for a while, then the corrosion sets in and you'll have no end of issues. Since it seems to have cleared, you may be OK, but I'd still encourage you to go ahead and weather proof the snot out of it.
Does sound like something simple like a bad connection.
 

k6cpo

Member
Joined
Dec 30, 2013
Messages
872
Location
San Diego, CA
The feedpoint was taped, but not weather sealed, so I'll have to remedy that at some point when I can get up there.

As a follow-up to this, the SWR readings last night were back to normal, so it looks like the snow/ice was the culprit.
Taped with regular electrical tape? That's not a good barrier to water or snow.

There are products available made specifically for sealing coax connectors against weather.

"Coax-Seal" comes in tape form but is more of a putty. It's very effective in sealing out moisture but can be a bugger to remove if and when the connections need to be broken. Coax-Seal - Products

Personally, I prefer a product called "Coax Wrap." This is also in a tape form, but sticks only to itself. It seals effectively against the weather (I've had no issues with moisture getting into any of my connections) but is easy to remove with a sharp knife. CoaxWrap - Non-Adhesive Silicone Tape
 

SCPD

QRT
Joined
Feb 24, 2001
Messages
0
Location
Virginia
I second MMckenna's suggestion---> look beyond the feed line.... if moisture is getting into the line, it usually doesn't clear up overnight-- I'd suspect something is amiss with the antenna.
.
I know-- this is going to involve some climbing- but if you are going to go up there to inspect the coax/connectors, I'd take down the antenna and give it a good going over too..... :)
.
...................CF
.
..........................CF
 
Status
Not open for further replies.
Top