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Author Topic: High SWR  (Read 4769 times)

Posts: 237

« on: September 20, 2012, 06:45:09 AM »

I have a Cushcraft A3S with the A-743 40 meter add on.  The SWR has been good all around with the highest being 2.6:1 @ 7.250 MHz.  Two days ago, we had a day of solid rain, heavy at times.  That evening after the rain had finally stopped, I got on the air.  The receive seemed good as usual but I wasn't getting any replies to my CQ's.  I decided to check the SWR.

On 20 meters and 40 meters, it was up in the 20:1 or higher range...almost at the 'set' mark.  On 15 meters, it was around 3:1 and on 10 meters, it was normal.  My understanding is that 10 meters won't be affected by the 15 meter or 20 meter traps...that makes sense and seems to follow suit.  Prior to all the rain, 15 meters was around 1.2:1.

The antenna is mounted on my roof so I was able to look at it from the ground and confirmed that the 'drainage holes' were still facing down.  My logic says that if the 15 meter trap had water in it, the SWR would be extremely high like 20 and 40 meters...not at 3:1.  If there was water in the 20 meter trap, would it not affect the 40 meter SWR as well?  That also makes sense and follows suit.

Another thought I had was that possibly water got into the coax (LMR400) at the connector.  The coax terminates with a PL-259 and is screwed into an in-line SO-239 'barrel' type connector.  On the other end of the connector is a small piece of coax that has a PL-259 on it.  It is screwed into the connector and the other end has been stripped for the 2 wire connection to the antenna.  All of this is then covered with 'butyl' tape.  There has not been a problem for the last 2 years.  Logic tells me that if there was water in the coax, the SWR would be affected on all bands.  But that is not the case.

Any suggestions as to what to do to fix it?  What to look for? The only piece of equipment I have is a multimeter.

Thanks much...

Posts: 1054

« Reply #1 on: September 20, 2012, 07:51:02 AM »

My initial thoughts are that the coax is OK, and that the 10 and 15-meter traps are OK, too. So I might check the 20-meter trap and 40-meter add-on. Here's why; I've seen a lot of baby spiders get into tight places and grow into grown-up spiders, weaving webs that can get water soaked from condensation and rain, and sometimes even plugging drain holes. My first inclination is to check the 20-meter trap.

What I think may have happened is that the end cap failed and allowed water into the antenna. You said the rain was heavy at times, and end caps do go bad with lots of UV from the sun and from any hail that you may have had over the past several years. It may look fine from the ground, but it could have holes or cracks in it that you can't see.

Another possibility is that corrosion has slowly set in somewhere around the 20-meter trap. The resulting aluminum oxide crud has effectively removed the trap and/or top element from electrical continuity with the rest of the antenna. I'd check that, too.

You'll probably have to take the antenna down and dismantle it. If you can, pop the end caps off the traps and check that everything is alright inside--chances are you'll have to clean and seal each one. Check the caps as well--they could be cracked and/or about to disintegrate. You can order replacements from the manufacturer, which I would probably do anyway. If you do have to disassemble the antenna, clearly mark which element is which, and which end is toward the fed end (or outer end), so when you put it back together you get it right.

You will probably need to check for oxidation corrosion at each joint as well, and might have to scrape each down to base metal again. If that's the case, use a ScotchBrite pad or something like it, and something like NoAlOx compound at each joint when you reassemble the antenna.

It's doubtful that you will need to replace the coax, but it wouldn't hurt to do it anyway. I would check it with a 50Ω dummy load at the antenna end to insure that the SWR is close to 1:1. If it is, the coax is still good,  but if you're going to make the antenna work as if it were a new antenna, then you might as well make the coax new as well.

Good luck troubleshooting it. Let us know what you find--your feedback will help others in a similar situation. GL

Posts: 21764

« Reply #2 on: September 20, 2012, 08:43:13 AM »

If the SWR is really >20:1 in the shack, which means it would be even higher at the antenna, I kind of suspect something became disconnected on one or both sides of the driven element at the 20m trap location.   That could be inside a 20m trap, or even a bad tubing connection (loose clamp).

Rain can certainly accelerate a pending problem.

Posts: 17476

« Reply #3 on: September 20, 2012, 08:53:30 AM »

When coax gets waterlogged the SWR is BETTER, not worse.  That's because the
loss increases.  If you are seeing a high SWR on some bands, that isn't the problem.
(But if the SWR never gets over 3 : 1 or 5 : 1, even on bands where the antenna isn't
supposed to be resonant, that is when you suspect lossy coax.)

I recently pulled apart my TA-33 traps and was surprised at the amount of mud, dust,
spider webs, and other debris collected inside.  The traps should show DC continuity,
which made it quick to find the one with a broken wire (or poor contacts due to
corrosion) with an ohmmeter.  It's still not a bad idea to check the rest of them
as well, cleaning the joints and giving them a new coat of OxGard and removing
debris. while you have it down.

In my traps the turns of the coils are quite close together, and anything that gets
between the turns can change the self capacitance even if it doesn't cause a short.

Posts: 6741

« Reply #4 on: September 20, 2012, 09:44:10 AM »

GUY:  I mean no offense..... but I don't understand this "wondering" what went wrong!  The bottom line is, the antenna or coax is "broke."  The only logical thing to do is fix it.

If you want to be a purist for procedure, take your dummy antenna up on the roof and connect the end of the coax to it and then check the SWR.

If it's OK, then tear the antenna down and take it some place where you can comfortably work on it and field strip it like a rifle.  Whatever is wrong will be obvious..... in 99% of the cases.

Oh, I suggest marking everything where they come apart and save yourself a lot of grief when reassembling it.  That would include minimizing the need to re-tune it.

A Pessimist is Never Disappointed!

Posts: 7718

« Reply #5 on: September 20, 2012, 09:49:40 AM »

The evidence presented says it is not the coax.

This version does have 20 meter traps on the driven element to perform isolation and loading of the 40 meter element tips.

The problem area is water in one or more 15 meter traps and one or more 20 meter traps. The 15 meter traps affected could be on the driven element or a parasitic element.

Time should solve the problem. A few hot days (weeks) will dry out the traps. Or do what an impatient person such as myself would do and load it with an antenna tuner, key the rig, and cook out the water.
« Last Edit: September 20, 2012, 10:02:30 AM by WX7G » Logged

Posts: 237

« Reply #6 on: September 20, 2012, 01:07:35 PM »

Thank you all.  Good suggestions/advice.  I was hoping to avoid having to bring the antenna down...but alas!  This weekend's forecast is for 2 days of rain so I guess it will have to wait.  Hopefully not too long cuz winter feels like it is coming early this year!

I will try the dummy load at the antenna first, just to confirm there isn't some really weird thing going on with the coax (I don't think so tho).  If all looks good, then the antenna will have to come down.

I have a Mosley TA-33 in the garage so I will end up having to put that up.  The only reason it isn't up now is because I can't find a TK-40-KR 40 meter add-on kit for a good price.  And with the long winter evenings coming, 40 meters is good to have.

Thanks again...

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