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Author Topic: Random SWR spikes  (Read 2280 times)
N8CNJ
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« on: March 31, 2011, 02:37:36 PM »

Hi,

I have an Icom 706mkiig installed in my 2009 Freightliner Cascadia.  I have a Jetstream 40 meter "stick" antenna mounted to a bar screwed into one of the mirror bolt holes.  I have tested the ground at one of the other bolts and it proved to be a good ground.

The problem I'm having is that even when sitting still and the antenna is sitting still, I can manually tune the antenna to a 1.7:1 SWR and it will just randomly spike to above 4:1 SWR with no change in my surroundings or antenna movement.  Then, just as suddenly as it spiked, it will drop back down to where it was tuned to.

I have replaced the 12' coax with 9' coax (both are RG8 mini).  I replaced the mount.  Insured that the coax connections were tight.  And I have added a 10 gauge stranded ground to the bolt that was tested for ground. 

When the SWR is down, I get good reports.  Even when traveling down the road in all directions.

Any ideas about why my SWR would just spike one minute and be good the next?
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KD8DEY
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« Reply #1 on: March 31, 2011, 03:18:19 PM »

sounds like you replaced everything except the antenna itself.

what are the chances that where the antenna is bonded to the screw in connector, that maybe the antenna wire coiled around the mast connected to it has a bad solder connection hidden under the wrapping and is loosing connection with a breeze when standing still or moving.
« Last Edit: March 31, 2011, 03:19:50 PM by KD8DEY » Logged
N8CNJ
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« Reply #2 on: March 31, 2011, 06:44:21 PM »

That is a VERY good possibility.  I'll be taking my soldering kit with me when I leave in the morning along with some high test fishing line to use as a guy wire if I can fix it.

This antenna flexes A LOT while in motion.
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K1CJS
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« Reply #3 on: March 31, 2011, 07:03:59 PM »

I would also take a close look at the connector at the back of the radio.  Sometimes a bad solder joint can develop, either at the connector itself or at the board end of the connection.  A little heat because of transmitting and the connection can open--and as it cools, it closes again.

I'm not saying that that is the problem, I just suggest this as another place to look if the antenna seems to be OK.
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N8CNJ
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« Reply #4 on: April 01, 2011, 04:03:45 AM »

I would also take a close look at the connector at the back of the radio.  Sometimes a bad solder joint can develop, either at the connector itself or at the board end of the connection.  A little heat because of transmitting and the connection can open--and as it cools, it closes again.

I'm not saying that that is the problem, I just suggest this as another place to look if the antenna seems to be OK.

Actually, thanks for reminding me to check that.  I have a 10 meter antenna on the truck also and have just reconnected it to test.  No problems, so it is probably just that cheap Jetstream antenna.
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AC4RD
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« Reply #5 on: April 01, 2011, 04:25:36 AM »

K1CJS has a good point there--I was having a similar problem with my IC-706 mobile, and found the HF antenna plug was worn and the center connection wiggled a great deal.  Replacing the plug (which is NOT easy for people with short fat fingers!) fixed the problem.  :-)  I've also had SWR spikes from bad bonding to the car body, but it sounds like you've looked at that.   Good luck finding the problem!
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KE3WD
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« Reply #6 on: April 01, 2011, 10:39:45 AM »


Actually, thanks for reminding me to check that.  I have a 10 meter antenna on the truck also and have just reconnected it to test.  No problems, so it is probably just that cheap Jetstream antenna.

Be advised that the test does not rule out the SO-239 at the rig yet. 

Could be a situation where the center pin for your 10-meter setup may be a wee bit larger in diameter than the one for the other antenna, making a better connection at that point. 

Should be easy enough to examine both plugs side by side, looking for wear or a difference in the pin diameters.  Also eyeball the socket side on the radio, paying particular attention to the center fingers and also to look for the possibility of a bit of corrosion there, easily cleaned with a wee bit of Deoxit spray and then inserting the connector a few times while still wet to get some mechanical wiping action in play.


GL es 73
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K1CJS
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« Reply #7 on: April 01, 2011, 08:07:58 PM »

Thanks for pointing out what I forgot to--the 'spring' fingers in the center contact of the SO 239.  73!
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N3PM
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« Reply #8 on: April 02, 2011, 06:22:38 PM »

I see a similiarity between the Ham Sticks and the Jetstream antennas. Almost all of our Hamsticks opened up the connection at the base fitting. The solder didn't stick to the plating well.
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