SWR Problem

[ctvanover]

Don't know if this goes here or not, but here goes. I got a 5k magnet mount, like the one used for an Wilson 5000, with a RM Z-180 screwed on to the base. With the antenna in the center of my extended cab Silverado and the ant. coax stretch out, I used my MFJ-259B SWR Analyzer, tuned it to 27.185 and the meter said that I had a 1.5 reading. After I ran the coax, keeping it nice and neat and tucked out of the way, I put my meter back on it and it said I had a 1.6 reading. So I hooked up my Magnum S9, plugged it into the cig. lighter, turned it on and the swr light started blinking the moment that I keyed down the mic. OK, so what did I do wrong and how do I fix the problem? Would it be where I used the cig. lighter and not getting a good ground?

 

[W5LZ]

ctvanover,
The first thing I wonder about is what were the 'R' and 'X' readings on the '259 when you got that 1.5:1/1.6:1 SWR readings? Also, since you said "after you ran the coax" the radio wasn't all that impressed with the antenna. Do I take it that you had not run the coax before, when you did the test with the '259? That really will/can make a difference.
- 'Doc

 

[ctvanover]

1st of all I put the ant. in the center on top of my extended cab. I then stretched out the coax, making sure that there were no coils or kinks in the line, and hooked it up to the '259 and tuned it to 27.185 and got a 1.5 reading. Then very carefully I ran the coax over to the drivers side back corner, ran the coax down along the weatherstripping, tucking it in nice a neat, down along the bottom of the door under the plastic kick plate, up to the bottom of the dash and across to where I have the radio. I then hooked the '259 back up and got a 1.6 reading. Screwed it into the back of my Magnum S9, plugged in the power cord, plugged the power cord into the cig. lighter, turn on the S9, and just after I keyed down the RK-56 the swr light started blinking. After looking closer at the 5k mag mount I noticed some kind of clear rubbery looking silicone type stuff on the bottom, I guess who ever had it before me was trying to protect their paint, would this cause a bad grounding problem for the ant.? And another thing is the mount that I'm temporarily using is one of those saw tooth mounts, like the in the pic below, would it be causing a bad ground for the radio since it's plugged into the cig. lighter?

 

[Robb]

I have a 1.3 match with my Wilson 1000 mag mount and a Galaxy 99V plugged into my cigarette lighter outlet. I use the sawtooth mount and have experienced no ill effects whatsoever.

Describe more about the silicone adhesive on the bottom of the mag mount. First of all it shouldn't be there. Reason is, that mag mounts use a 'capacitive ground' to the sheet metal on the vehicle. You wouldn't want ANYTHING to get in the way of the capacitive surface that the bottom of the magnet supplies to the sheet metal. The antennas come with a special thin-film mylar on the base of the mag mount. Nothing else should be present!

Just keep a nice coat of wax on the vehicle where the mag mount goes. Also, be sure that there are no metal particles on the bottom of that mount. The magnet itself will naturally pick up shards and scraps of very small metal particles before it is put on the vehicle. You must wipe the mag mount off very carefully before puttting it on the roof - first! Or else it may marr the sheet metal surface of you vehicle, as it will scratch the surface of your paint - and won't allow the mag mount to be flush with the surface of the sheet metal.

Make those changes first, and then re-check and adjust your SWR.
I would...

 

[ctvanover]

It looks like some kinda of clear silicone. It doesn't affect the pulling of the magnet what so ever. I'm going to re-check the swr tomorrow. I may have to turn the switch off on the back of the radio for the swr light.

 

[ctvanover]

ctvanover,
The first thing I wonder about is what were the 'R' and 'X' readings on the '259 when you got that 1.5:1/1.6:1 SWR readings? Also, since you said "after you ran the coax" the radio wasn't all that impressed with the antenna. Do I take it that you had not run the coax before, when you did the test with the '259? That really will/can make a difference.
- 'Doc

Doc,

This is with the coax ran and ready to hook up to the radio. So I hooked up my MFJ-259B again and this is what I got.

On ch. 1 (26.965) the R=33 and the X=14 with a 1.6 swr.
On ch. 19 (27.185) the R=36 and the X=14 with a 1.5 swr.
On ch. 40 (27.405) the R=39 and the X=15 with a 1.5 swr.

So if the antenna and coax checks out then it must be on the radio end, right?

 

[Robb]

It looks like some kinda of clear silicone. It doesn't affect the pulling of the magnet what so ever. I'm going to re-check the swr tomorrow. I may have to turn the switch off on the back of the radio for the swr light.

I'll bet dimes to doughnuts that the silicone is the problem. It shouldn't be there. It is wreaking havoc with the capacitive ground - thereby screwing your SWR!

 

[W5LZ]

ctvanover,
That 'X = 13 ~ 14' is the problem, the antenna isn't really near resonance at the frequencies you check it on. Get that '259' out again and run up/down to see where that 'X' is zero, or very close to it. That's where the antenna is resonant. If it isn't too far from 27.200 Mhz you might be able to re-tune it by the whip length. If it's really out in left field somewhere, then sorry 'bout that. That silicon may certainly have some effect, no idea how much.
Good luck.
- 'Doc

 

[mackmobile43]

It looks like some kinda of clear silicone. It doesn't affect the pulling of the magnet what so ever. I'm going to re-check the swr tomorrow. I may have to turn the switch off on the back of the radio for the swr light.

It shouldn't affect it unless its on there really thick, they come with a mylar film over the magnet so by moving the magnet away from it's counterpoise would it affect swr.

 

[ctvanover]

It shouldn't affect it unless its on there really thick, they come with a mylar film over the magnet so by moving the magnet away from it's counterpoise would it affect swr.

It was only around the edges, I guess it was put there to help keep out moisture?

 

[W5LZ]

You know, that silicon being as you described, it just dawned on me that it may be there because the antenna was taken apart for some reason, then 'glued'/siliconed back together maybe? That's just one'a them S.W.A.G. thingys, so don't put a lot of effort into polishing that idea up, sort of. You think?
- 'Doc

 

[ctvanover]

ctvanover,
That 'X = 13 ~ 14' is the problem, the antenna isn't really near resonance at the frequencies you check it on.
Good luck.
- 'Doc

Just how do you adjust it to get the "X" to where you want it?