Absurd SWR with Wilson 5K

[iceman78]

I have a Wilson 5K Magnamount and got a new car (2009 Nissan Altima) and thought a trunk lip mount would look a lot better so I got a trunk lip mount (originally from a Wilson 1000) from a friend. I changed out the coax to LMR 400, installed the trunk mount and screwed on my 5K base & whip. when checking the swr's it pins the needle and the receive is quiet. Checked it against MFJ and pinned that as well!. Here's what I did do:

-Checked thoroughly coax, solder joints, connector, & Assembly viasually and with continuity meter. Everything checks out as it should.
-Bonded/Grounded the doors, hood, trunk lid, radio, and the set screw on the antenna mount.
-Tried the trunk lip mount antenna on another car (2007 Acura TSX) with same results

Oddly enough when I throw the magnamount on the trunk the swr's drop to around 1.7 Right now I have the magnamount on the roof and the swr is 1.2. I would really like to have this trunk lip mount work properly because it's such an eyesore having the 5K on the roof. A knowledgable friend of mine suggested that perhaps the rear window defroster might be messing with the antenna but it's just too strange how after trying the antenna on another car it still didnt work.. As I said the Wilson 5K works just fine with the magnamount. I would really appreciate any insight anyone may have.

 

[W5LZ]

How are you getting that LMR-400 inside the car? The only trunk mounts I'm familiar with sort of curl the coax around and through the seal around the trunk lid. I would think that doing that with LMR-400 would be quite a trick.

Anything that has current going through it (or not) near an antenna, such as that defroster, can have an affect on the antenna. But I honestly don't see that defroster having -that- much affect. I would have to think that seeing what you are seeing with that meter could also be a sign of an open circuit, just not making connection to the antenna.
It'll be interesting to hear what you find out.
- 'Doc

 

[Moleculo]

Maybe you didn't get the screws on the base of trunk lip mount all the way through the paint to make good contact with the metal?

 

[Robb]

You might be able to do it with LMR240 coax - maybe.
But - even then - you are asking it to make a huge bend that may distort the core and lose its internal integrity/continuity.
BTW-wasn't there a similar thread like this a week ago?!?

I would have to say nix to LMR400 for this job.
But if you need better specs out of your coax for running power, then LMR240 and keeping a mag mount will probably get you where you want to go with it.
Additionally, putting it on the roof will yield a better SWR - IMO.
I run a Wilson 1000 on my new Honda on the roof - no problemmos.
But I do have a Brand-new piece of LMR240 that I could put on it, so thank you for the idea!

 

[Cajun Invader]

How are you getting that LMR-400 inside the car? - 'Doc

My thoughts exactly. That LMR 400 has a bend radius of 1 inch. The solid center conductor can and will break if you are not careful.

The Wilson trunk mount had RG 59U originally. How where you able to get the LMR 400 into the base? Did you have to drill it out or did it fit?

Maybe your ground is not sufficiant. Did you try running a ground strap from the trunk mount to the frame of the car?

Also check your continuity. It should show a closed circuit on a base loaded antenna.

 

[iceman78]

actually we didnt bend the lmr in that hair-pin bend. just stuck it right through the hole (the part where you'd normally make that u-turn bend) and ran the lmr into the trunk.

I checked continuity with the set screws to the chassis and it all checked out and I tried strapping a ground to the set screw itself--no help.

just for a goof i put the original wilson coax back on and the swr went down--to 2.5 which is quite better than pinning the needle. however there wasnt much receive in the radio.

lastly i was thinking could it be that possible that piece of lmr is bad? the solder connections all check out...nothing is shorted, etc. also I am using a wilson 5000; yet the trunk lip mount originally came from a wilson 1000; that shouldnt make a difference right?

 

[W5LZ]

From the sound of it, things seem like they ought to be better than that. I wouldn't think the coax is the problem, and if the mount is halfway right it should be better than what it is, and that leaves the mount and antenna connection. Just for grins, take a good look at how the antenna is mounted on that trunk mount, insulators, gaskets, whatever, etc. Running out of places to check! Check the freon level in the AC, that might be it!
- 'Doc

 

[Robb]

Just for grins, I would put the mag mount back on the roof and see if it all falls into focus...

 

[iceman78]

i have the mag on the roof and the r's are 1.2 problem is that i really wanted the antenna on a trunk lip mount for aesthetic purposes. i tried the mag on the trunk and the r's were 1.7 for some reason the antenna simply doesnt like to be mounted with the trunk lip

 

[mackmobile43]

i have the mag on the roof and the r's are 1.2 problem is that i really wanted the antenna on a trunk lip mount for aesthetic purposes. i tried the mag on the trunk and the r's were 1.7 for some reason the antenna simply doesnt like to be mounted with the trunk lip

Is the trunk lid metal or fiberglass? this is important and may very well answer the burning ? that plagues your swr meter.

 

[hotrod]

i have the mag on the roof and the r's are 1.2 problem is that i really wanted the antenna on a trunk lip mount for aesthetic purposes. i tried the mag on the trunk and the r's were 1.7 for some reason the antenna simply doesnt like to be mounted with the trunk lip

wilson mag mount seem to just work better on
the roof.if ya really hooked on the trunk mount id try the wilson trucker 5k with
a 10inch shaft.i honestly believe the wilson coils need/like to be above the roofline
to achieve a good swr.believe me your not the first to run into trouble
with a trunk mount

 

[loosecannon]

just a thought, but maybe something is happening to the coax when the trunk is closed.

im thinking that when you are doing all your testing, the trunk is open, then when you go to check the SWR the trunk is closed.

that bend that the coax has to make when using a trunk lid mount just plain sucks no matter how you look at it.

using RG58 coax is the best place to start.

remember that your coax is only 50 ohms because of the distance relationship between the center conductor and the shield.

if that coax gets crushed when the trunk is closed, even if it doesnt short out, that distance has changed, thereby changing the impedance of the coax.

todays cars are built with very tight tolerances and especially around the trunk area to help the water go where its supposed to when the car gets wet.

my guess is that the coax is getting crushed against something when the trunk is closed.

maybe try putting some white-out on the coax where it touches the trunk, and while its still wet; close the trunk.
then open the trunk again, you should see some white out on the trunk where the coax touched it.

that will help tell you the story.

no real solutions here, except to drill a hole in the trunk of your brand new car and hard mount the wilson there.

no matter what you do, you DEFINITELY have to add some ground straps from the trunk to the trunk lid.

oh, one more thing. remember that your meter can only tell you that there is a connection between two points, it cant tell you if its a sufficient RF ground.


hope this helps,
LC