Can't get SWR on 102 whip below 2.0:1

[Turbo T]

I just recently installed an aluminum roll bar in the bed of my truck. There are two 6 inch round metal lamps that sit atop the roll bar. In between the lamps sits a 102 whip on a Radioshack 21-1118 barrel spring. This all sits in a gumdrop mount.

The antenna, spring and gumdrop are all brand new.

I am running an 18 ft. piece of RG8X coax into the radio. I know this coax is good, it works great with my base set up.

Pictures of my install are below. The antenna sits above the roof and bed of the truck. The roll bar is bolted to the bed, and the lamps work if only the red wire is attached to a 12VDC source.

I have tried running a ground wire from the round knurled area of the PL259 to a good ground.

I have tried different a different whip. I have tried adjusting both whips. I have removed springs. I have added both springs plus an extension.

I have done my SWR testing with the doors closed, me inside the vehicle and I even had the vehicle in an open area away from trees, poles and buildings.

I even tried placing cloths over both of the lamps thinking maybe they were reflecting some of the signal causing a higher than usual SWR reading.

However no matter what I do, in any of what I have mentioned above, I cannot get the SWR below 2.0:1. And that's across all 40 channels.

So in other words right now with the brand new 102 whip, brand new 21-1118 spring and brand new gum drop mount, I am sitting at a 2.0:1 SWR on channel 1, 20 and 40.

The only other thing I can think of is the roll bar is only maybe 8 feet wide by 4 feet long.....if it's possible that the antenna is using just that little bit of area to reflect the waves off of....which I would have thought the antenna, while sitting maybe 3 inches higher than the roof of the truck, would be using the roof, hood and bed as the ground plane.(my last truck, same make and model, I had this same antenna and spring mounted in bed on bulkhead, SWR was always 1.2-1.4:1)

Any thoughts on this? I really want to get my SWR to 1.5:1 if not lower than that.

Thanks in advance.

 

[loosecannon]

im afraid it is most likely just what you posted.

lack of counterpoise for the whip.

a counterpoise needs to be a horizontal metal panel of some sort.

just because all the metal is connected doesnt mean that it is a good groundplane/counterpoise.

the good news is that a 2:1 SWR is not too terrible that you cant transmit with it.

you still have a big honkin' antenna way up high on your vehicle which should get out pretty well.

i do have one last ditch effort that might be worth a shot, but i cant guarantee that it will work.

you could put a couple of big fat braided copper straps (1" wide or more) going from either side of the bed to the underside of the cab.

remember that truck cabs are on rubber bushings to help stabilize the ride a bit, and these dont make the best ground connection to the frame sometimes.

just an idea.
good luck.
LC

 

[Robb]

Is that truck bed made of high-impact plastic?
Doesn't help you if it is.
Try the ground straps as LC said.

I would take the lights down and see if it doesn't change too.

 

[Turbo T]

Nope, the truck bed is made out of good old U.S. steel. So is the rest of the truck.

As I was always told, for a mobile antenna, you need metal all under the antenna. I figured even if the light bar sits a few inches higher than the cab, technically, there is metal under the antenna. Guess it just needs to be closer?

I'll try the straps.

 

[mackmobile43]

As long as your roll bar has good dc bonding then straps will not help one bit.

I do not like gumdrop mounts as they are notorious for shorting even when new and they will not take the stress of a large antenna or a long whip such as the 102''.

This is your best bet for the dollar in mounts.

 

[W5LZ]

It certainly won't hurt to ground the cab to the frame, maybe even the bed to the frame. If that roll-bar is bolted to the bed then it's grounded. I think I'd also remove that spring. The lights shouldn't make any appreciable difference (whether they work or not).
That 'groundplane' does not have to be flat by any means, that's a misconception. Those droopy radials under some base antennas certainly aren't flat.
And you should definitely make sure that mount is assembled properly, the insulator in the right spot(s). Mounting the thing on that curved roll-bar may have something to do with things, not the best contact in the world maybe?
You should be seeing better than that 2:1, can't say specifically why you aren't. So, check everything again. It could be really something simple and you've just missed it. It happens a lot...
- 'Doc

 

[loosecannon]

That 'groundplane' does not have to be flat by any means, that's a misconception. Those droopy radials under some base antennas certainly aren't flat.

good god Doc, thanks for the antenna lesson. :headbang

because of the lack of cone shaped vehicles on the road these days, i said flat metal panel. try not to confuse the OP just because you see an opportunity to put your 2 cents in. LOL
LC

 

[Se7en]

As long as your roll bar has good dc bonding then straps will not help one bit.

I do not like gumdrop mounts as they are notorious for shorting even when new and they will not take the stress of a large antenna or a long whip such as the 102''.

This is your best bet for the dollar in mounts.

who sells those bolts mack, .. barjan?

 

[15minigrass]

who sells those bolts mack, .. barjan?

Sparky has them!!!

 

[Se7en]

Sparky has them!!!

thanks

 

[Turbo T]

Pretty sure I've seen those mounts in truck stops too. Was not aware gum drop mounts were known to short.

Also the first I've heard a gum drop isn't any good for a 102. As big as they are you'd think they'd be able to handle some abuse.

W5LZ there is no insulator really on a gum drop, just a few star washers to bite into the metal for a ground, and a 1 inch hex nut.

 

[AudioShockwav]

The problem I have run across is pulling the center out of them.
I spend a lot of time up in the mountains and I am very hard on antennas.
The gum drop mount would get loose, I would tighten it up... couple of days go by and it will get loose again, out comes the wrench and tighten again.
After a few time`s the center would pull through and the antenna would wobble all over.

This is what I am using now it is called A
ProCom Extra Heavy Duty Stud Mount, you can find them on E-Bay
procom mount | eBay


Or here

ProComm Super Heavy Duty Stainless Steel Stud Mount with Allen Wrench - jbc930ss
So far so good, no more problems, and a decent price.

73
Jeff

 

[AudioShockwav]

Make sure the bed is bonded well to the frame.
Make sure the cab is bonded well to the frame.
A lot of times the cab and bed sit on big rubber donuts with metal bolts going through them, ok for an electrical ground but not so good for RF because that bolt(s) is the only good path to the frame.

I would also bond the Neg terminal of the battery to the frame.
If you have some wide braid, that is better than wire, even the outer shield of RG8 will work better than wire.

73
Jeff

 

[AudioShockwav]

The thing that makes this hard, is you do not have an antenna analyzer, so you do not know the reactance or impedance of the antenna the way it is mounted now.
With a less than perfect ground right under the 1/4 wave the impedence is going to be low to start with.
SWR meters do not tell the whole story to what can sometimes be a troublesome antenna install.
It is most likely that the coax is making up for some of the not so good ground plane in this case.

73
Jeff

 

[Marconi]

Pretty sure I've seen those mounts in truck stops too. Was not aware gum drop mounts were known to short.

Also the first I've heard a gum drop isn't any good for a 102. As big as they are you'd think they'd be able to handle some abuse.

W5LZ there is no insulator really on a gum drop, just a few star washers to bite into the metal for a ground, and a 1 inch hex nut.

Turbo can you post and image of a gum drop without an insulator? I don't see how it could work a 1/4 wave ss whip otherwise.

Jeff, can you post a close up picture of the gum drop you use with it taken apart a bit so we can see how the insulator works?