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Beginner Question

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Wagen

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Jan 23, 2012
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3
Hello,
I recently purchased a cobra 29lx and have been attempting to install it in my pickup.

I have created and grounded a mount on the drivers side hood, directly across from the factory antenna.
the antenna that was suggested to me when purchasing the radio was a larsen base loaded antenna with a nmo mount. after setting up everything i discovered that my SWR is bordering 3.0 across the board.

I dont see any grounding problems and tested the radio on my Tandem plow truck at work and it reacted fine.

My question is does the coil location have much effect on SWR? does the coil have to be above my roofline?

Any helpful tips would be appreciated.
 

W2NJS

Member
Joined
Mar 27, 2006
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1,938
Location
Washington DC
No, the problem likely is the skewed radiation pattern caused by having a decent ground plane (the hood) on one side and nothing, or almost nothing, on the other side. CB is lowband and needs a decent ground plane to operate at low SWR. A shorter, loaded, antenna mounted in the center of the roof would do much better. If by chance the radio has a loading adjustment available on its back for operator use then you should try changing the setting; that might help some.
 

Wagen

Member
Joined
Jan 23, 2012
Messages
3
Why is it that a tandem truck with an antenna mounted to the side mirror can receive such good swr from a similarly lopsided ground plane?

Would a ngp antenna be a better option or would it severly affect transmit strength?
 

SmitHans

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Oct 12, 2009
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112
Location
Arizona, USA/Sonora, MEX
I am not sure the radiation pattern would affect SWR that significantly. How's your coax? Is it getting pinched under the hood? How did you create a grounded mount? Are you sure your antenna is the correct length?
 

Wagen

Member
Joined
Jan 23, 2012
Messages
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My mount was made myself from mild steel, bolted to two bolts that mount the fender to the truck. I then further grounded the mount with 16ga wire directly to the antenna base and to the chassis.

The coax I tested with a multimeter and had no resistance problems that I detected. As for any pinches or tight bends, the only place I can think of would be where it runs through the fender, otherwise nothing has been permanently mounted in the cab, I simply let it spiral out ib the cab to be sure it wasn't coiled up or bent.

As for the antenna length, I have yet to trim any of the wire whip, but didn't think it would be that drastic of a change to start of with a swr of 2.8 all across the board.
 

LtDoc

Member
Joined
Dec 4, 2006
Messages
2,145
Location
Oklahoma
The first thing that comes to mind is did you tune/adjust that antenna? No antenna is ever 'tuned' correctly to start with (unless you're luckier than everybody else!).
Will it placed on that fender produce a sort of directional radiation pattern? Sure, but not that bad of a one. It should certainly be usable, and probably not that noticeable anyway. Will one antenna affect the other one? I really doubt it will be adversely affected, either one.
Since you've already tested the thing and it worked, don't waste a lot of good worry about it. Anything can be made 'better', but how much better is a debatable question.
Antennas like being above metal rather than beside metal, and don't much care to be under metal. Speak firmly to it, explain that it's in the best place you can find for it, and that you will punish it if it doesn't shut up and work. That works most of the time...
- 'Doc

(Threaten to paint it pink! That works almost 100% of the time.)
 

AC9BX

Member
Joined
Jun 11, 2011
Messages
326
Location
Lockport, IL
Being mounted on a mirror or at the fender will certainly affect the signal pattern but will have little affect on the SWR. A vertical antenna with no ground plane can perform reasonably well with reasonable SWR. Grounding has little to do with anything in such a system because ground is everywhere.

It is safe to assume the metal body panels are at the same ground potential as the radio and the coax shield. There's a possibility of some phase error or ground loop noise but that wont bother the SWR. What this means is even though there isn't a connection it works. Suppose you rest a magnet mount antenna on the hood or roof. The metal under the paint is electricly at the same potential as the shield of the coax at the antenna base. If it wasn't a mag mount would function as a vertical with no counterpoise. So while it isn't ideal it is a very effective ground plane and will work as expected. The roof will be better than the fender for signal pattern.

One thing that comes to mind is the coax and connectors. Can you test this with a length of coax draped around the vehicle? If gives similar results you've probably eliminated the cable and connectors as a problem. This brings us to the antenna. These things are almost never properly tuned out of the package. Use whatever adjustments you have to lengthen or shorten it as much as possible and test SWR. Is longer better or vice versa? Try to find a trend. Be sure you know which way it needs to go before you start cutting. Perhaps best is to experiment with a length of wire, start out several inches longer, cut off an inch at a time and retest. I used a mobile antenna once that was 5 inches too long. (It was this fiberglass piece of junk from decades ago. But it had a very strong mount and large guage wire loading coil. Replacing the fiberglass whip with a length of brass welding rod made it into a very fine antenna.) There is typically a small range of an inch or less that offers the proper SWR. When you get closer cut less each time.
 

k3cfc

Silent Key
Joined
Feb 27, 2011
Messages
715
Location
Beavertown Pa.
No, the problem likely is the skewed radiation pattern caused by having a decent ground plane (the hood) on one side and nothing, or almost nothing, on the other side. CB is lowband and needs a decent ground plane to operate at low SWR. A shorter, loaded, antenna mounted in the center of the roof would do much better. If by chance the radio has a loading adjustment available on its back for operator use then you should try changing the setting; that might help some.
These loading adjustments went out with the tube radios years ago.
 

k3cfc

Silent Key
Joined
Feb 27, 2011
Messages
715
Location
Beavertown Pa.
As long as i have repaired CB radios i have never seen a nmo mount for them. and i may be wrong but if i was you i would Google Larsen antennas because from what i can find they do not make antennas for CB RADIO.
 

k3cfc

Silent Key
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Feb 27, 2011
Messages
715
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Beavertown Pa.
AS long as i have repaired CB RADIOS i have never seen a nmo mount for them. and i may be wrong but i don't believe Larsen makes a CB RADIO antenna. if i were you i would Google Larsen antennas.


Larsen® is a leader in the design, development and manufacture of technologically advanced antenna systems for wireless applications. Larsen has provided mobile, portable and base antennas for two-way radio, public safety and military markets for over 40 years.
 

k3cfc

Silent Key
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Feb 27, 2011
Messages
715
Location
Beavertown Pa.
Hi Tom. thanks for the link. i found on ebay an item number 200712144687 that sells this antenna. it has some interesting info there. the only thing is they list a mag mount for this antenna. the wire is 53 inches long so he has some cutting to do.

later chuck
 
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