Amateur Radio & CB antenna - redux

axlesandantennas

Formerly Double-Desert
#1
Was not really liking my previous antenna location for either antenna. Ground plane was terrible.

Got out my multimeter and grinder. Figured out that the front bumper did indeed show continuity to the frame.

Mounted the CB whip on the passenger side of the bumper. <1.5:1 SWR. Not even going to mess with tuning with that that reading.

The dual band 2m/440 antenna on the driver side is giving me some issues. Will not tune below 1.7:1. When I set it along with the mount on a aluminum step stool, it's about 1.2:1 just about across the entire 2 meter band. I guess it just needs more metal below it. But, this is for another time. For now, 1.7:1 is ok. I'll run about 20 watts and not much for endless conversations.

De N4EGA / Axles And Antennas

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DaveInDenver

Expedition Leader
#2
Keep in mind DC continuity does not mean the RF path is good. With a CB antenna the truck as a counterpoise isn't as critical, or rather it's more tolerant of poor installations and probably the coax shield is doing the work. VHF and UHF especially will highlight issues much more readily. You usually need to bond mounts and around bolts with braided jumpers. Also at VHF+ frequencies any piece of conductive material has the opportunity to become a passive element. It's possible with that location the bow shackles and light bar have become part of the antenna.
 
#3
Also keep in mind with line of sight communication that height is everything. The VHF antenna may be better off on a fender mount. It will still be below the roof line but will at least be above the hood.
 
#5
That antenna is mounted way too low. It's too close to all of the metal in the front end, which is likely reflecting back and causing poor SWR. I'd move it up higher on the Jeep (like on one of the front fenders) and it'll likely work much better.
 

axlesandantennas

Formerly Double-Desert
#6
Keep in mind DC continuity does not mean the RF path is good. With a CB antenna the truck as a counterpoise isn't as critical, or rather it's more tolerant of poor installations and probably the coax shield is doing the work. VHF and UHF especially will highlight issues much more readily. You usually need to bond mounts and around bolts with braided jumpers. Also at VHF+ frequencies any piece of conductive material has the opportunity to become a passive element. It's possible with that location the bow shackles and light bar have become part of the antenna.
Very likely indeed!
 

axlesandantennas

Formerly Double-Desert
#7
Without having to reply to everyone:

Y'all are right, this is a terrible location. I was attempting to use the big bumper as the ground plane as it worked well for the CB antenna. Between the front grill (and the raidator directly behind it), there is a massive effect on the antenna. Imma see if I can get it up over the hood before my trip next week. It works as is, but not enough to really blow my dress up.
 
#8
On the back seam of the hood will produce excellent results. On the TJ there a "pocket" in the bracing , just outboard of the hinge that's a perfect spot.
 
#9
The dual band 2m/440 antenna on the driver side is giving me some issues. Will not tune below 1.7:1. When I set it along with the mount on a aluminum step stool, it's about 1.2:1 just about across the entire 2 meter band. I guess it just needs more metal below it.
That dual band antenna looks small - is that a quarter wave on 2m? What frequency are you getting an SWR of 1.7:1 at?

As Dave says, electrical ground is not equal to an RF ground plane. Those are two totally different things, though sadly they have become conflated in the world of internet car forum folklore.

Is there a reason why you're running the dual-bander as a sloper? More common in HF, not so much in 2m/440 :)
 

axlesandantennas

Formerly Double-Desert
#10
Ok, fixed this monstrosity. I rummaged around my shed and found a 1/8 inch sheet of aluminum. With a jig saw, file and drill press, I managed to make a mount that slips between the hood and fender. Well....1.2:1 !! Thanks y'all!

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#13
I'm generally a big fan of the side of hood mounts like that. Easy enough to keep the antenna away from vertical metal (provided it's not too close to the windshield), usually decent ground plane (but not as good as the middle of a metal roof), gets the antenna up fairly high (but not as high as the roof) and you can see what you're gonna hit the antenna on if it's a tall one.
 

axlesandantennas

Formerly Double-Desert
#14
I'm generally a big fan of the side of hood mounts like that. Easy enough to keep the antenna away from vertical metal (provided it's not too close to the windshield), usually decent ground plane (but not as good as the middle of a metal roof), gets the antenna up fairly high (but not as high as the roof) and you can see what you're gonna hit the antenna on if it's a tall one.
That was one of the reasons I went with a 1/4 wave as opposed to half wave. Did not want to get above the roof line.
 
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