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Jk antenna problems

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sjans

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So I have a jku with an Arizona rocky road mount which mounts to the spare tire bracket. I realize this isn't the optimum location however I'm hoping I can get my swr's lower than they are now. Here's my issue, after having all components checks professionally (cb, coax, etc), I have an swr reading with my firestik of 2.1 and 2.7. My 102" whip is 3 and 3.3. I've tried both with springs which actually make them worse. I've carefully removed all necessary paint and have various bonds from the antenna bracket to the tailgate then to the chassis. I've also installed bonds ( flat braided strapping) from the body to the chassis on both side of the jeep around the driver and passenger doors to the chassis. One also on the exhaust to the chassis. All this and the swr's really haven changed. I've even bonded the radio bracket/mount to the chassis. Is this as good as I'm going to get?
 

prerunner1982

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I am surprised the 102" whip has a worse SWR. The 102 should need the spring to get it closer to 108" (1/4 wave length for CB). The 102" whip should have a generally broad mostly flat SWR curve.
Is the nylon washer above or below the mount? It should be above to separate the antenna from the mount.

Is the coax pinched?

You could try taking a 108" length of wire and connecting the bare ends to the mount (bare metal), lay it out straight, and test your SWR again. This will act as a ground plane and if your SWR goes down you will know you need to recheck your grounding from the antenna mount to the tailgate and the tailgate to the body.
 

lmrtek

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worst case you can always use an antenna matcher

it could be that you need to shorten the wire on the firestick
until you lower the swr
 

sjans

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Prerunner,

Thanks so much for the reply. I'm also surprised the whip has higher swr's, that's what initially made me think it was a ground issue. As for the 108" wire, just attach one end of the wire to bare metal on the mount below the stud? As for the coax, I don't think it's end since I needed all 18' to reach the radio. The guy attached his meter directly to the antenna stud and got the same readings as I did. This is also why I believe it's not the coax. I hope it's not a ground plane issue because I'm clueless as where to put any additional grounds. I've added 5 and can't really see difference. Let me know if I'm understanding you correctly regarding the counterpoise.
 

GROCKSHD

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Intresting. I have the teraflex mount on my JKU and with a 4' Firestik. I get a 1:2-1:5 swr reading. The only thing I did was remove the paint from the bolt holes and from the hole for the antenna.
 

cmdrwill

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The Jeep is not big enough to provide a 'groundplane' at 27 mhz., or more correctly a counterpoise or other half of the antenna system.

Most use the 'no groundplane' Firestick antenna. Probably equivalent to a one half wave antenna.
 

prcguy

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That's basically what's going on, the Jeep is small and since the top is fiberglass the only groundplane you have right under the antenna is the thin edge of the tailgate and tops of the rear quarterpanels. I've done dozens of installs in Jeeps many years ago and remember a ball mount on the rear quarter panel worked better than a spare tire mount but it was still troublesome.

As someone else mentioned, take some wire attached right to the grounded side of the antenna mount very close to the antenna and stretch it out sideways and see if that improves your match. You could try a 9ft wire or a couple of shorter ones, just make sure its right at the antenna base and goes out sideways from the antenna. If this gives you a good match then you know its purely a ground plane problem.

Grounding the mount to the chassis and other methods that do not provide a large surface of metal right at the antenna are not the same as having the antenna mounted on a large flat surface of metal. If lack of ground plane is your only problem you can fool the antenna by soldering a small value capacitor across the antenna mount from hot to ground. Try values from maybe 25pf to 100pf using a 100v or more rated disc ceramic or dipped silver mica and this will usually get the match in a reasonable range.

Another thing to look for is grab the coax with your hand and move your hand up and down the length of coax while measuring the SWR. If its stable then you have one less thing to worry about. If it changes noticeably then the lack of ground plane is forcing RF currents onto the braid of the coax and it will also radiate some. In this case you would need an RF choke in the coax and that's a separate topic.
prcguy

The Jeep is not big enough to provide a 'groundplane' at 27 mhz., or more correctly a counterpoise or other half of the antenna system.

Most use the 'no groundplane' Firestick antenna. Probably equivalent to a one half wave antenna.
 

sjans

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Swr's

Intresting. I have the teraflex mount on my JKU and with a 4' Firestik. I get a 1:2-1:5 swr reading. The only thing I did was remove the paint from the bolt holes and from the hole for the antenna.
That's what I've heard others say. I don't get it. I was very careful to make sure all paint was removed when installing my mount. The only thing in common to the two swr's readings (when I check and when the shop checked) was both readings were taken when the antenna was on the mount. To me, this means my radio/coax is not the problem and it's a ground plane issue. I'm almost sure my swr's were 1.1-1.3 with the firestick when first installed and before I did all the bonding. However the whip was still high but I don't think as high as now. May need to remove all the ground straps and check again. If the firestick is low then I'll just cut the whip however I read this shouldn't have to be done typically with a whip and spring.
 

w0fg

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I don't have a solution, but your situation is pretty much opposite of mine I have a Cool Tech Ultra Mount license-plate mount that I've had a terrible time getting a proper ground, but my spare-tire bracket mount, with my Lil' Tarheel screwdriver antenna gives me less than 1.5:1 on all frequencies with my Icom 7100, including 27 Mhz,and this with no additional ground straps.
 

sjans

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Thanks PRCGuy,

So would the no ground plane firestick be as effective as the regular ones. I'm assuming not as good as the whip.
 

sjans

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So it's sounding like it may be in my mount installation. Thanks to all who submitted solutions. I really appreciate it!!
 

prcguy

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I don't know but according to Firestick it should. I was given one to install on a truck bed tool box and it looked like a regular Firestick but I think the coax was very specific to the no ground version and the braid had no connection to anything or something similar. I used a grounded mount and my own coax which tuned ok and did not use the Firestick coax assembly.
prcguy


Thanks PRCGuy,

So would the no ground plane firestick be as effective as the regular ones. I'm assuming not as good as the whip.
 

prerunner1982

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The Jeep is not big enough to provide a 'groundplane' at 27 mhz., or more correctly a counterpoise or other half of the antenna system.
That's basically what's going on, the Jeep is small and since the top is fiberglass the only groundplane you have right under the antenna is the thin edge of the tailgate and tops of the rear quarterpanels. I've done dozens of installs in Jeeps many years ago and remember a ball mount on the rear quarter panel worked better than a spare tire mount but it was still troublesome.
I guess that's a bonus of driving a Jeep Cherokee... the redheaded sibling of the Wrangler. Not as cool, but there is enough ground plane that I can get a good (1.4-1.5) SWR on my 20m hamstick, closer to 1.1 on my 10m hamstick and 102" whip all mounted on the drivers rear corner by the taillight.
 

sjans

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Antenna

I guess that's a bonus of driving a Jeep Cherokee... the redheaded sibling of the Wrangler. Not as cool, but there is enough ground plane that I can get a good (1.4-1.5) SWR on my 20m hamstick, closer to 1.1 on my 10m hamstick and 102" whip all mounted on the drivers rear corner by the taillight.
I may have to relocate mine as well. I really liked the tailgate location since it was centered and the mount is almost higher than the spare tire.
 

sjans

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Well, I took stranded copper wire (108" insulated) and attached it between the antenna stud and mount (on the bottom of mount just for clarification). Stretched it out and swr's actually jumped way up. Should I have removed all the bonding I added to the jeep before doing this? Holy crap this is getting ridiculous lol.
 

sjans

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So here is an update. After changing to a teraflex mount which isn't as tall, the whip swr's came down just a bit but still 2.3-2.5. I did connect a 108" counterpoise perpendicular to the jeep and swr's came down to 1.5 and lower when I held it. however I can't seem to keep that number after routing the cable inside the jeep. It goes back up. So I guess this means it's a ground plane issue. How do solve this?
 

prcguy

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Read my post #7 and try a 50pf, 50 or 100v disc ceramic or dipped silver mica capacitor across the hot and ground of the mount, or right across the coax at the mount. If you find some 25pf caps you can try one, then two then three in parallel to find the value that provides the best match, then put in a single cap of that value or leave the paralleled ones.

This will fool the antenna into thinking its got some ground plane under it and it should match fine. Antennas are not very smart and its easy to fool them.
prcguy


So here is an update. After changing to a teraflex mount which isn't as tall, the whip swr's came down just a bit but still 2.3-2.5. I did connect a 108" counterpoise perpendicular to the jeep and swr's came down to 1.5 and lower when I held it. however I can't seem to keep that number after routing the cable inside the jeep. It goes back up. So I guess this means it's a ground plane issue. How do solve this?
 

sjans

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Thanks prcguy. Wish I was knowledgable enough to do that. Are you saying to solder them so one end is connected to the top of the mount and the other to the bottom?
 
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