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Old 05-03-2018, 06:19 PM
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Antenna tuning troubles...part 2

So, after ditching my high lift jack and using the antenna mount on the Teraflex HD hinge jack mount, I got SWR reading of 1.5.

I wanted to ditch the big Teraflex jack mounting bracket, in favor of making a bracket that mounted the antenna behind the spare tire. I made a bracket which set the antenna at the same height of the other mount. It's a 2' Firestik that extends about 2" above the roof line and it's mounted about 8" off the centerline of the Jeep behind the spare tire.

I also figured I'd need to get a ground strap to the mount because the only real opportunity to establish a good ground was coming from the bolts that mount the tailgate to the Teraflex HD hinges.

So I went to a Ham shop and picked up a 5' piece of 1/2" braided tinned copper grounding strap. Picked up a couple #6 ring terminals, used my hydraulic crimping tool and made my bullet proof ground strap.

Snaked it around and through the tailgate, in behind the interior trim, drilled a 3/8" hole in the sheet metal below the rear roll bar mount, cleaned it to bright shinny with a dremel tool and terminated the ground strap. Same termination process on the home made antenna bracket. Used my multimeter to check for any shorts on the antenna mount or cable, all seems good. Cable tied everything down and moved it outside away from the house for tuning.

No love... The best SWR reading I get is channel #1 at 2.0, with channel #40 at 2.75. The antenna tip is screwed in all the way down to make the antenna as short as possible. Now I am stumped again.

This location is grounded, the old one was only getting ground through the hinge pins. This location in just off the centerline of the Jeep, the old one was just inboard of the left rear corner. I got better readings before.

Any ideas which way to go?

Thanks as usual for any help.

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Old 05-04-2018, 01:07 AM   #2
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Photos please. As detailed as possible.

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Old 05-04-2018, 01:26 AM
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Photos please. As detailed as possible.
Here they are:
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Old 05-04-2018, 01:28 AM
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Old 05-04-2018, 01:30 AM
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Old 05-04-2018, 01:32 AM
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Old 05-04-2018, 11:52 AM
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I went to the Firestik website and read their help section about electrically log tuneable antenna tips. Then went out and played around with mine removing different combos of the tuning parts on top of the antenna.

Best SWR I could get is CH1 = 2.0 and CH40 2.1.

I guess, based on what I read on Firestik, it is about location, location, location. I'm going to put my jack mount bracket back on, use the ground strap, and see what SWR I can get?

I could have replaced the whole suspension faster than getting this darn radio system to work. Frustrating. Nothing is black and white.
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Old 05-04-2018, 12:02 PM   #8
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Running this kit.. 1-1.5 swr across the channels with a 4' firestik firefly.

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Old 05-04-2018, 12:30 PM
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So I put my other bracket on, pics below, I got SWR CH1 + 1.6, CH 20 + 1.0, CH40 = 1.6. I guess may antenna location more centered on the Jeep just isn't a "good" location?
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Old 05-06-2018, 04:51 PM   #10
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So I put my other bracket on, pics below, I got SWR CH1 + 1.6, CH 20 + 1.0, CH40 = 1.6.
Those are great SWR values, I wouldn't touch a thing.
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Old 05-06-2018, 05:59 PM
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Those are great SWR values, I wouldn't touch a thing.
They are good for using the antenna on the jack mount I'm trying to eliminate.

I'm just stumped as to why moving the antenna 2' to the left, would cause so much higher SWR readings? Can't be the spare tire, it wasn't mounted on the Jeep.
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Old 05-06-2018, 07:02 PM   #12
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I'm just stumped as to why moving the antenna 2' to the left, would cause so much higher SWR readings? Can't be the spare tire, it wasn't mounted on the Jeep.
Nearby metal masses easily detune antennas. Which is why I like to mount CB antennas on a tail light mount which gives more clearance away from spare tires, spare tire mounts, Hi-Lift (unnecessary) jacks, etc.
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Old 05-06-2018, 09:14 PM
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Thanks Jerry.
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Old 05-14-2018, 09:18 AM   #14
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I mounted a piece of aluminum to the tire mount location that extended my antenna about 15 inches to the right. About as far as it could go and still allow the rear door to open completely. My SWR readings dropped about .5 across the channels compared to mounting the antenna right at the tire.

I just hope it can stand up to some rough country and not break!

It is 3/4" angle stock. I had to drill out a larger hole than the 3/4" would allow comfortably so I have a break in the antenna mounting end. A 1" angle stock would have been better but was not available at the hardware store I purchased from.

If it doesn't last I will upgrade
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Old 05-14-2018, 09:54 AM   #15
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mount it somewhere away from the brake light. The antenna is coupling with the metal. Or try changing the length of the coax making it's length not an increment of a 1/4 wave of the frequency of use. try to add a 3 foot coax jumper in line and see if that works
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Old 05-14-2018, 11:31 AM   #16
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... Or try changing the length of the coax making it's length not an increment of a 1/4 wave of the frequency of use. try to add a 3 foot coax jumper in line and see if that works
The length of coax cable is absolutely 100% not important with this type of antenna. Some antennas have critical feed line lengths but the correct coax length for our Jeep antennas is simply the amount needed to connect the antenna to the radio. Any reasonable length coax works as well as any other reasonable length coax with the type of antenna most commonly installed on our Jeeps.

The only type of antenna that is very rarely (and incorrectly) used on ourJeeps whose coax length is critical is a no-groundplane antenna which is more appropriate for installations on things like wooden or fiberglass boats that can't provide the required groundplane. NGP antennas perform poorly anyway.

The 18' cable length thing that gets mentioned once in a while is nothing but an old wive's tale.
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Old 05-14-2018, 12:16 PM   #17
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mount it somewhere away from the brake light. The antenna is coupling with the metal. Or try changing the length of the coax making it's length not an increment of a 1/4 wave of the frequency of use. try to add a 3 foot coax jumper in line and see if that works
Try the 3 foot or so jumper. It's easy enough, see if it fixes your problem. A simple google will tell you why :-)
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Old 05-14-2018, 02:17 PM   #18
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Here's a photo of my extension that was enough to reduce my SWR readings by .5.
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Old 05-14-2018, 05:22 PM   #19
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Try the 3 foot or so jumper. It's easy enough, see if it fixes your problem. A simple google will tell you why :-)
The right Google search will explain why it won't help. Only those who don't truly understand antennas continue to cling to the myth that coax length matters with the type of antenna we install on our Jeeps.

Here is one web page that does a good job explaining this oft-repeated myth...

http://www.radiomods.co.nz/coaxlength.html

Here's one more that gets into it with a bit more of the math explained... http://www.stu-offroad.com/misc/myth-1.htm

The bottom line is the coax length has absolutely no effect on our Jeep CB antennas.
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Old 05-15-2018, 07:12 AM   #20
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I never said anything about the 18 foot of coax. I agree with you there. What I'm saying is FootSoldier has an issue with not being able to tune his antenna to an acceptable SWR because of it coupling with a metal object in it's near field. As now it is established that his antenna is unable to be tuned to an acceptable SWR the coax length becomes important as to where the 'zero voltage point' that is talked about in your links needs to be moved. As where dealing with just one band, this is doable. The accepted fix is to Just add or subtract a small amount of feedline, or move the antenna. Adding or subtracting from the length of coax isn't going to make the antenna resonant, it's just going to let the transceiver 'see' a good SWR. Moving the antenna to a place where it is resonant is the preferred fix, but maybe not doable. I myself would move the antenna if possible, as it would with the now resonant antenna achieve full forward gain....

I can't make this any clearer on the physics of what's happening. It is what it is....
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Old 05-15-2018, 11:24 AM   #21
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I never said anything about the 18 foot of coax. I agree with you there.
Those articles weren't confined to just debunking the 18' myth, they also debunked the myth that coax cable length makes any difference. Don't just read the headlines. Adding that additional 3' length of coax won't help because, again, the length of the coax has zero effect on anything to do with tuning the type of antenna we use on our Jeeps.

Read both articles completely, you'll see they both debunk the coax length myth entirely.
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Old 05-15-2018, 11:43 AM   #22
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Here they are:
Dumb question, on the mount you made, did you remove the paint under the ground strap and under the mount to make sure you have good electrical conductivity all the way through?

It is closer to the 3rd brake light than the hi-lift mount, but aside from your brake light extension bracket, the 3rd brake light housing itself is plastic, isn't it?

The one thing I do see that your troubled location is right next to is the rear wiper motor... Could you flip your mount and try it on the passenger side using the mounting bolts on the opposite side of the spare tire mount? That's about where I have mine and have no issues with a 3' firestick. (But I am using the AEV 3rd brake light in the rim of the spare.)

Josh
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Old 05-15-2018, 11:03 PM
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Dumb question, on the mount you made, did you remove the paint under the ground strap and under the mount to make sure you have good electrical conductivity all the way through?

It is closer to the 3rd brake light than the hi-lift mount, but aside from your brake light extension bracket, the 3rd brake light housing itself is plastic, isn't it?

The one thing I do see that your troubled location is right next to is the rear wiper motor... Could you flip your mount and try it on the passenger side using the mounting bolts on the opposite side of the spare tire mount? That's about where I have mine and have no issues with a 3' firestick. (But I am using the AEV 3rd brake light in the rim of the spare.)

Josh
Yes, removed paint/coatings between the spare tire mount and my bracket, as well as there the ground strap attached.

The 3rd brake light bracket is metal. I'd have to make another bracket to flip it to the right side of the tire mount.

I reinstalled the jack mounting bracket (without the jack) on the right side again and put the antenna on it. Then tuned the antenna and got CH1=1.5, CH20=1.0, and CH 40=1.5 SWR.
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Old 05-15-2018, 11:50 PM   #24
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Great SWR numbers that are nicely centered on ch 20, nice work getting that figured out!
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Old 05-17-2018, 07:08 AM   #25
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Hey Jerry, I’m at 1.0 on 40 and 2.0 on ch1. This means my antenna needs to be longer? I’m using a firestik, it doesn’t have the screw tuneable tip. How do I get it longer if that’s what I need to do? By the way, this is what the meter is reading on my President radio. I have a real meter somewhere, just have to dig it out.
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Old 05-17-2018, 09:03 AM   #26
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Where are you on 20. If it's under 2, then leave it as is. It will work fine.
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Old 05-17-2018, 09:18 AM   #27
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I’ll check it today, thank you.
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Old 05-17-2018, 01:56 PM   #28
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Hey Jerry, I’m at 1.0 on 40 and 2.0 on ch1. This means my antenna needs to be longer? I’m using a firestik, it doesn’t have the screw tuneable tip. How do I get it longer if that’s what I need to do?
Well, that particular antenna for all intents and purposes can't be made longer once its internal wire has been trimmed. I'd bite the bullet and just replace it with a new Firestik FS series which has the tunable tip which uses a brass screw. They're usually under $20 IIRC on Amazon.

If you install a new antenna, just tune it for the lowest possible SWR on ch 20 which is dead-center in the band. That will give good balanced performance from 1-40 without need to also check/adjust it for 1 or 40 individually.
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Old 05-17-2018, 03:59 PM   #29
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Just checking SWR on Channel 20 won't tell you if your antenna is electrically long or short which means that you will end up spending as much or even more time fiddling around than if you follow standard tuning procedure and start by checking the SWR at Channels 1 and 40.

If the SWR on Channel 40 is higher than on Channel 1, your antenna is too long. If the SWR on Channel 1 is higher than on Channel 40, your antenna is too short.

Now that you know which direction to go you can check the SWR on Channel 20 and adjust antenna length accordingly to get the lowest SWR. However, if you are able to get the SWR readings on Channel 1 and Channel 40 to match, your SWR at the midpoint of the CB band (actually Channel 19) will automatically be at its lowest point and you need do nothing further.

This article by Firestik on off-center tuning may help bring things into perspective:

Off Center tuning

See also these articles on basic CB tuning procedure:

Setting the SWR of Your Antenna© - Firestik® Antenna Company

https://www.rightchannelradios.com/b...g-instructions

The video in the last link will be very helpful if you have never used a SWR meter before.
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'06 Jeep "LJ" Rubicon, '46 Bantam T3-C trailer
Past Jeeps: '49 Willys Wagon, '54 CJ-3B, '65 CJ-5,
'83 CJ-7, '97 ZJ
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Old 05-17-2018, 09:25 PM   #30
Knows a couple things...

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Join Date: Nov 2005
Location: Escondido, Calif.
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Quote:
Originally Posted by Mr. Bills View Post
Just checking SWR on Channel 20 won't tell you if your antenna is electrically long or short which means that you will end up spending as much or even more time fiddling around than if you follow standard tuning procedure and start by checking the SWR at Channels 1 and 40.
That's pretty silly claiming you would spend more time tuning an antenna by transmitting solely at the midpoint of the band to determine if the antenna is too long or too short than checking at two different frequencies... which is hardly a "standard tuning procedure" any more than an 18' coax length is the correct coax length for a CB antenna. Both are just internet rumors repeated by some as the gospel.

If the SWR goes up when unscrewing the tip to make it longer on ch 20, you instantly know it's electrically too long. Screw the tip in to reach the low-point/best SWR. And visa-versa. Stupid simple. Common sense too.

I tuned my first ham antenna (a 15m beam) in 1963 as a high school kid, then later supported USAF hf/vhf/uhf radio networks and installed/tuned many military antenna farms, then did the same thing with commercial transmitters and antennas. Military standards manuals I had to adhere during my USAF days specified tuning all antennas solely at the midpoint of the operating band. Over 60 years since I tuned my first antenna and know more than just a bit about the subject.

The bottom line is there's no need to spend more time to transmit at both end-band frequencies to determine if the antenna needs to be shortened or lengthened. That's just an old wive's tale. It's too simple and takes just seconds to do it solely at the mid-point. Tuned properly at the midpoint produces the lowest average across the band SWR possible. Doing it at both ends of the band just isn't needed or beneficial. Though I suspect your real point is just to continue arguing with me.

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