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Trimming antenna..how much is too much?

Here is an example with the popular Sirio 5000 mobile antenna scanned with a Rigexpert AA-54 analyzer to show bandwidth and why I tune antennas to bandwidth rather than the tired old way to even out channel 1 and 40.




img_20150316_141035-1-jpg.15412




Here I scanned for the lowest SWR and moved the cursor arrow to near the lowest point in the SWR curve. This was done with whip inserted about midway into the antenna for an initial check. It's hard to see in the pic but the Frequency was 26.320 Mhz which is now displayed after moving the cursor arrow to the lowest point..

The analyzer bandwidth was set for +/- 1000 Khz. So you can now see the SWR curve at 500 KHz per side centered at 26.320 Mhz.

The bandwidth you can see is about 500 Khz just below a 2.0 SWR on the left, and it's just over 500 Khz before it would exceed a 2.0 SWR on the right.

Total bandwidth to operate this antenna under a 1.8 SWR would be about 800 Khz. That's plenty of bandwidth to operate on.

You can now see to operate on the CB band that the antenna is too long since it's resonate at 26 MHz, so I have to shorten it some for 27 Mhz.

With this kind of bandwidth on the Sirio 5000 antenna, If I make channel 20 the lowest SWR at near flat, I should see channels 1 and 40 at a 1.4 or less SWR. More than ideal!





img_20150316_140956-1-jpg.15413



Here are the numbers with this scan.
 
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Short antennas on Wrangler style Jeeps are hit or miss. In my opinion, a 102' whip with a spring mount off of the rear bumper (if the bumper is steel, and is connected to the frame) is the best setup. Reasons: 1. 9 feet off a low Jeep bumper is not that tall and outrageous looking. 2. Slap on a tennis ball, and an antenna tie-down, and you can tie it down for the garage, and when you're not using the radio. 3. That 9 foot stainless whip will give you the best shot at a low SWR. 4. That 9 foot whip will get out better than any other antenna you can put on a Jeep Wrangler. Now, if you had a Grand Cherokee, I'd recommend a Wilson 5000 magnet mount, but you unfortunately have the challenge of getting an antenna to perform correctly on a Wrangler. My three cents, coming from myself previously owning a Jeep YJ, and a ZJ.
 
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fourstringburn said:
Here is an example with the popular Sirio 5000 mobile antenna scanned with a Rigexpert AA-54 analyzer to show bandwidth and why I tune antennas to bandwidth rather than the tired old way to even out channel 1 and 40.




img_20150316_141035-1-jpg.15412




Here I scanned for the lowest SWR and moved the cursor arrow to near the lowest point in the SWR curve. This was done with whip inserted about midway into the antenna for an initial check. It's hard to see in the pic but the Frequency was 26.320 Mhz which is now displayed after moving the cursor arrow to the lowest point..

The analyzer bandwidth was set for +/- 1000 Khz. So you can now see the SWR curve at 500 KHz per side centered at 26.320 Mhz.

The bandwidth you can see is about 500 Khz just below a 2.0 SWR on the left, and it's just over 500 Khz before it would exceed a 2.0 SWR on the right.

Total bandwidth to operate this antenna under a 1.8 SWR would be about 800 Khz. That's plenty of bandwidth to operate on.

You can now see to operate on the CB band that the antenna is too long since it's resonate at 26 MHz, so I have to shorten it some for 27 Mhz.

With this kind of bandwidth on the Sirio 5000 antenna, If I make channel 20 the lowest SWR at near flat, I should see channels 1 and 40 at a 1.4 or less SWR. More than ideal!





img_20150316_140956-1-jpg.15413



Here are the numbers with this scan.
Click to expand...

Thanks for that. A RigExpert is still on my “someday” list, but I appreciate the tuning approach. Makes sense.
 
Chicago Savage said:
Short antennas on Wrangler style Jeeps are hit or miss. In my opinion, a 102' whip with a spring mount off of the rear bumper (if the bumper is steel, and is connected to the frame) is the best setup. Reasons: 1. 9 feet off a low Jeep bumper is not that tall and outrageous looking. 2. Slap on a tennis ball, and an antenna tie-down, and you can tie it down for the garage, and when you're not using the radio. 3. That 9 foot stainless whip will give you the best shot at a low SWR. 4. That 9 foot whip will get out better than any other antenna you can put on a Jeep Wrangler.
Click to expand...

Go and look at K0BG.com's picture gallery and in the HOS (Hall of Shame) section you'll find antenna installs just like yours.

You have nothing but very lossy ground below the antenna.The only reason you have low SWR is due to massive ground losses. The part of the antenna where the RF current is highest is right next to vertical metal adding even more loss.

It won't get out better than something like a Wilson 1000 put on a steel hard-top or even the front fender.
 
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M0GVZ said:
Go and look at K0BG.com's picture gallery and in the HOS (Hall of Shame) section you'll find antenna installs just like yours.

You have nothing but very lossy ground below the antenna.The only reason you have low SWR is due to massive ground losses. The part of the antenna where the RF current is highest is right next to vertical metal adding even more loss.

It won't get out better than something like a Wilson 1000 put on a steel hard-top or even the front fender.
Click to expand...
He has a Wrangler. Not a Cherokee. There's no place for a magnet mount on a Wrangler. Looking back at my original reply, I would revise my statement, and say the best spot for a 102" whip would be a ball mount, and a six inch shaft, mounted above one of the rear wheel wells.
 
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Chicago Savage said:
View attachment 29938
Click to expand...
Borrowed photo. IMHO that is the best setup for a Wrangler. Next best would be off the rear bumper (if you care about hitting trees). I've seen 102" whips mounted over the spare tire with some kind of bracket as well. When I had my two Wranglers, I ran my 102" whip off the rear bumper and got out quite well. I do however take into consideration your findings about being so low to the ground using the rear bumper, so above the wheel well or spare tire are my new favorite mounting locations for a long whip on a Wrangler.
 
Chicago Savage said:
Borrowed photo. IMHO that is the best setup for a Wrangler. Next best would be off the rear bumper (if you care about hitting trees). I've seen 102" whips mounted over the spare tire with some kind of bracket as well.
Click to expand...

So your choices for an efficiency point of view are all the worst ones you can choose but hey they look good, right? Also in the picture above, once again I'll point out the adjacent vertical metal of the roll cage right where the current maximum is in the antenna so a lot of of your power is going into that roll cage. The top half of the antenna has little current flowing in it. Its literally the RF equivalent of transmitting with your antenna laying down on the ground having adjacent metal that close. Also if it weren't for the fact its a quarter wave so fairly wide banded it would be impossible to tune without serious cutting due to capacitive coupling to said roll cage.

Define getting out well. I do over 30 miles with 4W mobile to mobile. I'm S9 to the next town 12 miles away when mobile.
 
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M0GVZ said:
So your choices for an efficiency point of view are all the worst ones you can choose but hey they look good, right? Also in the picture above, once again I'll point out the adjacent vertical metal of the roll cage right where the current maximum is in the antenna so a lot of of your power is going into that roll cage. The top half of the antenna has little current flowing in it. Its literally the RF equivalent of transmitting with your antenna laying down on the ground having adjacent metal that close. Also if it weren't for the fact its a quarter wave so fairly wide banded it would be impossible to tune without serious cutting due to capacitive coupling to said roll cage.

Define getting out well. I do over 30 miles with 4W mobile to mobile. I'm S9 to the next town 12 miles away when mobile.
Click to expand...
 
Okay. Im done. Go ahead, and put a 4 foot fiberglass whip with a 2.5 SWR on a bracket above the spare tire and enjoy being able to talk for 1/2 to 1 mile. The only place to put an antenna on a Wrangler without adjacent metal blocking half of your signal would be to mount the antenna directly to the roll bar. Exactly how can that be accomplished unless you go topless 100% of the time. Peace out.
 
Chicago Savage said:
The only place to put an antenna on a Wrangler without adjacent metal blocking half of your signal would be to mount the antenna directly to the roll bar. Exactly how can that be accomplished unless you go topless 100% of the time. Peace out.
Click to expand...

How about the front wing? You can do it and have a good foot and a half to the nearest adjacent vertical metal.

JK_Fender_Installed-3_1024x1024.jpg
 
I don't have anything against that setup. It was exactly what I did on my first YJ, except I did it on the passenger side. 4 foot Firestick. I could never get the SWR below 2.5, and my range was 1 or 2 miles- if that. For communicating with fellow Jeepers on a Jeep trail, it will work, but in the city, don't expect much. You know, a couple of days ago, I saw a Jeep with an antenna mount that worked over the spare tire, and it put the entire antenna above the roof, meaning it's above all metal. Never seen that before. Take care, Dean.
 
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Chicago Savage said:
I don't have anything against that setup. It was exactly what I did on my first YJ, except I did it on the passenger side. 4 foot Firestick. I could never get the SWR below.2.5, and my range was 1 or 2 miles- if that. For communicating with fellow Jeepers on a Jeep trail, it will work, but in the city, don't expect much. You know, a couple of days ago, I saw a Jeep with an antenna mount that worked over the spare tire, and it put the entire antenna above the roof, meaning it's above all metal. Never seen that before. Take care, Dean.
Click to expand...

Several mounts like that. TERAFLEX. See Bells CB
 

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