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102" whip

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saber6

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I am installing a 102" whip on my older model extended cab truck.Can it be mounted off to one side front of my box or would it be better centered up above the cab and topper? I have plenty of steel around so mounts can be made.
 

Hydrogen18

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I believe having it centered above the cab would be the best for the radiation pattern.

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mmckenna

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Hydrogen18 is correct. There are two parts to the antenna, the vertical whip and the ground plane underneath it.

The ideal ground plane is going to be slightly bigger in radius than 1/4 wavelength in -all- directions. That will give you the ideal omni-directional radiation pattern.

When you set the whip off to one side of the ground plane, you will suffer from a distorted radiation pattern.

Keep in mind that even on a full size pickup, it's difficult to get a full ground plane under a 1/4 wave whip for CB. So, we do the best we can. In some cases, the lopsided radiation pattern can be used to make the antenna slightly directional. That might be beneficial in some cases, however for more common uses, it's better to have it in the center.

If your topper is anything other than metal, it's not going to play into the equation, if it's fiberglass, like mine, it's effectively RF invisible.

Ideal location is center if the cab roof. Your specific needs might make that not possible.
 

saber6

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It's an all aluminum topper extended cab long box so right behind the cab will be pretty close to the center. I'm planning on building a steel rack that comes off both sides of the topper with a cross bar to mount the antenna.
 

mmckenna

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That should work well.

While RF ground and DC grounds can be different, it's important to make sure everything is well grounded. That would include the aluminum topper and your antenna support. Often copper or tinned copper braid is used for this.
 

saber6

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That should work well.

While RF ground and DC grounds can be different, it's important to make sure everything is well grounded. That would include the aluminum topper and your antenna support. Often copper or tinned copper braid is used for this.
As far as grounding goes is it best to go directly to the frame or the batteries? It's a diesel truck with the two battery system.
 

prcguy

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For an antenna you want to ground it to the closest flat sheet metal near the antenna, a ground wire to the battery has nothing to do with an antenna.

Just remember the antenna will be happiest mounted in the center of the roof or other large flat metal object. We know that's not practical in most cases and the more you deviate from the ideal, the more problems may crop up.

I've encountered many antennas mounted on roll bars that were about even with a truck cab and only a few inches behind. That usually doesn't work very well as the antenna only sees the small amount of metal in the roll bar as its ground plane. At the very least you would try and bond something like that to the cab roof with braid as mmckenna mentioned and it might take several braids spread across the width of the roll bar or rack type mount to make it tune right.

Hopefully the rack you built will look and feel the same as the cab roof to the antenna and it will be happy, otherwise it can take a lot of work to fix it.
prcguy

As far as grounding goes is it best to go directly to the frame or the batteries? It's a diesel truck with the two battery system.
 

saber6

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My plan was to have a 2" square rack basically mounted on a plate bolted to the front corners of the truck box. 2" square welded to the plate coming up over and back down with the cross section about 2" above the cab and topper roof. However, I am open to any idea you all may have. Would grounding forward to the cab and back to the topper improve it?
 

prcguy

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I'm having trouble imagining it, can you point to a picture of something similar on line?
prcguy

My plan was to have a 2" square rack basically mounted on a plate bolted to the front corners of the truck box. 2" square welded to the plate coming up over and back down with the cross section about 2" above the cab and topper roof. However, I am open to any idea you all may have. Would grounding forward to the cab and back to the topper improve it?
 

saber6

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Picture 2" sqaure tubing up and over to the drivers side of the truck. Flat plate bolted under the topper with the 2" tubing welded to it on both sides of the truck


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jhooten

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don't forget the spring !


UHHHHH, NO! A 102" whip with a common 6" spring will put you 26.000, below channel 1. For channel 19 the antenna should be 103.3". SO, if you want to stay within the CB band and use a spring you need a 96" whip. Or for the best result you find a rod holder with set screws so you can remove the rod to trim to the length that puts your antenna system resonant on the specific frequency you operate on the most be it 19 AM or 38 LSB for example.
 

cmdrwill

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You would be better off with a Larson, or Comtelco CB 27 ntenna in the middle of the cab roof on an NMO mount.
 

saber6

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UHHHHH, NO! A 102" whip with a common 6" spring will put you 26.000, below channel 1. For channel 19 the antenna should be 103.3". SO, if you want to stay within the CB band and use a spring you need a 96" whip. Or for the best result you find a rod holder with set screws so you can remove the rod to trim to the length that puts your antenna system resonant on the specific frequency you operate on the most be it 19 AM or 38 LSB for example.
19 and 38 are what I primarily use.

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prcguy

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I still can't picture the tubing and flat plate thing. Bottom line is the antenna wants to be in the middle of a large flat metal surface like the roof of a van, or the center of your camper (topper) with the sheet metal right at the base of the antenna. We also know that can't work without major reinforcement of the cab or camper roof due to the antenna you've chosen.

This might help understand what can go wrong with what you might be planning. If you have a large flat ground plane that is otherwise perfect, then mount the antenna to some tubing or a plate several inches above the ground plane, then you will have problems matching the antenna. When you raise the feed point above the ground plane the impedance goes up and its very hard to compensate for that. When the ground plane is way too small like the thin structure you might install, the feedpoint impedance will also go up and the radiation pattern will pull up away from the horizon where you want it. Grounding your new structure with wires to other parts of the truck will probably not fix the problem either, the antenna wants a good ground plane right under it and you need to supply that.

If your plans include anything I described then be aware you might have difficulty matching a big whip mounted on a home built structure above or away from the cab. Other advice about putting a smaller antenna like a Larsen in the middle of the truck cab is a slam dunk, it will work great and tune up just fine.
prcguy
 
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jhooten

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Bottom line is the antenna wants to be in the middle of a large flat metal surface

prcguy
You are correct in that statement. The problem is the lower you go in frequency that less possible it is to accomplish on a vehicle. The large flat metal surface needs at least a quarter wave length diameter centered on the antenna whether it is a full quarter wave whip or a short loaded type like the NMO-27. Since a full sized pick up is slightly over six feet wide it is not possible to accomplish at CB frequencies. The 6 meter ham band is about as low as you can go to use the cab roof as an effective ground plane.

As you go lower in frequency, beginning at the upper end of HF, where you find CB, the antenna system begins to depend more on the earth for it ground plane. The vehicle body becomes one plate of an air gap capacitor coupled to ground which becomes the other plate of the capacitor. It functions on the same principle at those through the glass antennas.
 

sdeeter19555

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Use a common ball mount on the sheet metal, mount it where you want the antenna. Install a piece of 3/8" all-thread between the mount and the whip, cut the all-thread to adjust SWR (trial and error).

I had a 102" mounted to the back of a toolbox, took a 6-inch spacer (all-thread)...I had a flat 1.1:1 SWR on all 40 channels.

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jonwienke

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No. Not unless you want to bend the crap out of your antenna and/or the sheet metal, or rip the mount out of the vehicle if you hit something with the antenna. Total correct antenna length is about 108", and you are supposed to have a spring assembly between the mount and whip to prevent damage if the antenna hits something. Typical spring assembly length is about 6" so with the whip and spring together, the total length is correct.

If you have to use 6" of allthread, you are doing it completely wrong.
 

sdeeter19555

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No. Not unless you want to bend the crap out of your antenna and/or the sheet metal, or rip the mount out of the vehicle if you hit something with the antenna. Total correct antenna length is about 108", and you are supposed to have a spring assembly between the mount and whip to prevent damage if the antenna hits something. Typical spring assembly length is about 6" so with the whip and spring together, the total length is correct.

If you have to use 6" of allthread, you are doing it completely wrong.
Tell an SWR meter it's off...and further, you can double a 102 inch whip over without a hint of harm to anything. God knows, I hit enough stuff with mine. The only thing a spring does is allow the antenna to sail around, beat everything around within five feet of it, and be "non-vertical".

These know it alls are why I infrequent this forum...I've ran this setup, it worked well and the all-thread allows for fine tuning the SWR. I'm done here, you win...and other forum I'm leaving permanently.


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