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Isolate A99 Antenna from Mast Pipe

B

Brain the Dog

Active Member
May 18, 2016
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I'm fixin' to install my new A99 with the Ground Plane Kit. It will be attached to a steel pole that is 31-feet tall. Will have in-line polyphaser. Pole will be cemented into aground and bracketed to side of house.

Should I isolate the antenna from the mast pole? Is there a benefit? Is there harm if I don't? How do I isolate it?

In all of my CB adventures, I have never seen a base station antenna isolated from the pole. So, I'm just wondering.
 

if you don't have RFI issues theres no need, if you do, isolating it like this imax has worked several times for my locals.

 
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Heavy Metal said:
View attachment 21313
What I did
Click to expand...
Maybe, if you're trying to isolate the RF from the coax below the choke, you might also want to avoid laying the coax directly on the metal both above & below the choke. Should probably be an insulator stretching a little above & below the choke.

Also, it kinda appears the antenna isn't insulated from the mast because the U-bolts look like they connect both the antenna mounting plate & the mast.
 
Needle Bender said:
Maybe, if you're trying to isolate the RF from the coax below the choke, you might also want to avoid laying the coax directly on the metal both above & below the choke. Should probably be an insulator stretching a little above & below the choke.
Click to expand...



Maybe some pvc cut it and then use clamps
To recluse it around might be better to isolate it too.
 
I agree with SP5IT, no need to.

The antenna radiating part is aleady isolated from the U bolt bracket , this bracket is where ground radials go.

The metal mast provides a D.C.ground and your polyphaser will break the coax ground shield for extra protection from nearby strikes or high static discharge. Be sure to use at least one 8ft ground rod and preferably attach it to the polyphaser. 3 ground rods are better though.

Use a 1:1 balun or make one from coax just like you see in the above pics. This should keep common mode stray RF from traveling down the coax to your shack.

If you do all that, you shouldn't have any issues.

Can u get your antenna up to 36ft at the base?
 
fourstringburn said:
I agree with SP5IT, no need to.

The antenna radiating part is aleady isolated from the U bolt bracket , this bracket is where ground radials go.

The metal mast provides a D.C.ground and your polyphaser will break the coax ground shield for extra protection from nearby strikes or high static discharge. Be sure to use at least one 8ft ground rod and preferably attach it to the polyphaser. 3 ground rods are better though.

Use a 1:1 balun or make one from coax just like you see in the above pics. This should keep common mode stray RF from traveling down the coax to your shack.

If you do all that, you shouldn't have any issues.

Can u get your antenna up to 36ft at the base?
Click to expand...

Thanks. The base of the antenna will be about 31 feet above the ground.
 
Needle Bender said:
Anecdotal I know, but I've always had surprisingly good luck at 41', even with the GainMaster; YMMV
Click to expand...
Needle Bender , you have your feed point at 41 feet ? Are you using a push up mast or tower? Just curious as I have 3 ,, 10 foot masts telescoped 8" which gives me 27'. I'd like to try going higher .
 
I figure you already know it, but if you use those 10 foot sections over 30 feet, you will definitely have to guy the last two sections. That's if I'm understanding you correctly. If it's a mast and you've only got 8"s or so of 'overlap' you are really pushing it. Depends on the wind of course, but around here 2 or 3' is about as extended as you can safely go.
 
Brain the Dog said:
I'm fixin' to install my new A99 with the Ground Plane Kit. It will be attached to a steel pole that is 31-feet tall. Will have in-line polyphaser. Pole will be cemented into aground and bracketed to side of house.

Should I isolate the antenna from the mast pole? Is there a benefit? Is there harm if I don't? How do I isolate it?

In all of my CB adventures, I have never seen a base station antenna isolated from the pole. So, I'm just wondering.
Click to expand...

No, do not isolate the A99 from the pole. It will perform great with the radials and mounted directly to the pole. Additionally, mounting directly to pole will provide the DC ground to help bleed off any stray static charge ......
 
well, i know this is going to start a bit of a thing, but according to accepted antenna theory; a vertical ground plane antenna works off of its capacitance to ground, and if you don't isolate it from it's metal mast, you have a leaky capacitor.

that being said, there is good reason (safety) for having a metal mast going straight to the ground, and i believe the engineers at where ever decided what to put in CB antenna instruction books cared more about liability than correct antenna design.

also, from what i have read elsewhere on this forum, you have to isolate the antenna to the point where the metal mast is below where the radials hang or they will just induce their currents into the mast. If that is incorrect, someone please let me know.

LC
 
I'm sorry LC, that's wrong. I'm not going to try to explain it, it wouldn't be a short post at all (and I'm not much of a teacher anyway). The thing is designed to be bolted to a mast, and so at least partially 'grounded'. Why not do it as the instructions say?
 
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