Placement of Two Antennas on One Mast - Overlap ok?

badspell68

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one - Diamond X300A Base antenna, 2m/70cm, UHF, 10ft
one - 20' CB antenna

I'm hoping to avoid mounting a horizontal addition to my mast....Can I place the CB antenna just below the GMRS antenna on the same mast, with the CB entering the space of the GMS, without it affecting eithers transmit and receive capabilities of the two antennas, or do I need to offset one outside the physical space of the other?
 

prcguy

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No good answer to this. Put the CB antenna low on the mast where the mast extends up into it and the tuning will be messed up and the CB antenna won't really be an antenna until it reaches the point where its taller than the mast. Then the 2m/70cm antenna sitting right up against it will not be much different than sliding the CB antenna most of the way down the mast so its an inch away for most of its length. The operation of the 2m/70cm antenna will not be much different.

The 2m/70cm antenna is longer than 1/4 wavelength on CB and will have a definite influence on the CB antenna. If the 2m/70cm antenna were much smaller like 5ft tall then you could put it on an arm that sticks out away from the mast a good 6ft and the CB antenna would not no its there and the radiation pattern of the 2m/70cm antenna would be somewhat omni directional with some gain lobes and lossy nulls since it would only be about 1 wavelength out from the mast. You really need to get an antenna at least 2 wavelengths out from a mast if you want a reasonable good omni pattern and that would be about 12 1/2 feet out from a mast on 2m and 4ft out for 70cm.

one - Diamond X300A Base antenna, 2m/70cm, UHF, 10ft
one - 20' CB antenna

I'm hoping to avoid mounting a horizontal addition to my mast....Can I place the CB antenna just below the GMRS antenna on the same mast, with the CB entering the space of the GMS, without it affecting eithers transmit and receive capabilities of the two antennas, or do I need to offset one outside the physical space of the other?
 

badspell68

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Bummer...This stand even though I'll only be running one unit at at time, not having both on?

Or, is it best to mount the GMRS on the very top of the 20' mast then the CB several (how many feet away??) 20' under the GMRS mounted directly to the house.
 

prcguy

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A 1/4 wavelength at CB frequencies is about 9ft. If you were to mount the CB antenna 9ft away from the 20ft mast with or without another antenna on top, your omni CB antenna would be turned into a 2 element directional Yagi antenna. You would have to get the CB antenna a good 2 wavelengths away from the mast to enjoy anything close to an omni directional pattern, and that would be about 70ft away. If you placed the CB antenna 1 wavelength away from the mast or about 36ft it would have a clover leaf pattern with four gain lobes and four lossy nulls. You can't change the physics of antennas.

I would put the 2m/70cm antenna on top of your 20ft mast where it will work best up high and probably have a short run of coax to your radios then put the CB antenna on the other side of the house on another mast or even at roof level. Coax loss is less at CB frequencies so a longer run is not that bad.

Bummer...This stand even though I'll only be running one unit at at time, not having both on?

Or, is it best to mount the GMRS on the very top of the 20' mast then the CB several (how many feet away??) 20' under the GMRS mounted directly to the house.
 

badspell68

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Interesting Idea...

Going to use LMR-400 on the GMRS. But are you saying I could use RG-8X for a full run on the CB ?
 

prcguy

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No. For a given frequency range you need to keep things a minimum distance away in wavelengths. A full wavelength at CB is about 36 ft, so if you have a 1/4 ground plane or a 2ft tall Firestick or a 1/2 wave A99 or a Maco 5/8, they will all suffer the same problem with a four lobe clover leaf pattern at 36ft from a tall mast. Put any of them 6 to 10ft away from a 20ft mast and they will all become a 2 element beam.

A full wavelength at 70cm is about 2ft, so that would be equivalent to a CB antenna 36ft from a tall mast. Put a 70cm base antenna six or ten feet away from a tall mast and its just fine where a CB antenna would be pissed off.

Just realized that my CB antenna is 5/8th. Does that change any placement variables in my favor; to place the antenna closer to the GMRS?
 

badspell68

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New Hope - PA
The GMRS is also 5/8th and is 10' it's going to be on a 30' mast. The minimum distance for the CB from the GMRS should be?

Additionally, would placing the CB antenna, grounded of course, in a tall tree be an issue?
 

badspell68

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New Hope - PA
Considering all the issues with a steal mast. I decided to bolt the antenna directly to the wood house. In this scenario do I ground the antenna itself to a grounding rod with #6 copper or the RG-8X to the grounding rod?

Thanks for all the info!
 

WA0CBW

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Yes! The "ground" point on the antenna (the mast bracket) needs to be connected to the ground rod. The coax shield (braid) needs to be connected to the ground rod as well. And that ground rod needs to be connected to the main electrical ground as well.
Bill
 

badspell68

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Wont the ground wire become part of the antenna increasing its size and be part of the transmission? Connect the new ground rod to the houses ground rod? Wont that bring a lightning strike into the house?
 

mmckenna

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Wont the ground wire become part of the antenna increasing its size and be part of the transmission? Connect the new ground rod to the houses ground rod? Wont that bring a lightning strike into the house?
If your antennas are designed correctly, it won't be a problem.

National Electric Code —requires— bonding the grounds together. The energy will take the shortest path to ground. Bonding the two ground rods together keeps them at the same potential.
 
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