Antenna spacing?

[Blaise]

I'm thinking of putting antennae in the tower on my Victorian home.  It's indoors, but there's basically nothing around at that height for a mile or so (other than the half-inch of plywood/asphalt), and nothing significant for several more, so I feel like I should get pretty solid range out of the setup.  I have concerns, though, about having a cb antenna in close proximity to a gmrs antenna. 

Will they interfere with each other?  If so, how bad would that interaction be, and what kind of spacing is appropriate?

[SteveShannon]

They will affect each other, but much depends on the distance and direction.  The ffect might not be negative and I suspect nobody can accurately predict what it may be.  Each antenna could act as a reflector for the other, increasing gain slightly in the other direction, but the same is true of any metal, such as rain gutters.  

With all that said, I would try it.  Since it’s inside you could pretty easily try it with one antenna and again with the other antenna and finally with both.  You might be pleasantly surprised. There are lots of hams with wire antennas in their attics who make out just fine. 

[mbrun]

I'm thinking of putting antennae in the tower on my Victorian home.  It's indoors, but there's basically nothing around at that height for a mile or so (other than the half-inch of plywood/asphalt), and nothing significant for several more, so I feel like I should get pretty solid range out of the setup.  I have concerns, though, about having a cb antenna in close proximity to a gmrs antenna. 
Will they interfere with each other?  If so, how bad would that interaction be, and what kind of spacing is appropriate?

Yes they will absolutely interact with one another. Whether they interact with one another in an undesirable way under your specific circumstances is a complete unknown. However, given that the wavelengths of the GMRS antenna and 11m CB antenna are substantially different, I suspect you would be hard pressed to detect any adverse effect one had on the radiation or pickup pattern of the other.

The concern I would raise is the potential damage a transmitting radio could have on the receiver of the other. If too much power makes it into the receiving radio from an adjacent transmitting antenna you could damage it. The manufacturers do not widely publish a maximum input power rating on there radios so it is a bit of a guess. The number I use for basic max input calculations is 20dBm, which is based on a max permissible value I found from an ICOM service manual aways back.

If two antenna are used for both Tx and Rx and cannot be sufficiently separated horizontally then installing them co-linearly is preferred as it allows you to take advantage of the off-axis nulls in their radiation patterns to increase loss from one radio to another. That is how I fly, and I have ample loss from one to the other at the frequencies in question.

I wish I could give you a specific distance value, but absent the ability to measure the performance of the specific antennas proposed any numbers are a pure guess. If I where in your shoes, I would mock up my proposed mounting solution(s) in my back yard and take measurements to confirm which solution(s) yielded adequate RF path loss.


Michael
WRHS965
KE8PLM

[Blaise]

Hey, thanks for the responses. 

I may just start with one, and see how it goes.  I don't think the peak of the tower is tall enough to mount both antennae colinearly, but I still haven't cut the hole and measured yet!