I've got a gr300 repeater coming to me tonight, and I'm using a 2 antenna setup in a vertical configuration. Antennas will be mounted on a 20ft push up pole. The pole will only be extended roughly 15 ft. Property is 452 ft above sea level, and the average terrain is much lower than that.
Now to the questions. How far apart do my antennas have to be (keep in mind, vertical config)?
Is it necessary for my receive antenna to have a ground plane?
Lets put it in technical terms. In order for a repeater to play correctly, it needs some 90 to 100 db of isolation between the RX and TX antenna signals. How you obtain this is up to you. It can be cavities, vertical separation or horizontal separation.
I will say that horizontal separation is much greater than vertical separation. I am sure if you do a search on the internet for horizontal antenna separation for repeater antennas, you can obtain just what is needed for spacing between the antennas.
Your next issue will be just how good the coax cable is for keeping leakage down. Don't forget the shielding that is needed between the receiver and transmitter. The better the receiver pass band is the better your system will work. How well your receiver reacts to high level signals off frequency is very important.
A simple single pass band cavity on the transmitter will clean up any noise and crud it transmits. You don't have much control over this other than not using some cheap japtrack radio. Same goes for the receiver. A pass cavity on it will help a bunch.
You may not want to buy a duplexer, but investing on a couple of pass cavities will make a world of difference on just how well your package performs.
Trying to run your antennas at only 15 feet is a waste of time. Your not even above the local trees at that point. I doubt your going to hear the repeater more than a mile away at best. Getting back into the repeater with a portable will probably not even be that far.
