Our pilot boxes need to be replaced. We now have six, each 25 feet from its' neighbor. The prevailing wind is from the Easterly direction and 90 % of our takeoffs are toward the East end of the runway. The easternmost box gets buzzed occasionally when a plane rolls left on takeoff. The westernmost box gets buzzed sometimes on crosswind landings. There are times when pilots at one end do not hear the call outs from the other end because of the distance and background noise.
I would like to reduce the number of pilot boxes to four, which is our max for the number of flyers at one time and also reduce the spacing between the boxes. How close can the boxes be placed and still avoid radio interaction if we use only modern radios?

For the most part, the pilots can be almost shoulder-to-shoulder and fly safely. If there are going to be frequency problems, you'll get them when pilot A's airplane is close to pilot B's Tx when A and B are at the ends of the flight line from each other.

To get the possibility of intermodulation interference, you'll need at least 3 transmitters operating on evenly-spaced channels, such as 11, 13, and 15, for example, but 11, 22, and 33 could also cause a problem. If a LOT of people are flying, take any 3, and a possible problem could result.

Even so, this type of interference has been extrememly rare at flying fields. It's only a potential difficulty.

The formula involves any three channels: X, Y, and Z.

If 2X - Y = Z, you may have a difficulty. Any channel number can be used in any one of the variables. There is no order implied.

Again, this only dictates a potential, and not actual problem. Modern systems work very well in our R/C environment, and actual problems have been extremely rare, and usually with older equipment.

Thank you for your prompt reply.

Your first paragraph implies that our present layout with the pilot boxes far apart may pose more of a potential radio control problem than having the pilots closer together.

You stated: "For the most part, the pilots can be almost shoulder-to-shoulder and fly safely. If there are going to be frequency problems, you'll get them when pilot A's airplane is close to pilot B's Tx when A and B are at the ends of the flight line from each other."

Do you feel then, that having the pilots closer to each other reduces the likelyhood of control problems due to radio interaction?

Actually, the main point I was trying to make was that it wouldn't make much difference either way. Most types of interference caused by multiple transmitters being used wind up hurting old, cheap, or unmaintained equipment. Good equpment has few problems of any kind, except when you get a signal right on your channel, such as when someone on your channel turns on their transmitter in the pits.

The layout you mention would only pose a bit of a problem if the two guys at the far end from you are using channels that would wind up causing interference on your channel, and your airplane was much closer to them than to you.