My CP's all had 6812's Specified. But the Equiv
> was the SWR FM-1
> It also specifed 1 bay.. I would Love to Lower the TPO and
> Put up a second bay
> But Feel I would be In some sort of Violation.
You are allowed to change your antenna without filing for pre-approval, assuming the change does not modify HAAT over 2 meters, and the modification does not change your specified ERP. You will need to recalculate TPO to give the same end result that your current license calls for. That calculation must include compensation for any increase in height. (For LPFM, more height equals less power). Once you have changed the antenna, you have 10 days to file a new Form 319 where you apply for a Minor Modification to an existing station. Then you wait for FCC approval, which may take many months. But you can stay on the air. It is a good idea to post a copy of the application next to your current license in the transmitter area, in case you are inspected.
As for what antenna to use, that has a lot to do with what you are trying to do, and where you are trying to do it. In many instances a single bay works just fine, especially if the intended audience is located nearby. On the other hand, if the antenna is on the outskirts of town, a two bay antenna close spaced (like 1/2 wave) will tend to put more signal towards the horizon and less on the ground or up in he air at the transmitter site. The same two antennas will give you gain, (or more properly stated: increase their efficiency) if you space them further apart, say 7/8 or full wave. Half wave spacing is not very efficient. You get your choice of some pattern control or efficiency, but not both. There is no free lunch.
Exactly how 1/2 wave spacing performs will vary form one antenna design to another. The best bet is to consult with various manufacturers to see what they predict their antennas will do in a real world environment. They are usually more than happy to share very detailed information about their product. If they can't or won't, I suggest finding another vendor.
Keep in mind that how the antenna is mounted on the tower will also affect performance. Usually antenna specs are measured by mounting them on a mast that sticks above the tower. If you mount the antennas on the side of a tower, you will get different results. It is a good idea to let your antenna vendor know how it needs to be mounted. They may have some very good suggestions and may be able to save you a lot of grief. I also suggest talking with the manufacturer’s actual engineers, not just a sales guy at a supply house. The engineers do this for a living. Keep in mind that you can tell if a salesman is lying by observing if his lips are moving.
I've found out the hard way that antennas seem to vary quite a bit from one manufacturer to another. Maybe you really do get what you pay for. When our station first signed on, we used a pair of inexpensive (but well known) antennas 7/8 wave spaced. They usually worked fairly well, but we did have a lot of "picket fence" problems. Even close by, there were some areas where coverage was very spotty. When it rained, our coverage went to next to nothing, although the SWR meter on the transmitter never indicated a horrendous mismatch. (Yes, the meter works).
We eventually got tired of paying the tower climbers to come out and inspect for a problem, which they never could find. The cure was two new antennas from a different, and well respected, manufacturer. They were quite a bit more expensive than our first set. And yes, they are half wave spaced. Used this way, they are not very efficient in the TPO department. While we were at it, we changed the 1/2" foam feed line, "just in case." So far they have been quite satisfactory. The "Picket Fence" effect has vanished, and we stay on the air when it rains.
Since this episode, I’ve had experience with translators that are located within the city limits of two nearby communities. Both of these use single bay antennas that are of similar design to the ones we use at the station. No weird spacing, no black magic. Just single bay antennas connected to the transmitter with 1/2" foam coax. They both do a great job
I’d have to conclude that half wave spacing can be a good idea under the right circumstances, but I doubt that it is an “instant cure all.” It you really want to get into the 1/2 wave spacing game, a better idea might be to use three or four bays, so you get gain and pattern control. But somewhere along the line, you have to ask yourself if the extra cost and complexity is worth a small change in the end result? The difference we experienced may be as simple as changing from a crappy designed, cheaply built antenna to a piece of quality equipment.
YMMV
