TPO vs Antenna Gain

[Fieldtech1]

As someone still learning the art of broadcasting I have a question. What advantage would you have going with a high TPO vs a higher gain antenna in a non directional setup. I know the pattern becomes more pancaked looking, but are there any other caveats?

IE ;

KLTN Houston, TX Makes 100kw ERP with 51kw TPO

KXLI Moapa, NV Makes 93kw ERP with 9kw TPO

 

[Lazy J]

Higher TPO with less antenna gain gives much better building penitration. The Houston stations are very close to town, so you would want more power going down and building penitration is very important. Now KLXI in moapa is a rim-shot station, so you want very little power going down and you can forget about any building penitration.

Plus, on a side note, KLXI is run completely on generator. So they need as much coverage with as little TPO as they can get.

 

[TomT]

When you start to get to the extremes in high gain antennas, e.g., 10 or 12 bays, the signal is focused more on the horizon. End result may be to overshoot close-in areas. Sometimes you may want to do this,such as a mountain-top site where there is no significant population close in; and the antenna can be designed with some "beam-tilt" to spray the signal into the population centers in the valley.

In less extreme terrain, such high-gain, multi-bay antennas become a handicap. They look good sometimes on paper--since it is often cheaper to buy a multi-bay antenna & drive it with a small transmitter. Once built, they just don't seem to perform as well as similarly-situated stations with fewer bays and higher TPO.

Over the years, my observations are that 2 to 3 bays seem optimum for Class A stations, while 4 to six bays seem to be a good formula for Class B-1 or Class B stations (which we have here in the east).

 

[radiosaur]

When you start to get to the extremes in high gain antennas, e.g., 10 or 12 bays, the signal is focused more on the horizon. End result may be to overshoot close-in areas. Sometimes you may want to do this,such as a mountain-top site where there is no significant population close in; and the antenna can be designed with some "beam-tilt" to spray the signal into the population centers in the valley.

In less extreme terrain, such high-gain, multi-bay antennas become a handicap. They look good sometimes on paper--since it is often cheaper to buy a multi-bay antenna & drive it with a small transmitter. Once built, they just don't seem to perform as well as similarly-situated stations with fewer bays and higher TPO.

Over the years, my observations are that 2 to 3 bays seem optimum for Class A stations, while 4 to six bays seem to be a good formula for Class B-1 or Class B stations (which we have here in the east).

I never liked a Class C with more than an 8-bay, either. I’m a firm believer in more TPO, less antenna gain. If you can deal with the power bill.

There was one notable exception. WMC-FM Memphis is one of the “superpower” stations. We installed two BE 35 units at WMC-FM in Memphis in the early 90’s in an alternate / main configuration. 35 KW into a specially designed antenna for the 300 Kw H 100 Kw V signal. A 12 or 14 bay. Don't recall.

I tested the second transmitter into the dummy load weekly…until I found out (in a meeting with the GM) what a “demand charge” from the power company is.

 

[BobOnTheJob]

When you start to get to the extremes in high gain antennas, e.g., 10 or 12 bays, the signal is focused more on the horizon. End result may be to overshoot close-in areas. Sometimes you may want to do this,such as a mountain-top site where there is no significant population close in; and the antenna can be designed with some "beam-tilt" to spray the signal into the population centers in the valley.

In less extreme terrain, such high-gain, multi-bay antennas become a handicap. They look good sometimes on paper--since it is often cheaper to buy a multi-bay antenna & drive it with a small transmitter. Once built, they just don't seem to perform as well as similarly-situated stations with fewer bays and higher TPO.

Over the years, my observations are that 2 to 3 bays seem optimum for Class A stations, while 4 to six bays seem to be a good formula for Class B-1 or Class B stations (which we have here in the east).

If that was around 1970, I used to hear that blowtorch in Cincinnati a few times a year after 99.7 in Louisville shut down for the night.

I never liked a Class C with more than an 8-bay, either. I’m a firm believer in more TPO, less antenna gain. If you can deal with the power bill.

There was one notable exception. WMC-FM Memphis is one of the “superpower” stations. We installed two BE 35 units at WMC-FM in Memphis in the early 90’s in an alternate / main configuration. 35 KW into a specially designed antenna for the 300 Kw H 100 Kw V signal. A 12 or 14 bay. Don't recall.

I tested the second transmitter into the dummy load weekly…until I found out (in a meeting with the GM) what a “demand charge” from the power company is.

 

[radiosaur]

When you start to get to the extremes in high gain antennas, e.g., 10 or 12 bays, the signal is focused more on the horizon. End result may be to overshoot close-in areas. Sometimes you may want to do this,such as a mountain-top site where there is no significant population close in; and the antenna can be designed with some "beam-tilt" to spray the signal into the population centers in the valley.

In less extreme terrain, such high-gain, multi-bay antennas become a handicap. They look good sometimes on paper--since it is often cheaper to buy a multi-bay antenna & drive it with a small transmitter. Once built, they just don't seem to perform as well as similarly-situated stations with fewer bays and higher TPO.

Over the years, my observations are that 2 to 3 bays seem optimum for Class A stations, while 4 to six bays seem to be a good formula for Class B-1 or Class B stations (which we have here in the east).

If that was around 1970, I used to hear that blowtorch in Cincinnati a few times a year after 99.7 in Louisville shut down for the night.

I never liked a Class C with more than an 8-bay, either. I’m a firm believer in more TPO, less antenna gain. If you can deal with the power bill.

There was one notable exception. WMC-FM Memphis is one of the “superpower” stations. We installed two BE 35 units at WMC-FM in Memphis in the early 90’s in an alternate / main configuration. 35 KW into a specially designed antenna for the 300 Kw H 100 Kw V signal. A 12 or 14 bay. Don't recall.

I tested the second transmitter into the dummy load weekly…until I found out (in a meeting with the GM) what a “demand charge” from the power company is.

I'm sure you did. It has been 300 KW since sign on way back when.

 

[radiogooroo]

KLTN Houston, TX Makes 100kw ERP with 51kw TPO

In the case of this particular station, the supporting tower may have something to do with it too. It is on an extremely short guyed tower just east of downtown Houston. I think the antenna is on a modeled section bolted to the top. The tower might not be able to support anything taller up there.

The answer to the original question is simple - the fewer bays you have, the fewer nulls you have in the area immediately around the tower.

 

[Kmagrill]

KLTN Houston, TX Makes 100kw ERP with 51kw TPO

In the case of this particular station, the supporting tower may have something to do with it too. It is on an extremely short guyed tower just east of downtown Houston. I think the antenna is on a modeled section bolted to the top. The tower might not be able to support anything taller up there.

The answer to the original question is simple - the fewer bays you have, the fewer nulls you have in the area immediately around the tower.

This is a double edged sword. Antennas very close to the ground can suffer from significant multipath as a result of terrain reflection. In those cases, more bays are often better than less. Special interbay spacing may also be employed. I personally know of one class C station with a 5 bay antenna mounted on a 150' tower that suffered terribly from this effect. The station had far better coverage at 1kW ERP than at 100kW. The solution was to go with a 6 bay, .8 wave spaced antenna and the results were quite noticable. For high powered stations, I tend to believe that an antenna gain of around 3 (60' aperture) is good. For lower power, a gain of .5 to 1.5 is good. All of these are conditional upon the parameters of the specific installations. Back in the 1960s, most class C stations used 10 or 12 bay antennas with 20kW transmitters. In places where the antennas were situated in population centers, null fill was added and sometimes, beam tilt. Coverage was really pretty good in most cases. Starting in the late '70s and early '80s, combined transmitters with fewer antenna bays became popular. It became pretty common to see a 40kW or 50kW transmitter driving 6 or 8 bays. I installed a 70kW transmitter driving 4 bays in 1987, but was dissapointed with the results. The antenna was on a 300" tall tower, atop a tall mountain. I found that 38kW driving an old ring/stub 6-bay from the same site, actually produced better coverage than our shiny new ERI rototiller. That's when I started asking about other stations with similar problems and got plenty of stories back. Some attention to the unique character of the sites is required to get the best results.

 

[OKCRadioGuy]

We went from a 3-bay side-mount at 300 ft to a baby panel at 640ft (which is the equiv. of a one-bay). Building penitration isn't as good, but the overall coverage is much better. We achieved what we wanted. The fluttering and goofy stuff while driving mobile simply went away.