FM Transmitter Power vs. FM Antenna Gain

Ok, I'll admit I'm mostly a programming guy trying to learn more about broadcast engineering -- so forgive my question if it sounds dumb, but here goes:

If hypothetical FM station "Z" is authorized as a non-directional C3 with 13kw ERP at 450 feet HAAT, is there any advantage to the signal to having fewer bays on the FM antenna and greater transmitter output power?

Or is there an advantage to having less transmitter output power and more bays (more gain)--or is it a wash?

In other words in this example: A 5 kw transmitter with a six bay antenna at 450 feet? Or a 10 kw transmitter with fewer bays at 450 feet? Either configuration would need to equal 13kw ERP.

By "advantage", I'm referring only to the signal strength - does one configuration or the other provide a signal that travels farther, a signal with less multipath, or a signal that penetrates buildings better?

Thanks!

Assuming that the radiation patterns and max ERP of these antennas in the horizontal plane is the same for whatever number of bays is being considered, then the biggest difference in their coverage performance will take place close to the transmit site.

If the antenna is on top of a building in a major metro area, then the penetration into the nearby buildings will be better with fewer bays. Example: the FM stations on Sears (Willis) Tower in Chicago all use single bay antennas -- to maximize the fields near their antennas.

If the transmit site is located away from the center of the metro area, then more bays with less transmitter power might be a better choice.

The ground elevations around the transmit site also have an effect (smooth, hilly etc). Each situation really needs to evaluated for its specific conditions.

A quick read of the papers posted at http://rfry.org will give more background on this, especially paper 6.

In a different post, I described it like this... A single bay antenna will give you a signal that is spherical (like a basketball). So you get a lot of power going straight up and down. As you add more bays, it takes the signal and flattens it out, like an oval.

As R. Fry described, you want more bays if your out of town because you don't want to waste your power going up and down.
There is no quick equation (because antennas are different), but you can use a smaller transmitter with a bigger antenna to get 13kW. Just a quick guess, but a 2-3kW transmitter into a 10-12 bay antenna will get you 13kW. Where a single bay antenna might actually need 14-15kw transmitter to get 13kw ERP. So, your power bill will be alot higher with the bigger transmitter/single bay setup.

Also, as R. Fry pointed out, if you are in a hilly area, a really flat signal will get a lot of dead spots because it won't fill in the valleys very well. So, in that case, the bigger transmitter/less antenna setup might work better.

Lazy J said:

In a different post, I described it like this... A single bay antenna will give you a signal that is spherical (like a basketball). So you get a lot of power going straight up and down. As you add more bays, it takes the signal and flattens it out, like an oval.

Click to expand...

Just to note that this conclusion is true only when each bay is an isotropic radiator. But such isotropic radiators do not exist in the real world -- they are only a theoretical concept.

The free space radiation patterns of a one-bay, and vertically-stacked arrays of more than one bay of most of the "circularly-polarized" FM transmit antennas such as the Dielectric/SPX DCR series, the ERI "Rototiller," and the Jampro "Penetrator" and their copies have very low relative fields at +/- 90 degrees in the elevation plane (e.g. they are non-spherical) -- because none of those elements has even the theoretical, free-space az/el radiation patterns of an isotropic radiator, particularly after being installed on a tower.

I was trying to not over-engineer an answer, just give a general idea to explain the pros and cons.

Sometimes less is more.

But thank you for the clarification, correct information is the best information!

We have a 2 bay "6 kw" Class A in hill country, we share a tower with a "3 kw" Class A using a single bay. There's no real significant difference in height between the two antennas, the antennas are the same make (ERI), though mounted on slightly different sides of the tower, and it is a narrow tower (18" Rohn 45G).

Yet some places in town they have a better signal than us. Farther out, of course, the difference in ERP is more obvious.

Physics and theory aside, as a practical matter 1, 2, or 3 bays will generally work best for Class A stations, three to six bays for higher power stations. Above that, some installations will work just fine, others just don't seem to work at all. Especially these multi-bay 12+ monsters. Much of it has to do with antenna design and installation planning. And some antenna designs just work better than others.

In the end, especially for lower power stations (Class A, B-1, C-3, and educational equivalents) the advantage, one way (Higher power transmitter/fewer bays) or another (lower power transmitter/more bays) really lies with the total economics of the project.

Great information. I have also often wondered the differences and reasons why some stations choose multibays vs some that do not. Thanks guys. We need more posts like these.

Also note that if you are using a 2 bay full wave spaced antenna there is a significant amount of the RF going down not out. The better choice is a 3 bay half wave spaced antenna, same gain and tower real estate needed as a 2 bay full wave but the RF lobe out is much better. I never install 2 bay full wave antenna's anymore.

I suspect it might come down to a dollar andd cents thing with ownership. Antenna's are cheaper than transmitters, so I know a couple of stations I worked for opted for lower power transmitters and muti bay antenna's to supposedly make up the difference. In my years when I sold two way radios I perferred RAW GUTS POWER to antenna gain anyday. Just my take on this.

Just one further comment. I'm amazed at the number of stations that don't range test their antenna. Yes it costs more, but you know what you're getting that you're going to live with for the next 30 years. Only ERI and Jampro can do full range modeling.

CleanAudio said:

Also note that if you are using a 2 bay full wave spaced antenna there is a significant amount of the RF going down not out. The better choice is a 3 bay half wave spaced antenna, same gain and tower real estate needed as a 2 bay full wave but the RF lobe out is much better.

Click to expand...

At least in the US, the peak, omni, RMS ERP in the theoretical, free-space, "RF lobe out" is the same for all numbers of vertically-stacked bays for all spacings from full-wave to half-wave.

US stations are licensed for a given ERP, not the input power to their transmit antennas (whatever their peak h-plane gain).

The biggest difference in the field intensities that any of those configurations can produce over their coverage areas occurs relatively close to the transmit site.

An illustration of this is shown in paper 5 at http://rfry.org.

To make sure I'm getting what's being said here: all other things being equal (which I know they never are!), you are trading horizontal distance for downward signal strength as you reduce bays, yes?

If I've described this somewhat accurately, is there a predictable, expected "shadow" from a multi-bay antenna installation? In other words, is there a certain number of degrees angle you can expect reduced signal at, and is it a slow fade or is there more a "wall," a point where suddenly the signal improves as you're driving away from the tower?

Also, would it be fair to say that the lower the antenna, the safer more bays are because you're not having to radiate as far down to the ground?

Is it possible to use SO many bays that you're actually putting more signal vertically into the air than horizontally?

I've certainly understood the concept that more bays require less input for an equal ERP which saves the owners money... I'm curious what happens if you have the money to do either with no consequences. (Maybe the owners have a windmill farm on the transmitter property! lol.)

Sadly, I looked at the above mentioned link and it flew right over my head... I suspect my answers are there, but I didn't "get" what I was looking at. Any further "dumbing down" of this for me would be greatly appreciated.

NightAire said:

...you are trading horizontal distance for downward signal strength as you reduce bays, yes?

Click to expand...

If all FM transmit antennas really had the standard, theoretical radiation patterns calculated/expected for them, and if the station met their FCC license, then all of them would radiate equal power in the horizontal plane. The only differences that would occur would take place relatively close to the transmit site. In average FM broadcast installations most of the coverage area served by the transmit antenna lies within the narrow sector of its elevation pattern between zero (the horizontal plane) and -3 degrees, where the relative field of the elevation pattern is nearly the same, no matter how many bays are used.

The link below leads to a graphic comparing the field intensities produced by a 14-bay and 6-bay antenna when both radiate the same ERP from the same height above average terrain (HAAT). This is a fairly extreme comparison to illustrate the point in the above paragraph.

...is there a predictable, expected "shadow" from a multi-bay antenna installation? In other words, is there a certain number of degrees angle you can expect reduced signal at, and is it a slow fade or is there more a "wall," a point where suddenly the signal improves as you're driving away from the tower?

Click to expand...

Over most of their coverage areas the two antennas in the graphic would provide equal signal strengths (other things equal). For this 1,500 foot antenna height the signal nulls shown in the graphic all are located within 4 miles of the transmit site.

However in practice the nulls are not as deep as this due to signal reflections that tend to fill them. That could produce narrow zones of multipath distortion in some FM receivers in those nulls -- but in general the field intensity close to the transmit site is very high no matter what antenna is used, due to the short distances involved.

Also, would it be fair to say that the lower the antenna, the safer more bays are because you're not having to radiate as far down to the ground?

Click to expand...

Not really, because the distance to a receive site miles away from the transmit site is virtually the same no matter how high the transmit antenna is installed.

Is it possible to use SO many bays that you're actually putting more signal vertically into the air than horizontally?

Click to expand...

Not if the antenna is designed, installed, and operating correctly.
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Typical side-mounted FM broadcast antennas seldom have the theoretical patterns predicted for them, due to pattern distortions from the mounting structure. For this reason it is probably best to limit the number of full-wave-spaced bays used to around 8 or 10 (opinion).

Each situation is unique, and for best system performance the choice of the antenna should be well investigated, including the purchase of pattern testing by the antenna manufacturer using a model of the antenna bays proposed when installed on a tower section similar to that for the final installation.

http://i62.photobucket.com/albums/h85/rfry-100/CoverageComparison.gif

VERY interesting... thanks for the info!

rfry said:

The link below leads to a graphic comparing the field intensities produced by a 14-bay and 6-bay antenna when both radiate the same ERP from the same height above average terrain (HAAT). This is a fairly extreme comparison to illustrate the point in the above paragraph.

Click to expand...

Update to fix URL issue: Here is the graphic referred to above.

(Click on image to expand its size)