How does FM directionality work?

[johnbasalla]

A non-engineer is asking this question. How does the pattern set-up work? If a station gets approval to change its pattern, let's say to provide better coverage in one direction (Northeast, or whatever), what needs to be done?

 

[OKCRadioGuy]

From what I understand a tower has to be documented as much as possible with it's respect to placement of the face of it with respect to north (surveyor) and careful documentation of where things are on the tower such as other antennas, coax, and othr stuff like that. Also they need to know the size or type of tower and if it's uniform all the way up. Then, the litterly replicate what you have on your tower at the antenna manufacure's plant. They work with modeling and real-world until your limits are matched with resoectto the da license. Then you hang the antenna in exactly the right spot onthe tower exactly so many degrees with respect to north, and have a surveyor certify it. Then you can get your license to cover etc. You'll have to go through all of this if you change the DA in any way or something close to your DA changes. Fun times .

 

[RadeoEngineer]

From what I understand a tower has to be documented as much as possible with it's respect to placement of the face of it with respect to north (surveyor) and careful documentation of where things are on the tower such as other antennas, coax, and othr stuff like that. Also they need to know the size or type of tower and if it's uniform all the way up. Then, the litterly replicate what you have on your tower at the antenna manufacure's plant. They work with modeling and real-world until your limits are matched with resoectto the da license. Then you hang the antenna in exactly the right spot onthe tower exactly so many degrees with respect to north, and have a surveyor certify it. Then you can get your license to cover etc. You'll have to go through all of this if you change the DA in any way or something close to your DA changes. Fun times .

There are only two companies that can do full scale modeling, ERI and Jampro. All others use scale modeling which IMHO is less accurate. To make an FM antenna directional, you employ passive reraditors to shape the pattern. It is correct that you have to have the entire support structure replicated to do this correctly, and it can take days if not weeks to get the pattern to work on the test range. Then you have to rely on the tower crew to place everything exactly as prescribed on installation at the working site. Any antenna hung on any structure made of metal will have some kind of directionality to it. It can't be avoided.

I think the only way to get a truly non-directional FM antenna would be to have it constructed as such that the feed line is also the support structure and runs right through the middle of the bays. I don't know if that has been or even could be done. Jampro makes a monster, circularly polarized spiral antenna that may come close.

If you think about hanging a lantern up and how it lights in a 360 degree pattern, then hold a mirror up to one side of the lantern, it would reflect the light in the direction of where the mirror is pointed, increasing the light intensity in that direction, and block the light that would have gone behind the mirror. That's sort of how a directional antenna works.

 

[johnbasalla]

Thanks for the explanation. Now, if a station suddenly isn't getting the coverage its supposed to in a certain direction, would that mean the problem is in the antenna system, the tower and/or those units that insure the directionality? It wouldn't be a transmitter problem?

 

[RadeoEngineer]

Thanks for the explanation. Now, if a station suddenly isn't getting the coverage its supposed to in a certain direction, would that mean the problem is in the antenna system, the tower and/or those units that insure the directionality? It wouldn't be a transmitter problem?

Tough question to answer without more info. Maybe you're having a problem you need to be more specific about? You could have a transmitter that isn't making power and that would result in a coverage loss in all directions of its pattern. You could have something local to the receiving point that suddenly caused a loss of reception, such as an interfering factor, a crane that just went up, a local 1st adjacent that just put on HD, a broken receive antenna and so forth. You could have some kind of damage to the transmit antenna that could be lightning strike, a big guy with a boot on an element up the tower, a burned element, a meteor hit to some part of the system, a bullet hole somewhere and the list goes on forever. Gotta be a little more specific about what's going on mang.

 

[w9wi]

A non-engineer is asking this question. How does the pattern set-up work? If a station gets approval to change its pattern, let's say to provide better coverage in one direction (Northeast, or whatever), what needs to be done?

A bit off-topic but, you can't use a FM directional antenna to improve coverage beyond what would be possible with a non-directional antenna. You can use it to *reduce* coverage in the direction of another station, to allow the use of a transmitter site that would interfere with the other station if a non-directional antenna were used.

 

[PTBoardOp94]

A bit off-topic but, you can't use a FM directional antenna to improve coverage beyond what would be possible with a non-directional antenna. You can use it to *reduce* coverage in the direction of another station, to allow the use of a transmitter site that would interfere with the other station if a non-directional antenna were used.

I've never worked on an FM DA, but ... well, a scenario:

If I have a 6kW class A, then I'm told that my current mast is in poor shape and should be replaced. So I have a tower crew come and install a new tower to the same specifiacation on the same site. I also install a new directional FM antenna.

I'm licensed for 6kW ERP, so if the new antenna is less efficient, I can boost the TX input power. Would that not improve coverage in the main lobe of the antenna and reduce it in the nulls? I have 6kW coming out of the antenna, and that power has to go somewhere.

I guess I'm missing something.

 

[bossbill]

If I have a 6kW class A, then I'm told that my current mast is in poor shape and should be replaced. So I have a tower crew come and install a new tower to the same specifiacation on the same site. I also install a new directional FM antenna.

I'm licensed for 6kW ERP, so if the new antenna is less efficient, I can boost the TX input power. Would that not improve coverage in the main lobe of the antenna and reduce it in the nulls? I have 6kW coming out of the antenna, and that power has to go somewhere.

I guess I'm missing something.
[/quote]
If you install a directional FM antenna, that would be only because the station is short-spaced and in that case the antenna specs would be part of your license. Assuming that's the case, then the transmitter power is adjusted to provide the 6kw ERP in the main lobe. It's not like AM, where you have a power output which may end up with more power in the main lobe and less in the null. With FM the main lobe cannot exceed the ERP for the class. You don't lose anything. The antenna "gain" is calculated for the main lobe. That number is used in conjunction with transmitter power to reach the kw ERP.

 

[w9wi]

I've never worked on an FM DA, but ... well, a scenario:

If I have a 6kW class A, then I'm told that my current mast is in poor shape and should be replaced. So I have a tower crew come and install a new tower to the same specifiacation on the same site. I also install a new directional FM antenna.

I'm licensed for 6kW ERP, so if the new antenna is less efficient, I can boost the TX input power. Would that not improve coverage in the main lobe of the antenna and reduce it in the nulls? I have 6kW coming out of the antenna, and that power has to go somewhere.

I guess I'm missing something.

May not have made it clear that this is a FCC thing, not a technical thing. It is certainly possible, from a technical standpoint, to use a directional antenna with an existing transmitter to increase coverage in the main lobe. But the FCC won't let you do it. At least not without upgrading the station to a higher classification.

You're a 6kW Class A with a non-directional antenna. You replace that non-directional antenna with a directional unit. Your license will specify a transmitter output power that will provide an ERP of 6kW in the direction of maximum radiation. Your coverage will not change in that direction. The ERP will be less than 6kW in every other direction -- your coverage will be reduced.

_________________________________________________

Now, what you may be able to do... is use the directional antenna to allow an upgrade to a higher class, say, C3.

Say that your 6kW non-directional Class A is just barely adequately spaced from another station on the same frequency.

You might install a directional antenna with a 6dB null in the direction of that other station. Now, you can increase ERP in all other directions to 25kW - the 6dB null limits the power in the direction of the other station to the same 6kW that just barely fits.

There's more to it than that (from a regulatory standpoint) but that's why you would want to use a FM DA.

 

[ChiefEngineer]

The FCC has looked at good engineering practice in the past concerning mounting of antennas.

In Florida there are DA and non DA using the same antenna. This is ebcause the envelope pattern of the DA does not exceed the omni pattern by xx%.

Many stations mounted on a 12 foot face tv tower use this to their advantage. A 6 db gain is real and possible. 6 kw becomes 24 kw in one direction. While the station is licensed for 6 kw operation, the signal performs as a B in the direction of gain. This is used if your station is serving an area that directioality is not a problem.

 

[PTBoardOp94]

Thanks guys. I'd known that FM DAs required commission approval, but had forgotten about it.

 

[rfry]

The bottom line here is that the ERP of a directional FM station is licensed for its authorized radiation envelope in all compass directions. The maximum ERP of directional FM stations cannot exceed the maximum for that class of FM station, and the minimum ERP(s) must meet the criteria required to protect other FM stations.

An omnidirectional FM station is licensed on the RMS gain of its radiation pattern, regardless of its real gain and ERP in various compass directions -- even if, as a result of pattern distortion by the tower and other parasitic re-radiators in and near the antenna aperture, that ERP may exceed the RMS value of the azimuth pattern in some compass directions.

As others have pointed out, this means that an "omni" FM station legally can radiate more than its licensed ERP in some directions, while a directional FM station cannot.

These points are developed in Paper 6, and in slides 12 and 13 of Paper 10 at http://rfry.org .

RF

 

[littlejohn]

Any sidemounted FM antenna should be optimized to the tower for best circularity. Unless you are very lucky, optimization will result in much better coverage. Of course, once optimized, one oght also to point the abberations left - and there will be some - in the most advantaqgeous direction. ERI builds a section they call a lambda section which is to the degree possible noinresonant. You can get a fairly nice pattern out of one of them. Tricks used to obtain circularity on large support structures also include antenna spacing from the structure, placement on the structure, parasitic radiators both horizontal and vertical, and in some cases (about ten percent) reverse CP. ERI keeps records of everything they do. I first learned of optimization from a gent named Dan Dowdle in the 80s, we spent a week getting a really good pattern out of a G-5 antenna mounted on a ten foot face tower. Aboiut as important as getting the pattern round is getting the horizontal and vertical components as close to each other as possible. I understand Dowdle is still doing it for them, the intervening 25 odd years experience plus records mean that it goes a lot faster now.
W9WI is correect. Empirically, I find the directionals end up reducing transmitter power over the full class facility by anywhere from 20-22% if you're lucky to about a third if you aren't. Remember, the 'round' (non-D) facility contours are calculated using the tables in Part 73. For a directional array, you have to measure the antenna in the real world (either full scale or modelled) and cannot exceed the calculated power in >any< direction. The calculated contours always allow for more power than the measured ones.

 

[radiorob2.0]

I tried to operate a FM directional. It work well until the bush hog mowed down the quarter wave radiators. ;D

 

[karasak]

I use directional fm (vertical polarisation) now but i'd like to use circular polarisation with 4 stack crossed dipole (http://www.bdnow.com/UploadFiles/ACP1.pdf) . I don't need omnidirectional horizontal pattern because big mountain chain is behind my transmittter site so i need let's say 180 degrees in horizontal.Here's image of my mast http://picasaweb.google.hr/lh/photo/9rroZgwHf6h4MsO9EkZJAw?feat=directlink . I have problem: 3 dipole could be mounted on thinner must on the top(where dipoles are now) but fourth must me side mounted (where panel antenna is now). Old antennas will be detached from tower.Could this 4th dipole change horizontal pattern, i don't need omni pattern.
Kreso

 

[Nostalgia]

I use directional fm (vertical polarisation) now but i'd like to use circular polarisation with 4 stack crossed dipole (http://www.bdnow.com/UploadFiles/ACP1.pdf) . I don't need omnidirectional horizontal pattern because big mountain chain is behind my transmittter site so i need let's say 180 degrees in horizontal.Here's image of my mast http://picasaweb.google.hr/lh/photo/9rroZgwHf6h4MsO9EkZJAw?feat=directlink . I have problem: 3 dipole could be mounted on thinner must on the top(where dipoles are now) but fourth must me side mounted (where panel antenna is now). Old antennas will be detached from tower.Could this 4th dipole change horizontal pattern, i don't need omni pattern.
Kreso

From the picture shown all I see are 2 omnidirectional vertically polarized ants up on the upper mast, and two seriously directional panel ants below them. Assuming yours are the ones up on the mast, they are not directional, no reflector, no parasitic elements. If you are counting the mast as the reflector, well then thats another story.

As to your question, based on the element spacing, .5 or full wave or something in between, you would need to keep all the elements together in the space allowed. I dont see any scenario where you could mount your 4th element among those panel antennas without causing pattern distortion, for both you and that user.

And dont confuse polarization with directional, two different things. That RVR antenna is omni DIRECTIONAL as shown, but circularly POLARIZED. I would get a good local engineer involved and come up with a solution that will meet your requirements- available space on the tower, and not wasting power toward the mountains. Good Luck!

 

[karasak]

Dipoles and directional panels below WILL BE removed from mast, because they are over 25 years old, so on empty mast we will attach 4 stack circullar crossed dipole RVR from the link above. I know the difference between polarisation and pattern and i say that i don't need omni pattern because of mountain chain 500m behind transmitter site (wasting power too much), some kind of directivity will be good for us. My concern was about 4th dipole (the lowest in stack) who will be sidemounted on lower, bigger mast (where directional system is mounted)

 

[rfry]

My concern was about 4th dipole (the lowest in stack) who will be sidemounted on lower, bigger mast (where directional system is mounted).

1. What is the outer diameter of the mounting pole?
2. What is the face width of the triangular tower?
3. In what compass directions do the tower legs point from the tower vertical centerline?
4. Over what compass sector is the mountain chain with respect to the tower site?
5. What mechanical loads can be supported by the mounting pole and the tower structure at the elevation of the antenna array, allowing for the maximum wind velocity and icing conditions at that site?

The radiation pattern from an element mounted on the triangular tower will be considerably different than the ones mounted on the pole -- even if they all are mounted on the same vertical centerline.

Some acceptable configuration may be possible, based on the responses to the above questions.

Suggest that you gather the above information, and then contact R.V.R. Elettronica or other antenna manufacturer to request a quote from them for your specific requirements.

You might investigate Kathrein-Scala as a vendor. They manufacture a c-pol yagi ( http://www.kathrein-scala.com/catalog/CA5-FM-CP-RM.pdf ) that possibly they could use in an array design that would meet your needs. They also manufacture c-pol FM panels that could produce a ~180 degree radiation pattern in the horizontal plane that might be acceptable to you.

Richard Fry (with no business connection to any antenna manufacturer)
http://rfry.org

 

[knowbetter]

Thanks for the explanation. Now, if a station suddenly isn't getting the coverage its supposed to in a certain direction, would that mean the problem is in the antenna system, the tower and/or those units that insure the directionality? It wouldn't be a transmitter problem?

just guessing, based on your question, I would guess that you are a low power, and lacking coverage in a specific area...

first thing I would check is the type of antenna, and it's relationship to the tower. It is very easy, and since it is very noticeable with Low Power stuff, to get antenna re-radiation, due to the tower structure, or the proximity of the antenna placed on the tower. A low power vertical, for example may well work better as a "driver source" for a larger tower sturcture, which essencially send the station in the opposite direction..

So if the antenna is on the east side of the tower, and the city is to the east, you may well find the better signal is to the west, since the signal is being picked up and re-radiated by the steel in the tower.. The smaller antenna is using feeding the tower structure, making it a direction antenna of sorts!..

so, make sure you observe the required separations when mounting the antenna, and these vary, based on the size of the tower leg, or the side mount.