Directional FM Antenna for non-D Station

[Fieldtech1]

I think i know the answer from a question I posted months back but im still learning from the pro's. The station in question has a CP to go from A to C3 remaining non-directional. The tower is 4 miles from the coast, could a caroid patterned directional antenna be used on a non-D CP to avoid wasting RF power over water. The gain from the directional antenna would also allow the existing BE 3kw transmitter to be used to make the 11kw in the CP. Furthermore if the directional antenna is used, does it have to be "proofed" as if it were a directional station?.

Thanks for your responses..

 

[Engineer]

If the CP is non-directional, you must build it with a non-directional antenna. Directionalization not in accordance with the CP is not permitted. You could modify the CP for directional operation, but it would be cheaper just to buy a new transmitter by the time you paid for consulting engineering, antenna design, range test, surveyor's affidavit, etc.

 

[rfry]

The net H&V radiation patterns from a typical, sidemounted, non-panel FM transmit antenna licensed for non-directional use will show some directional effects due to re-radiation from the tower and other conductors in/near the antenna aperture.

This reality can be useful in your situation by choosing a mounting configuration that directs any reduced fields toward an unimportant sector. Knowing how to do that will take a pattern study by the antenna OEM using the elements you install, on a section of tower similar to the tower you will use to support it.

The gain of that antenna will be based on the r.m.s. value of its h-plane radiation pattern (theoretically a perfect circle). Its actual peak gain in the horizontal plane will not apply when calculating the tx power needed for your licensed ERP.

In order to use the real, peak gain of an FM transmit antenna when calculating ERP, the FCC will need to authorize the directional pattern that you apply for. The process of specifying, OEM pattern measurement, and installation adds considerable cost to the antenna part of project, but some of that might be recovered if you don't need to replace the transmitter, and/or with reduced TPO when using the (licensed) directional antenna.

 

[Fieldtech1]

In Lake Charles, LA, 3 Cumulus stations are combined into a directional antenna, but only 1 is liscensed directional. How does the fcc view that situation differently?

 

[rfry]

In Lake Charles, LA, 3 Cumulus stations are combined into a directional antenna, but only 1 is liscensed directional. How does the fcc view that situation differently?

That is possible as long as the antenna OEM developed and supplied the directional pattern needed, the antenna was installed and verified to meet the installation requirements for the DA pattern, and the non-D users "took what they got" with the respect to patterns of that antenna on their frequencies.

The h-plane gain of the DA pattern would be based on its peak value, and the other two on their r.m.s. values.

 

[Kmagrill]

In Lake Charles, LA, 3 Cumulus stations are combined into a directional antenna, but only 1 is liscensed directional. How does the fcc view that situation differently?

Is it really a directional antenna? Sometimes a station is licensed under the 73.215 DA rules even though it is non-directional. There can be several reasons for this. Sometimes there is terrain that provides the shielding. Sometimes the station reduces power. Reduced power is very common when addind a 73.215 station to a combined antenna.


Regarding the original question, you do have to submit a new form 301, so depending upon the power savings over time, it may be worth doing, or it may not. It really depends on how much power you save vs. the cost of filing the paperwork.

 

[BobOnTheJob]

Most of the Miami stations are licensed as directionals simply to avoid wasting power over the ocean and over the interior parts of South FL which have more alligators than human.

 

[Fieldtech1]

Is it really a directional antenna?

Regarding the cumulus antenna in Lake Charles Here are the three stations using it.

KYKZ 96.1 Non-D
http://fccinfo.com/CMDProEngine.php?sCurrentService=FM&tabSearchType=Appl&sAppIDNumber=1054962
KKGB 101.1 Non-D
http://fccinfo.com/CMDProEngine.php?sCurrentService=FM&tabSearchType=Appl&sAppIDNumber=1054943
KBIU 103.3 Directional
http://fccinfo.com/CMDProEngine.php?sCurrentService=FM&tabSearchType=Appl&sAppIDNumber=1054933

I learn so much from you guys, Thanks

We are weighing options before proceeding. We can add another PA module to our BE and get to 5kw which with a higher number of bays could make the ERP. Something that we will be discussing with our consulting engineer. Was just curious for the sake of saving money on a new transmitter and electricity and the precedence set by the Lake Charles stations non-d on a directional antenna.

 

[radiode]

Just a comment from personal experience of an FM antenna being simultaniously both non-D and DA: We had an ERI rototiller licensed as a non-d. Wanted to add another station to it as an aux site, but needed a DA to hold the 60 dBu contour of the main while maintaining as much ERP as possible on the Aux. Had ERI put an identical bay on the range and rotated the bay around the pole until the slight null fell where it was needed. Rotated the actual antenna on the pole to match and licensed as a DA for the NEW station. Since there was no actual change in the original antenna, no re-filing required for the original station. Also no discrenable change in coverage. It's also possible to have different HAAT's for two or more stations on the same antenna with a common feed line. It took me a while to accept that reality... :>)

 

[Dock Trash]

Geez, just look at patterns from antenna manufacturers. An alleged omni mount from a major antenna company actually had a major lobe towards the intended center of population of close to 6dB higher than a true omni. It's done all the time... An ERP of around 300,000 watts. 6dB=4 times ERP authorized.

 

[ChiefEngineer]

All those tuned towers at the top of untuned towers must mean something.

There are MANY Non DA directional antennas. Any omni mounted on metal becomes directional in some mode. The question is the intended purpose. There are enough people who regularly do this that you should look at the options.

A true DA will result in lower ERP most times. A "fake" omni or "fake" DA (same antenna) will not require lowering ERP and result in greater real gain over population areas you may wish to serve, other than the ocean.

Have ERI do a study. Your extra 6 db of power from a C3 will go inland. Just by the mounting on the tower.

 

[RadeoEngineer]

I am always amazed that people will buy an antenna that they're going to rely on as their final link to their audience and probably live with for 30 years, and not have it pattern studied. Yes it costs a few extra thousand, but isn't it quite critical that you get the best performance possible?

 

[Kmagrill]

I am always amazed that people will buy an antenna that they're going to rely on as their final link to their audience and probably live with for 30 years, and not have it pattern studied. Yes it costs a few extra thousand, but isn't it quite critical that you get the best performance possible?

Well, just to play Devil's advocate here, pattern studies are not perfect either. In fact, I've found them to be wrong a significant amount of the time. For example, we once hung a modelled ERI 4-bay rototiller antenna that went on a 1200' tower with several other FM stations. Within days, the staff was complaining about the signal, especially compared to the others on the same tower. We solved the problem by getting three FIM-71 meters and positioning them around the town. We then had the tower guys make a second climb and rotate the antenna, stopping every 10 degrees. When we got the maximum readings on the three FIM-71s, we locked the antenna down. The actual pattern had been off by about 35 degrees. After that, we made it a practice to measure the actual antenna on the real tower whenever we did installations. I never had another complaint after starting that practice.

Pattern measurements can be wrong for a variety of reasons. For starters, there are several types of antenna range. Some are better than others. In every case that I know, the modelled antenna is used as a receiving antenna and transmitted RF is sent from a calibrated distance. As the antenna is rotated, a graph is drawn of the recieved signal. Although the system should perform equally regardless of which antnenna is the receiver, in practice, near-field effects around the target antenna can have an unpredictable effect rendering the measurements wrong because when the antenna is used to transmit, there can be additional structure induced multipath that can't be easily modelled at low power or as a recieve antenna. Additionally, most of the time, a single bay is placed on a mock-up or approximation of your tower. When you place your actual antenna on the tower, it's probably not going to be a one-bay and even if it was, you probably won't mount it at exactly the same vertical distance to the cross memebrs as was done on the test range. The relative position of each bay compared to the cross members of your tower are going to change, so each bay will have its own pattern distortion.

At some companies, a 1/4 scale model is used at 4x the target frequency. Theoretically, this also works, but it's one more change from the ideal. So, I'm not suggesting that pattern measurements are useless, but do keep in mind that it's only a model. There are limitations (only some of which I've mentioned here) and you must be prepared to deal with the possibility that you won't get what you expect. If you're going to model, use it as your baseline and go from there.