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FM Transmit Antenna -- Wrong Frequency?

E

everydayguy

Newbie question here -- a station owner told me that the 6-bay non-directional FM antenna he's using for his FM station is not tuned to his actual assigned frequency.

His station broadcasts at the FCC-licensed 99.1 mhz, but he says an engineer ran a test on his antenna when he bought the station about six months ago and found that it was actually tuned for a station that operates at 98.7 mhz.

* Is this significant?
* Does he need to get the antenna retuned to 99.1?
* Should he buy a new antenna? (The current one is about 12 years old.)
* Would having an antenna not tuned to your exact frequnecy result in less coverage? More tropo ducting?

Thank you for your advice.
 
It isn't significant. That isn't a large shift. Probably adds a little reflected power, but nothing to worry about.

Example.

We have a backup transmitter which we use for stations ranging from 100.1 to 105.3. The antenna is tuned right in the middle of those frequencies. We have slightly more reflected when we use it on 100.1 or 105.3, than when it is used on 104.1 (which is closer to the tuned frequency).
 
Recently a translator a few miles north of Boston switched from 92.1 to 102.9 utilizing the same antenna. It doesn't appear to be very complex: going up the tower from near the top with a few vertical elements, to nearer the top with a few horizontal elements. What's really odd, the array is supposed to be directional, but the former 92.1 allocation was to the east, the 102.9 allocation is supposed to be more northerly.
 
Laurence Glavin said:
Recently a translator a few miles north of Boston switched from 92.1 to 102.9 utilizing the same antenna. It doesn't appear to be very complex: going up the tower from near the top with a few vertical elements, to nearer the top with a few horizontal elements. What's really odd, the array is supposed to be directional, but the former 92.1 allocation was to the east, the 102.9 allocation is supposed to be more northerly.
Click to expand...


That sounds extremely suspect to me. A 10.8 MHz jump is quite a bit to make, depending on the antenna. Also, any directional antenna has to be oriented on the tower at exactly the azimuth specified on the construction permit, and verified by a surveyor. It would be very odd if you could make the change you describe and not have to reorient the antenna on the tower.

Of course, people do the wrong thing all the time. So what this station somewhere north of Boston did may or may not be correct.

Now, back to the original poster. Antennas are often routinely mistuned by one channel. They are usually 1 channel high (i.e. yours should be tuned to 99.3), so that when they begin to ice up their tuning actually becomes better before it becomes worse. Most antennas have a bandwidth of several channels. Typically you can operate at 1 MHz or more from the tuned frequency without a lot of difference.

The big question is not the center frequency to which the antenna is tuned. The question is- how well is the antenna tuned at the frequency I want to operate? If you have good VSWR (Less than 1.1:1) I wouldn't worry about it. (But given that yours is tuned low rather than high, I'd keep an extra sharp eye on it when it ices up.)
 
We had a 2-bay ful spaced ERI that was "tuned" for 93.1. Fed it with a McMartin 25. Put a frequency agile Bext on it and it would run 10 KW into the antenna on 92.3, 93.1, 96.9 and 97.9 at various times with no excessive VSWR.

The antenna's now for an AUX in another market.

The real **** was the chain drive tuning on that McMartin. It's loading would just make it to 97.9 if it didn't come apart on us.
 
Interesting topic...we have an antenna originally on 102.9. It was re-slugged by ERI to 100.3 with all but no reflected power. The allocation game changed (thanks in part to a certain station near Louisville on 103.1) and 102.9 was re-assigned to that community and the re-slugged antenna was checked on 102.9 before bringing ERI back for another re-tune....never mind. It had identical reflected power at 102.9 and 100.3...something on the order of 5-10 watts with 3.5KW going out.
 
Only thing nobody's bringing up is the inter-bay spacing.
Operating way off-frequency could give you an un-wanted beam tilt up or into the ground!
 
boiseengineer said:
Operating way off-frequency could give you an un-wanted beam tilt up or into the ground!
Click to expand...

Below is a link illustrating this point for a 6-bay antenna array designed for 94 MHz, used with no modification on a frequency of 104 MHz.

Assuming that the input Z of the array could be matched to the main transmission line, and ignoring the VSWR that likely will be present on the interbay line sections, and the pattern distortions from the tower -- the peak gain of the elevation pattern will be directed 5 degrees below the horizontal plane, and radiation in the horizontal plane will be about 5 dB down from its maximum value. Peak, c-pol gain will be about 2.9 X, down from about 3.3 X for a standard configuration.

If this antenna was designed for 104 MHz and used on 94 MHz (no mods), then peak c-pol gain would be 3.1 X, and occur about 7 degrees above the horizontal plane. Gain in the horizontal plane would be 5.4 dB down from its maximum value. Also the major sidelobe shown in the link would occur at a large negative angle instead of a large positive angle.

http://i62.photobucket.com/albums/h85/rfry-100/Antenna_Parameters.gif
 
boiseengineer said:
Only thing nobody's bringing up is the inter-bay spacing.
Operating way off-frequency could give you an un-wanted beam tilt up or into the ground!
Click to expand...

This would not be the situation on the original posters antenna. It is only .4 Mhz off. Even if he is in an ice prone area with no heaters, He will probably only fight another .4.

He will be fine. The directional that is over 10 Mhz off not only has problems, it is probably illegal.
 
Most FM antennas are 'segmented' meaning the manufacturer had several antenna designs/sizes for different parts of the band (for example they build one dimension of antenna for 92-94 Mhz, a slightly smaller one for 94-96 mHz, etc.). The fine tuner at the bottom of the antenna is then used to tune for minimum return loss (ie: reflected power, SWR). So, the chance is small that a radio station gets an antenna exactly tuned to their frequency anyway. What might imnprove things is to adjust the fine tuner.

When I worked for 105.1 in RI, they had installed an antenna without ever fine matching it. The first thing I did when I got there was rent a network analyzer and have a rigger adjust the tuner. Most of their multipath simply went away....

You do use a network analyzer to tune your antenna don't you? Tuning for minimum reflected power is crude at best and wrong at worst (since the reading can change based upon where the sampler is placed in the line).
 
LA_Guy said:
Most FM antennas are 'segmented' meaning the manufacturer had several antenna designs/sizes for different parts of the band (for example they build one dimension of antenna for 92-94 Mhz, a slightly smaller one for 94-96 mHz,)... So, the chance is small that a radio station gets an antenna exactly tuned to their frequency anyway.
Click to expand...
The above is true for most non-panel FM arrays built on a "spine" of rigid coaxial line, but still the lengths of the interbay lines in that spine are cut for the vertical spacing of the elements needed for the center frequency of the operating channel. So such an antenna array is designed/built to operate on a specific channel, even though the elements themselves are designed for use over some portion of the FM band.

Side-mounting the antenna on a tower can change the input match set at factory checkout. That is the reason that most antenna OEMs supply some means to optimize the input match after the array is installed.

If that antenna multiplexes several channels, then some vertical spacing is chosen for the elements of the array that is ~most compatible with the group operation. But almost always this will change the antenna specs for one or more of the users vs using a single channel antenna.
 
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