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LPFM antenna on a AM Tower. Is this possible?

F

fl-lpfm

Guest
Hello All!!!

Is it possible to install an LPFM 2 bay antenna on a AM Tower? If so, what would be needed? And how will this affect the AM station or the LPFM if anything?

Thanks!
 
Need more information...

- Is it a directional AM?

- How tall is the tower?

- What wattage on the AM TX?

- Is the tower series fed (on an insulator) or shunt fed (wire skirt)?

All those will factor in to the cost/pain in the neck factor of mounting the FM bays.

In short, yes you can mount it on a series fed AM tower provided you have an isolator for the antenna. A shunt fed tower will not need an isolator, but the FM bays can't be near the feed point of the tower.

If it's an AM/DA, you will need to do a partial proof after mounting the FM bays as the tower impedance and reactance will change, thus the parameters of the pattern will change along with the tuning of the phasor.

Give Kintronics or LBA a call and tell them what you need to do. They can get you pricing for the isolator if you need it.

It can be done.
 
If no other site is available (zoning problems) then mounting on an A.M. tower might be the answer.

The detail you left out of your question: What would be the cost of the "de-tuning device" or "isolation device" needed to keep the LPFM feedline and antenna from disturbing the A.M. signal. A 100 foot tower for the LPFM could possibly be cheaper.

If your LPFM transmitter site in on a hill, you might only need a 30 or 40 foot tower.
 
An FM antenna on an AM tower is not that big a deal. Two primary considerations: 1) wind loading capability of the tower, and 2) if it's a multi-tower directional array.

At a minimum, the condition of the tower should be determined by a tower contractor to ensure that it will not be compromised by adding a FM antenna and line.

If it's a series fed tower (insulated base) an isocoupler will be needed in the FM line. If it has a grounded base and is shunt or skirt fed, no isolation is needed. In either case, after the installation is completed, a base impedance measurement of the AM station feed point would need to be conducted and a FCC filing requesting direct measurement of power for the AM be completed (it would probably be a condition of the FM license anyhow).

If the AM is a directional, as mentioned in another post, a partial proof of performance would need to be conducted and filed with the FCC (again probably a condition on the FM license). The caveat here is, it is possible that the AM array is not within licensed parameters already, and a partial proof would serve to prove it. To restore the AM array to licensed values and pattern could be very costly - 10s of thousands. That's a can of worms not worth opening unless this tower is absolutely the only one available, and land use regulations prohibit a new tower.
 
We have an AM 4 tower parallelogram DA here with two FM antennas on it and a bunch of cellular. The tower is shunt fed today, but that's only been the case for about 10 years. For the previous 50 it was series-fed using isolators. One seldom used trick for feeding series towers is to stand the FM feedline off from the tower, using insulators (we used PVC), until it reaches the quarter-wave point on the tower where the AM rf voltage is zero. The coax is then bonded to the tower at that point and every few feet thereafter. This is practical and cheap if the FM antenna doesn't need to go much higher than the 1/4wave point. If it goes too much higher, the bonding costs eat up the savings. Also, some experimentation is usually required to find the exact zero voltage spot on the antenna because velocities are never exactly as predicted.
 
We did that trick with KOL-FM on one of the AM towers back in the 70's.

A problem with using an AM tower for an FM antenna is that usually an AM tower site is not at an elevation that would work best for an FM. While in some cases (such as an LPFM), that would not matter, it can surely cause disappointment in a General Manager trying to save tower space rental costs.
 
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