Diplexed FM antennas

[bossbill]

Anybody experienced with two-station diplexing, with a standard multi-bay antenna? Specifically, using an end-fed design with standard rigid interbays, or a power-divider configuration?

 

[rfry]

If this is a new installation for two non-D FMs, then the answer will depend on the frequency separation, and choice of antenna OEM. Closer frequency spacing may make such projects more feasible, but also may require a fairly long length of rigid line at the antenna input used to optimize the antenna system for the best Z match at those frequencies. The downside here is that close frequency spacing also can drive up the cost of the hardware needed in the transmitter room(s) to combine the two channels with acceptable isolation/r-f IM products.

If this is an existing installation to which a 2nd FM is to be added, then there may be additional costs -- which best are addressed by getting a specific quote from that antenna OEM.

 

[WNTIRadio]

What kind of antenna is there now? Some are also more broadbanded than others. If it's a ring-stub, forget it.

 

[bossbill]

If this is a new installation for two non-D FMs, then the answer will depend on the frequency separation, and choice of antenna OEM. Closer frequency spacing may make such projects more feasible, but also may require a fairly long length of rigid line at the antenna input used to optimize the antenna system for the best Z match at those frequencies. The downside here is that close frequency spacing also can drive up the cost of the hardware needed in the transmitter room(s) to combine the two channels with acceptable isolation/r-f IM products.

If this is an existing installation to which a 2nd FM is to be added, then there may be additional costs -- which best are addressed by getting a specific quote from that antenna OEM.

I was hoping you'd pop on here. This is a new install, and no additional tower space available. Frequency separation is 5.2 MHz. One TPO is 9 kw, the other 3 kw, into a dual-three-pole branch combiner, utilizing existing 1-5/8" air Heliax. Five-bay half-wave spacing, slant V. One vendor is offering a traditional configuration with rigid interbays, end fed, cable and tuner at feed point. The other is offering a five-way power divider with cabling to the bays. It's been so long since I've dealt with this end of the business, and never with a diplexed FM before.

 

[rfry]

I was hoping you'd pop on here. This is a new install, and no additional tower space available. Frequency separation is 5.2 MHz. One TPO is 9 kw, the other 3 kw, into a dual-three-pole branch combiner, utilizing existing 1-5/8" air Heliax. Five-bay half-wave spacing, slant V. One vendor is offering a traditional configuration with rigid interbays, end fed, cable and tuner at feed point. The other is offering a five-way power divider with cabling to the bays. It's been so long since I've dealt with this end of the business, and never with a diplexed FM before.

Hi, Bill. Wasn't sure you remembered me from your days as a broadcast equipment salesman for our former, joint employer.

A separation of 5.2 MHz shouldn't be much of a diplexing issue to most OEMs of FM transmit antennas.

There are two kinds of branch combiners -- notch, and bandpass. Notches are OK as long as there are no other possible external, off-freq signals that could be coupled into either transmitter to produce illegal signals, and ~no possibility that any such could be present in the future.

The most secure solution in this respect would use bandpass filters. However for 5.2 MHz separation, bandpass filters have more insertion loss than notch filters, so the system math needs to be done carefully to be sure that the licensed ERPs are possible from the system hardware.

Probably your best answer/decision would follow from the detailed quotes from the OEMs of such diplexed systems for your specific applications, including the tower aperture, antenna weight & windload available, available TPOs, combiner installation space, and (of course) the quoted prices and reputations of such OEMs.

Rgds,

Rich

 

[bossbill]

Of course I remember you. I have always been impressed with your expertise and knowledge, and enjoyed reading your papers.

I think we're okay on the combiner, but I'm trying to wrap my arms around the reason why using a parallel feed would be better (or not) than the end-feed design. Pricing is about the same. The end-feed would have a 30' coax below the antenna feed, and an additional tuner. The parallel-feed would have a five-way power divider, and air line to the elements for pressurization. That would be more like a center-feed, right? So the electrical spacing differences between the two frequencies would be lower? But we're only working with a 5% frequency change, so is it that important?

 

[rfry]

The benefit of using parallel feed (separate cables to each bay, fed by a power divider) is that each bay can be driven with exactly with the same r-f phase across each FM channel, which means that the elevation patterns can have maximum theoretical gain at zero degrees elevation. The downside is that it may be more expensive to install, and the cables need to be dressed and bonded to the tower properly, per the mfr's instructions.

The bay spacing of a conventional end-fed antenna probably would be cut for the median frequency. This means that the r-f phases at each bay will not be identical, and the peak gain of the elevation patterns would not lie exactly in the horizontal plane.

Running the numbers for a 5.2 MHz separation and 1/2-wave bay spacing, the lower freq elevation pattern would have about 1.4 degrees of beam tilt, and the gain in the horizontal plane would be 0.994X that at -1.4 degrees. I didn't run it for the upper freq, but expect it would be about the same except the tilt would be in the opposite direction. So all this is rather trivial.

From a design standpoint these two approaches should give virtually the same performance, and the choice will more depend on other factors.