Combining two signals at the opposite ends of the FM band

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iowegian3
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Combining two signals at the opposite ends of the FM band

Post by iowegian3 » Mon Jan 16, 2017 1:04 pm

Friend's station in IA was approached by a non-comm station for tower space. Problem: tower too light to support any more bays and coax runs, combining the two stations into one antenna would cost almost as much as a new tower.

Eventually they'd like to build a joint tower elsewhere. Given they're over 18 MHz apart it seems like it would make more sense and cheaper to design a that's tower with a 6 or 8 bay antenna for each station...they're both C3. Since the non-comm may be able to run more than 25 kW, it would get the top position.

For just two stations that are C3 or lower, does combining into one antenna make sense only if you don't have any other options to build a tower?

TPT
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Re: Combining two signals at the opposite ends of the FM band

Post by TPT » Mon Jan 16, 2017 2:27 pm

That would just about sum it up--the expense of a combiner & broadband antenna would probably exceed the cost of a pair of moderate sized towers ( or a tower hefty enough to hold two separate antennas) in a rural area.

What are the ERP's and antenna height above ground desired? Given the hassles of getting past the environmental, Indian bribes, historic preservation, it might make more sense to erect a new, sturdier tower on this site to allow for separate antennas for the two stations.

iowegian3
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Re: Combining two signals at the opposite ends of the FM band

Post by iowegian3 » Mon Jan 16, 2017 5:53 pm

Both stations only need about 100 m HAAT (88 m AGL) and 25 kW each. If tower space was tight in the area it might make sense to combine to maybe save on high lease terms. Beefy tower with separate antenna systems seems like the best bet. Or maybe two lighter towers 15 or 20 ft apart?

TPT
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Re: Combining two signals at the opposite ends of the FM band

Post by TPT » Tue Jan 17, 2017 7:51 am

Expensive--but if you were able to build a 400' tower (assuming typical Iowa terrain) and use 4 bay antennas, then you could stagger the antennas, keeping the TPO to around 10 kw for both stations.

Anything over 200' means hiring the "environmental" consultants to pay off the Indian tribes & do meaningless archaeological surveys. So that expense is baked in--better to save $5K by not having to do it twice.

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KPJL FM
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Re: Combining two signals at the opposite ends of the FM band

Post by KPJL FM » Tue Jan 17, 2017 8:51 am

It would be cheaper and faster to replace the existing tower with one that can handle the load.
Zoning and Planning already approved for a tower, replacing with the same height shouldn't require new studies, existing owner gets new, more capable tower with little or no cost. Both stations should be able to work out a lease deal that's favorable, considering the money spent to give owner a new tower. If you could upgrade existing tower, stations would have to pay all the cost, anyway.
All concerned should get together for lunch and chat about it.
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Kelly
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Re: Combining two signals at the opposite ends of the FM band

Post by Kelly » Tue Jan 17, 2017 9:40 am

As the other guys indicated, combining stations isn't cheap. That being said, you can always contact a couple vendors like ERI or Shively and ask for a quote for soup-to-nuts combining setup including turnkey hybrid combiner, filters, and master antenna for the stations involved. ERI could quote you on the replacement tower as well. That way you'll have the whole cost analysis with numbers done.
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kkiddkkidd
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Re: Combining two signals at the opposite ends of the FM band

Post by kkiddkkidd » Tue Jan 17, 2017 11:29 am

Also, if one or the other is DA, all bets are off...

If a combiner is all you can do, then so be it. But I would recommend staying w/ discrete antennas if at all possible.

I just participated in the freq change on one of my clients combined stations (ERI 2 station combiner & 6 bay antenna). They thought that it only required programming a new freq in the Nautel... $32+k later... They are on the new freq and operating normally and are starting to get over their nausea when I explained what was involved in changing freqs.

Tower space was the limiting factor in this case.
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iowegian3
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Re: Combining two signals at the opposite ends of the FM band

Post by iowegian3 » Thu Jan 19, 2017 12:47 am

TPT wrote:
Tue Jan 17, 2017 7:51 am
Expensive--but if you were able to build a 400' tower (assuming typical Iowa terrain) and use 4 bay antennas, then you could stagger the antennas, keeping the TPO to around 10 kw for both stations.
Sounds like this is the simpler approach, keeping separate antenna systems, one station on top, then the second set of bays would start maybe thirty feet below the bottom of the first.

The existing tower cannot be rebuilt at the present location. Too close to occupied buildings, plus the stations will cover the market better at a new site.

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Re: Combining two signals at the opposite ends of the FM band

Post by rich wood » Thu Jan 19, 2017 8:36 am

Howdy,
Doing a tower analysis and seeing what the cost for reinforcing the existing structure would be can save on a replacement or combining senerio. I have had good luck with Ehresmann Engineering in South Dakota on this type of work.
http://ehresmannengineering.com/
Also, if you put up two antennas you will need to look at the inter-mod products and the filters needed (at least 80 db down) for the spurs. With one at the top of the band and one at the bottom, by subtracting the two frequencies then adding the difference to the upper channel you will probably see the result in the 124 MHz avionics band!! The FAA has no sense of humor on interference.

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Shane
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Re: Combining two signals at the opposite ends of the FM band

Post by Shane » Fri Jan 20, 2017 12:36 am

I can also recommend Ehresmann.
Mike Shane, CBRE
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