Antenna for multiple FM transmitters

We want to buy about a dozen fairly low-power FM transmitters:
SainSonic AX-7C (CZH-7C) 76-108MHz FM Transmitter FCC Stereo PLL Broadcast Radio Station 1W/7W

The purpose is to connect an MP3 player to each transmitter and broadcast through the FM band. We're a remote site and located a few hundred miles from any civilization. We'd like to be able to reach out about 10 miles or so. The provided antenna doesn't reach that far. Would we need to buy and connect 12 separate antennas or could we hook all of them up to one antenna? We have a big (around 100ft) radio tower available.

Couple of assumptions here
1. You are not located in the U.S.
2. This would be operating in the FM broadcast band, if so will these be licensed or unlicensed

A commercial antenna combining network is expensive and very lossy, in other words you lose quite a bit of your transmit power output. A practical maximum of 4 transmitters per combiner/antenna would probably be the max.
We would really need more information on what you are trying to accomplish.

1. Correct, we are in Canada.
2. Unlicensed and probably completely illegal regarding CRTC rules.

We're a remote mine site and we want to broadcast some music from MP3 players to radio around the site, around 10km across. We're located hundreds of miles from anything (accessible only by plane), so we won't be interfering with any local signals.

So, from what you're saying, we're better off just buying a dozen cheap dipole antennas an hooking them up separately to each transmitter?

forget it. if you are going to do it, then do it right. those sainsonics are giant splatter boxes that put spikes and spurs in not only the FM band but all up and down the hf, vhf, uhf spectrum. it is not surprising that you are not getting good range. the harmonics are probably eating up most of the claimed rf output power.

if you want to do it somewhat cheaply, at least relatively speaking, still will be expensive by hobby standards, then look at used exciters in the 250 watt range and individual single bay per a transmitter vertical or circularly polarized dipoles and put them high up on the tower properly spaced being careful to observe wind loading of the tower taking into account not only the antennas but the coax used going up the tower. you will also need circulators and LP filters since you will be running these high power transmitters in a collocated configuration.

you are not going to be able to cheaply do what you want and if you are here asking these question are certainly not qualified to do such an advanced installation and good luck finding a reputable contractor to do what seems to me to be a very illegal installation.

my two cents on the matter.

i am a qualified RF engineer with years of experience in RF.

i grew up around B,cast engineers and their stations as well as studied and passed the CIE broadcast engineering course.

If you are located within the USA, then you will not be able to legelly do what you have indicated you wish to do.

Any AM or FM radio station working in the low power range( 100 milliwatts or less) per FCC can only cover about 1/4 mile max (if that).

Any thing else requires a FCC license and a lot of engineering.

frenaud said:

The purpose is to connect an MP3 player to each transmitter and broadcast through the FM band.

Click to expand...

This sounds like a a Rube Goldberg solution.

Why not simply use MP3 players in the field
with the day's mp3 selections synced each morning
or the beginning of the shift or some other opportune time...??
Or several times a day...

Why must you generate RF??

Is there Internet connectivity in the area? If so, utilize streaming to feed the audio.

If you have small pockets of population, then you could feed that stream to a low power transmitter in each area. (Within the laws of your area.)

[Edited to fix a typo.]

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James_Bond_007 said:

forget it. if you are going to do it, then do it right. those sainsonics are giant splatter boxes that put spikes and spurs in not only the FM band but all up and down the hf, vhf, uhf spectrum. it is not surprising that you are not getting good range. the harmonics are probably eating up most of the claimed rf output power.

Click to expand...

Are the Ramsey FM BCB transmitters better?

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part

15.239 (FM)

88.1-107.9 (88-108 Band Edges)

200 KHz steps

Maximum Deviation i believe 150 kHz in FM stereo or mono

field strength allowed 250 uV/m @ 3 meters from the radiating source which can be a short mono pole, or radiating coax or 3m from the outside walls of a building if the system is indoors

Part 15.219 (AM)

100 mW **INPUT** to final RF stage (there is an argument currently as to whether this is with modulation or an unmodulated carrier)

i forget how it is worded but you are limited to a 3m antenna including coax, ground lead (if used) and antenna

frequency range is the standard am broadcast band 530 to 1700

as far as i recall no limitation on modulation, bandwidth or anything else just antenna length and power input to final

then there is a 3rd option which is carrier current under 15.221, 15.223, 15.209

with this the different rule parts limit rf field strength radiated from the source which can be power lines, radiating cable, a single mono pole, etc

some of these cover medium wave and HF, some of these rule parts cover only the standard am broadcast band, again no limits on modulation, bandwidth under these rule parts.

you also have various other part 15 bands which can be used as STL links from your studio to remote part 15 transmitters under these rule parts quoted above.

these allocations exist in long wave, HF, 900 MHz, 2.4 GHz, 5.8 GHz and i believe recently some 20 something GHz range has been opened up under parts 15.

if you have money, ingenuity and a will to do it part 15 can give licensed facilities a run for their money and be legal.

for further information on part 15 broadcasting, i recommend this website / forum http://www.part15.us/forum and this forum has a resource libary (must be registered to access library) with scanned materials in pdf covering all aspects of low power broadcasting http://krocksradioone.com/forum/index.php?board=56.0

There are commercial products for combining multiple FM broadcast transmitters to use a common antenna. They're huge and very expensive. They're also not that easy to configure. The feedline used is that expensive large copper stuff (6 1/8" generally) as well. Look at the picture labeled "Combiner room" on this page (Site of the Week Bonus: Crestwood FM Master Tower, St. Louis, MO) for an example. The "transmission lines" pic shows what that copper transmission line.

Another combiner pic is here A selection from a decade of visits to tower and studio sites in the Northeast and beyond

While one of these would be overkill for your application, they're about the only thing on the market that will do what you want.

OK, someone posted the low power transmitting rules (part 15) above. Armed with that info you can make this work. I believe you said you have about a dozen locations to stream audio to (presumably) folks using FM receivers. I think the following might work for you:
A subscription to satellite based internet and a dozen installations
Each satellite receiver can be hooked up to a computer that is connected to a legal low power transmitter.
Advantage is that you can remotely log in and control each computer from anywhere with internet access.

All go back to the beginning of the OP's post, he replied that they are in Canada. I'm guessing he is still on moderation and his reply to my first post, was delayed sort of explains what he is trying to do.

for license exempt broadcasting on the am and fm bands in Canada look at BETS-1 and RSS-123

all their rules are on the IC website and with Canada you not only have to deal with IC but CRTC even when using their version of part 15.

i suggest you read and study real carefully their rules, they are a giant clusterf*ck compared to the usa because you have to deal with two agencies under their version of part 15 not one and since they have a much lighter workload up there vs the fcc here they tend to respond much quicker to piracy complaints.

One other option to consider would be a "carrier current broadcasting" system (https://en.wikipedia.org/wiki/Carrier_current). Basically instead of feeding an antenna, you feed the power lines (using a proper matching and isolation system). Instead of FM, it would use AM instead (but most radios have both so this shouldn't be an issue). The radio can either be plugged into the power lines that the signal feeds or be fairly close to them (a few hundred feet or so) to pick up the signal. Using multiple transmitters and separate feed boxes you can have multiple stations, so long as the transmitters are on different frequencies.

One issue is that the signal will not pass through a transformer so you'll need to feed the various sections of the power line individually. Also, of you have a multi-phase power, you'll need to feed each phase separately (the matching/isolation box can be made to easily feed the multiple phases from the same signal).

We fed well over 20 buildings using two 50 watt transmitters and a 10 watt one. The 50 watt units sent the signal over coax to the various buildings (we custom built splitter transformers to split the signal power based on building size) with the 10 watt unit feeding an isolated building that we couldn't route the coax to without huge issues.

Since, by definition, a carrier current system is a closed system (similar to a cable-tv system) it is legal by FCC rules (you should verify that Industry Canada rules are similar). There are distance limitations that you can't exceed, but those are fairly lenient and since you're feeding the powerlines, care must be taken when installing or working on the system (that matching/isolation box basically keeps the power on the power line side and the RF on the RF side so it's generally safe, but be aware of what you're doing and where you're doing it!).