Film-Tech Forum ARCHIVE: Maximizing FM Modulator range

This is topic Maximizing FM Modulator range in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Harry Robinson (Member # 265) on 10-04-2006, 11:15 AM:

I have an old Marbel FM modulator I bought for the church where I work. The reason for this is to transmitt our services to the other parts of the building, i.e., the nursery, offices.

The problem is there is a lot of metal in the building. The range indoors isn't more than 50 or 60 feet. I would appreciate any suggestions for boosting the transmition.

Posted by Jack Ondracek (Member # 1466) on 10-04-2006, 12:03 PM:

The simple fix is to just walk around with your modulator, and get what signal you have into the area you really need it.

If you have time to play, here are a couple other suggestions:

Assuming the modulator is working properly and puts out enough power for your application, take a look at your antenna. You need one that is rated for the output impedance of the modulator (75 or 50 ohms). A 75 ohm antenna will usually have a coax fitting that looks like the end of a TV cable. Coax wire is also built for one impedance or the other. To start, match the antenna, coax and connector to the output jack on your modulator. Avoid using adapters to make one kind work with another. You'll lose some power if you do.

If your antenna isn't at the modulator, try not to use a lot of coax. You probably don't have a lot of power to start with, and you'll lose some of it in the wire. Some antennas are "wideband", so you don't have to worry about tuning. Others may require the use of a SWR meter, so you can make sure they're putting out as much power as they can, and not just reflecting a bunch of it back to the modulator.

In the end though, don't be surprised if your building just isn't compatible with the amount of power your Marble box is putting out. To get a signal throughout the building, you may need more power, additional transmitters, a different antenna or some other creative idea.

Posted by Jeff Taylor (Member # 443) on 10-04-2006, 01:31 PM:

How about those 900 mhz FM/Video repeaters?

Posted by Kenneth Wuepper (Member # 1174) on 10-04-2006, 07:17 PM:

Many college campuses use the power grid as an antenna for their wired wireless radio stations. The building wiring is the antenna.

Some FM radio receivers use the power cord and house wiring as the antenna. Could be a good match for your needs. Seek help form a radio engineer before connecting anything to the power wiring though.
KEN

Posted by Jack Ondracek (Member # 1466) on 10-04-2006, 09:56 PM:

quote: Kenneth Wuepper
Many college campuses use the power grid as an antenna for their wired wireless radio stations. The building wiring is the antenna.

Ken, do you know if carrier current is effective at FM frequencies? I've only seen it used with AM.

Jack

Posted by Kenneth Wuepper (Member # 1174) on 10-05-2006, 06:27 AM:

Jack,
When I did some under grad. work at Ferris, they were using FM and the caps across the transformers were very small.

My clock radio had line antenna so it worked really well for me. The programming was not what I wanted but the signal was ok.
KEN

Posted by Chris Erwin (Member # 176) on 10-05-2006, 10:02 AM:

FM has had "leaky coax" applications (LPB had that at one time I believe alot like the AM couplers D/I's use/used on their in-car speaker wiring to transmit the AM Carrier)but I haven't seen an application that uses carrier current on FM.

You could also use an inexpensive FM transmitter that Ramsey Electronics offers.

Just like the old Real Estate maxium: Location,Location,Location any transmittion system is mainly Antenna, Antenna, Antenna. Check your match like Jack said,use as little coax as possible. A good antenna can make a world of diffrence.

You can legally free radiate AM on your own property like on a school campus-but the gear is $$$.

Posted by Harry Robinson (Member # 265) on 10-10-2006, 05:38 PM:

Just wanted to post a thanks for the helpful replies.
Harry