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Antenna Placement for FM Transmitter


jevely
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I just purchased a Ramsey 30B FM Transmitter kit and have started to put it together. Lots of little parts and tight spaces, I found assembling the Lightorama controllers much easier.

My question is about antenna placement. Will the antenna attached to the transmitter broadcast outside or do I need to attach a remote antenna which is mounted outside the house? If so, what type of antenna to use? Ramsey mentions a dipole antenna but I am drifting out of my element with this radio stuff!

Jim

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I use the same transmitter. I built a dipole antenna by taking 12-gauge wire, cutting it to the length specified in the Ramsey manual, and soldering one piece to the center conductor, and the other to the braid of a length of 75-ohm (TV) cable.

Then I formed a coil of about 6 turns of the cable near the end where the wires are soldered. This prevents radio frequency current from flowing down the outside of the cable.

I then hung it vertically in the rafters of my garage and brought the cable about 20 feet to plug into the connector of the transmitter.

I turned the transmit power down to near the minimum, so as not to violate section 95 of the FCC rules (I'm not sure what Canada's regulations are, but I would suspect they are similar). My signal will reach to the end of the block.

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Thanks for the input, Steven. I have a couple further questions.

Does it matter if the 12 guage wire has insulation on it or should it be bare?

What is the approx. diameter on the 6 turn coil of coaxial cable and how close should it be to the solder joint?

Jim

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EricFromKansas

I use the Ramsey 25B FM transmitter with my LOR display.

I found this web site that details how to build a simple FM antenna with a coil (Balun) as described in Steven's reply. I tried it last summer and it works very well. I used 10 guage electric wire instead of a coat hanger.

The antenna works better the higher it is off the ground, about 10 feet works well for me. Also had to turn the transmitter to lowest setting, as I was broadcasting all over town when it was turned to highest setting.

http://www.braincambre500.freeservers.com/open end half wave dipole antenna.htm

Good Luck, Eric

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I've just been using the included whip antenna that came with my FM30b. I have it inside the house, on the second floor, about 15 feet back from the front of the house, with typical wood frame, brick veneer, aluminum framed windows. I get a good block of coverage at the default, approximately half power setting.

- Kevin

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jevely wrote:

Thanks for the input, Steven. I have a couple further questions.

Does it matter if the 12 guage wire has insulation on it or should it be bare?

What is the approx. diameter on the 6 turn coil of coaxial cable and how close should it be to the solder joint?

No, insulation doesn't matter, unless you hang the wire near metal objects that may touch it.

Here's a picture of my coil. It's about 3-4 inches in diameter, but it's not critical.
attachment.php?id=8575
The white #12 wire goes through a couple of small holes in a 3/4" PVC pipe I had laying around. The coax goes through a bigger hole. The whole thing is secured with hot glue and a wire tie. It took me about a half hour to throw it together.
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I found the reception depends on the radio used. I first got the transmitter and discovered the radio headphones I use when cutting grass lost the signal too many times. But if I used the car radio the signal was strong enough to reach far enough down the street for my light show. So I purchased the Ramsey antenna to use when cutting grass. Works great.

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I finished assembling the transmitter last night and from within the house it is broadcasting at least one block. At this point, is there any advantage in installing an antenna outside the house? The sound quality seems good but will an external antenna improve it? I don't mind installing the antenna if there is a quality gain but I don't want to waste my time if the improvement is negligible.... So much work, so little time!

Any thoughts?

Jim

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The advantages to an external antenna are:

  1. The signal will get further. Since your signal already gets far enough, this is not a good reason for you.
  2. Your signal will be affected less by objects in the vicinity of the antenna. For instance, if your transmitter is right next to your computer, or a lamp, or even your body, the signal may change when you move any of those objects. If the transmitter is placed on top of a shelf, then this probably isn't a good reason your you either.
  3. The signal from the transmitter will be less likely to interfere with your computer, LOR controllers, wireless phones, etc., since you can run less power with an external antenna. Again, if this isn't a problem for you, then I wouldn't fix what isn't broken.

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Where is your sign telling people what station to tune to? I would recommend that as long a the signal is good at the sign location and within sight of your display, the antenna location is good enough.



L

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Based on the responses, the external antenna is still on my wish list but has dropped down in priority.

Thanks to everyone for their help.

Jim

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Jim, The link that Eric posted aboce is right on...

http://braincambre500.freeservers.com/open%20end%20half%20wave%20dipole%20antenna.htm

I just made this Antenna last night from spare parts I hade lying around the house, and I literally Doubled my range of my FM 25B to the point where I had to turn down the output power on my transmitter.

Also, Instead of Using 1/2 tube like the article, I had some 10/3 electrical whire laying around, and took the copper ground cable out and used that....FYI, the Diameter of that is @ 0.97inches (for the calcutions).

I cannot believe what a difference it made, and ti took about 20 minutes to build the whole thing.... If you need to buy the parts, it should be less than $10 for everything..

I owe you one Eric...

Thanks,
Scott

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I agree with you Scott, the link that Eric posted (thanks again, Eric) is exactly the information I was looking for. If time allows, I'll build one this year, if not, it's on the list for next year.

Jim

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  • 5 months later...

What are the chances anyone would share the information they got from the site? I've had limited success finding a site that puts the how-to in terms a non-radio person like me can comprehend.

http://braincambre500.freeservers.com/open%20end%20half%20wave%20dipole%20antenna.htm


Clicking on the link yields a message saying the site is disabled.

Tom B (The other one)

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hobbes wrote:

Basis21b,

Try this link:

http://web.archive.org/web/20070829054139/http://braincambre500.freeservers.com/open+end+half+wave+dipole+antenna.htm

Just an FYI for those not familar with webarchive. They are attempting to make backup copies of the Internet. The main site address is:
http://www.webarchive.org.

tom.


I had problems with the above link even.

Jim, I started with the 30B and since moved to the 100B. If you put the transmitter on a window sill or close to it, away from other items, you should be fine. Building an antenna is not going to be necessary. Even the replies here mentioned that they did, and then turned down the output power. They are inexpensive, and easy to build, they also expose you more due to your emissions. Again, not sure what the Canadian limits are.

Car radios, which is who your target audience is, have much better recievers than small hand helds.
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