FM Radio

[thedementedelf]

I'll be doing the presentaion on FM radio. Are there any area's you like to see covered specificilly?

[Jack Stevens]

Generically? FCC Part 15 compliance; audio and RF signal levels, antenna design and placement. Signal input methodologies - probably direct from sound card of computer, but is there a better way?
Specifically? Low power FM transmitters that are good and those to stay away from. I wish I had gotten that advice before I bought the one I currently have, and am looking to replace.

[scottmcl]

I know nothing. How would I start?

Is that generic enough?

[kidcole]

Antenna types might be a good topic to show Dipole antennas compared with quarter wave and half wave. FUll wave is probably not of much concern with this group.

[thedementedelf]

Just digging throuhg FCC Part 15 rules and Regs. If you want to stay complient with part 15. You need to stay with the antenna that came with the transmitter.

Antenna Requirement
Changing the antenna on a transmitter can significantly increase, or decrease, the
strength of the signal that is ultimately transmitted. Except for cable locating
equipment, the standards in Part 15 are not based solely on output power but also take
into account the antenna characteristics. Thus, a low power transmitter that complies
with the technical standards in Part 15 with a particular antenna attached can exceed
the Part 15 standards if a different antenna is attached. Should this happen it could
pose a serious interference problem to authorized radio communications such as
emergency, broadcast and air-traffic control communications.
In order to prevent such interference problems, each Part 15 transmitter must be
designed to ensure that no type of antenna can be used with it other than the one used
to demonstrate compliance with the technical standards. This means that Part 15
transmitters must have permanently attached antennas, or detachable antennas with
unique connectors. A "unique connector" is one that is not of a standard type found in
electronic supply stores.
Section 15.203
It is recognized that suppliers of Part 15 transmitters often want their customers to be
able to replace an antenna if it should break. With this in mind, Part 15 allows
transmitters to be designed so that the user can replace a broken antenna. When this
is done, the replacement antenna must be electrically identical to the antenna that was
used to obtain FCC authorization for the transmitter. The replacement antenna also
must include the unique connector described above to ensure it is used with the proper
transmitter.

[Waxhaw Larry]

I guess we won't be covering antennas.

[Jack Stevens]

We could still cover antennas, as in "placement of same." Not all transmitters come with antennas. Some, like the "Whole House Transmitter" I bought (and wish I hadn't) comes with an antenna that is not easily modifiable, reliable, or works that well. Yes, I had a lot of static in my transmissions, and the range was far less than FCC part 15 specification allowances; I got maybe 40 feet, as long as I could tolerate static.

Antenna placement - if I could have put my antenna outside, the signal would not have had to penetrate the house outer wall, which I'm sure contributed to the static issue. If I had done so, though, I would have had to weatherproof a unit that I was advised by the manufacturer would not work well below 50 degrees.

I intend to replace this unit this year; I'm looking forward to getting information about other units from people with hands-on experience.

[kidcole]

I will be glad to contribute my FM system details. I used a Ramsey FM25B transmitter, a Ramsey Linear Amp (1 Watt I believe), and a nice littel quarter wave antenna mounted about 14 feet in the air. The linear amp is actually mounted on a PVC pipe just below the antenna. It takes care of the cable losses when the transmitter was pushing 25 feet of cable. I had problems in the begining due to the output amp in the transmitter being bad (it was dead when they sent the transmitter kit). But once I replaced the bad part in the transmitter, the system easily pushed 3 or 4 blocks. I tweaked it down to about 2 blocks so as not to risk any complaints.

I also used a small mixer and a Behringer Audio Processor MDX1600, along with a pair of outdoor Bose speakers with the amps built in. The PC's audio signal went to the audio processor to do a combination of dynamic compression and enhancements. This gives a wide audio range with less sharp peaks and valleys. That's the key to getting a professional FM sound in the cars. Then the mixer is used to keep the FM signal constant while being able to change the volume of the speakers outside. I found I had to start the show at a lower volume and then bring it up as the cars increased. We had stead traffic of over 200 cars an hour on peak days, so you can imagine how much white noise from the engines I had to rise above ..

[John]

Kevin,

I'll be glad to bring an EDM tramsmitter for "Show and Tell"

[victory402]

I would love to learn anything I can on this topic. Poor sound = Poor show. I slid through last years show on a wing and a prayer. I think i just got lucky, because it sounded pretty darn clear. Denny hooked me up with a transmitter and I found an old FM antenna out in the shed. Not having any knowledge, initially i was like, "this is never gonna work", but like I said i think i got lucky.

[thedementedelf]

Kevin,

I'll be glad to bring an EDM tramsmitter for "Show and Tell"

Sounds Good

[Al in Raleigh]

Vic,
Everybody is talking about the Charlotte convention that will occur this summer. Maybe we can talk Kevin into coming to Raleigh to give his talk. If not then we will have a training session in Raleigh specifically on FM radios. It won't be on Kevin's topic because none of us are versed in the subject like Kevin. However, it will be in the art and science of transmitters. We have several people in our group that could present.
Al

[John]

A contibution for the "Art and Science of Transmitters",

the EDM will tolerate a 20:1 SWR mismatch with no damage. The final output parts will run hotter with a SWR this high, but it will be OK.

This info is direct from the EDM engineers on the EDM forum.