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Part 15 FM Transmitters

R

Robert Bass

Do any of you guys know of a good, Part 15 compliant, FM transmitter for “whole home” use? I had one of those C Crane transmitters a long time ago but it finally broke. I'd like to replace it with something a little better. What I have been able to find on the Internet so far, are transmitters that don't specify f they have the 75 ms pre-emphasis curve. I don't think the C Crane unit had it either.

Thanks,

R
 
If it's a real Part 15 FM transmitter, it won't cover your whole house. Under the actual rules, you're lucky to get about 15' of clear reception.

If they have an internal stereo gen, then most likely they have the 75us curve in them too.

Buy an old exciter on eBay and transmit into a dummy load. You'll get farther than Part 15. Will it be 100% Part 15 compliant? No, but if it really isn't going much farther than your home's walls and a little bit of the backyard, the FCC van isn't going to come and bust you.
 
Depends if you want to be legal or not. The PCS products aren't certified for sale by the FCC (at least given the information from responses to my past email queries from the seller calling themselves "pcs-electronics" on ebay). FM transmitters for Part 15 use are suppose to be certified as well as have their certification data prominently display on the unit.

Keep in mind that according to Part 15.239 that the maximum field strength of a legal Part 15 FM transmitter is limited to 250 µV/m (microvolts per meter) measured at a distance of 3 meters from the antenna. My tests with typical tabletop radios yield a useable distance at about 100 feet under the best of conditions (no obstructions, transmitter elevated and placed on some metal object like the top of a refrigerator, etc.).

Many of the typical Part 15 FM transmitters that operate in stereo operate with a 75µs pre-emphasis for audio. The few and rare exceptions are those that have mono audio inputs that double as "composite" inputs where you can feed the output of an externally located broadcast stereo generator.

I previously performed written reviews on the Decade CM-10, MS-100 (did a companion video review), C. Crane original and FM Transmitter 2 and they all operate in a similar fashion. I also tested the Whole House FM Transmitter 2 but I have issues with the company's marketing hype and what I consider questionable business practices.
 
So basically another C Crane unit would be my best choice?

R
 
I'm with WNTI on this. Just use an exciter. You will cover the home better and you are not trying to be a pirate and transmit for miles.
 
chriscollins said:
I'm with WNTI on this. Just use an exciter. You will cover the home better and you are not trying to be a pirate and transmit for miles.
Click to expand...

Too expensive for me, including used ones on E-Bay. I had excellent use with the old C Crane that I did have, and right now I'm leaning towards another one.

R
 
kenglish said:
Have you looked at the Ramsey Electronics kits?:

http://www.ramseyelectronics.com/hk/
Click to expand...


I built the Ramsey FM25 kit several years ago. It offers great frequency stability, great STEREO sound and separation and covers my entire house and my adjoining neighbor's homes, as well. Our houses are pretty close together. I can hear it faintly on my car radio a block away... LEGAL? I can't say, but no mods were made to mine. I assembled it per the instructions that Ramsey provided at the time. They DID offer a higher power version, as I recall, but would NOT sell it to anyone in the United States. It is "frequency agile" anywhere from 88-108 mHz.
 
Broadcast Warehouse made a great 1 watt freq agile exciter circuit board a few years ago with a composite input. A 47 ohm 2 watt resistor across the RF output and 4" of wire for an antenna blankets the house well and it dies a quick death 150' in any direction. I can't find it listed on the broadcast warehouse website. Must have sold too well :(
 
I've played with a bunch of part 15 (and a bit beyond) stuff, and the best I have heard, that has a very clean RF output and sounds great is from EDM in south africa. I use one here, and have never liked one more.
http://www.edmdesign.com/index.html

Beware of chinese transmitters on ebay. I've had some that sound awful, and one unit, which looked pretty slick, had really strong harmonics and other spurious output in the *aircraft* band. You don't wanna stir that hornets nest up.

dave
;D
 
RadioEngnr... What frequencies does that transmitter offer?

Speaking of synched light displays, I saw a killer one in Manitowoc, WI over Christmas. First class job.
 
The old Broadcast Warehouse pirate transmitters are awesome. I have one from years ago. The one I have isn't the one with the processing and XLR connectors. I bet that one is even neater. Ahhh the good 'ol days of Broadcast Warehouse (BW Broadcast)! On the PC board silk screen, in the corner, there's a little picture of a hand with the middle finger extended. LOL! I guess they've come a long ways into the "legit" world from back then...
 
I have a broadcast warehouse exciter in the rack case with the XLRs and switching power supply. Interesting, when switched on, the display says it's a TX4, though the only models they listed back then were the TX3 and TX5 for their smaller variable power exciters. The thing had no heat sinking and no fans, when placed on a dummy load for testing at 3 watts, the PA would overheat in short order and the power and modulaton would drop off rapidly. I ended up drilling holes in the front and back of the case and adding a two small fans; worked much better after that.

I will say this though, it uses the 1 watt board and the built in stereo generator and limiter work quite well when setup correctly, though, mine swaps left and right channels. That really doesn't matter especially since I fed the composite input with an ultramod.

I recently took that exciter out of storage after not using it for 4 or 5 years, fired right up, though I'm noticing a buzz when there's no modulation. Maybe the switching power supply is going bad or my fans are adding noise I never noticed before.

For what it is and what I got if for, I can't complain. At 20mW into a short antenna, it sends my computer's audio to my classic radios and that Compellor and Ultramod combo recreate that classic early 1980s radio sound.

The Ramsey kits are quite good as well. I built an FM25B and was impressed by the stereo sound. I did make one modification though, I replaced the ceramic cap that coupled the stereo generator's output to the modulator with an electrolytic cap to improve bass response.
 
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