Amazing How My Intuition Was So Accurate
As explained already, the EDM FM Transmitter in combination with a Decade GPL-32 Horizontal Antenna at 1/4-Wavelength was slightly above the RF field strength allowed by 15.239. We estimated that shortening the antenna to 1/8-Wavelength would reach accord with the FCC's guideline. We were right.
Before making the change the dBu reading 50' from the transmitter was 37.
After the change the dBu reading dropped to 23.
KDX uses the EDM Transmitter to supply audio for monitoring at the Main Studio location. According to tests the reduced field strength is providing the desired performance and is therefore another great success.
Thanks for the report on shortening the antenna.
Neil
After reducing the GPL-32 FM antenna to 1/8 Wavelength we became aware that the ROLLS HR78X Tuner at the Main Studio Control Desk was fizzling when ever I was physically moving in the middle of the room. It was of course a touch of multi-path interference. All other household radios were receiving a perfect signal.
We sought to resolve the problem at the ROLLS receiver without making any changes at the transmitter.
Previously, before changing the transmission antenna, the ROLLS received a solid signal with no antenna, so we added the dangly-wire dipole that came with the ROLLS and taped it to the wall in horizontal configuration. This moved the frizzly reception... now sitting in the operator chair caused the interference to occur.
We re-attached the reception antenna to the wall in vertical configuration and the result is perfect reception with no frizzling whatsoever.
Well that's interesting. The transmission is horizontally polarized while the reception is vertically polarized.
The antenna experience described in this thread was discussed at last night's ALPB meeting, and Chairman Bob Felmly noticed that the dimensions given for the GPL-FM Antenna in fact describe a 1/2 Wavelength antenna, not 1/4 Wave as initially supposed.
That being the case, our re-tuning to a smaller wavelength resulted in a 1/4 wave antenna, not 1/8 wave.
That's good news because it means that any mismatch is less.
Here is a nice BNC attenuator This one cuts the signal by 10db and will allow you to have your antenna matched and keep your signal down to your yard.
Carl,
Here's something to consider based on some experimental observations at my station. You attribute your problems with FM reception of your station to multipath and this may well be the case but maybe there is another mechanism at play.
I don't know the details of your setup but if you are feeding the antenna with a transmission line then it is likely that the line is radiating as well as the antenna. When the signal from the antenna and that of the transmission line combine at the receiver it is possible, according to position of the receiver and things in between (you), that the signals add or subtract to produce a boost or a null.
You might try to choke the signal from the outside of the feedline by coiling a few turns of the line into a loop. Here I use a type 12 toroid to do this but maybe just coiling the feedline near the antenna will have the same effect.
Neil
Will that help his SWR to be matched? I suggested the attenuator to simply bring down his power coming out of the back of the TX into his antenna line and then to the antenna. Since he is only trying to transmit within the house its possible that by lowering the amount of signal he could prevent front end overload as well as someone complaining about his unlicensed signal.
My 500 mW transmitter seemed to bring in noise at close range. But at -48dbm I was way less than part 15 and I could not even get to the porch. With an attenuator at the end of the TX I could lower my signal enough that it didn't overload my Radio's at close range. This was good for house casting.
Thank you TheLegacy and Radio8Z for tips worth keeping in mind.
The way things are as of now everything is working according to the best expectations.
The EDM Transmitter is about 30" above the floor (table level), the feed-line about 6-feet shielded BNC type cable up to the GPL-32 Horizontal Dipole mounted at the top of a double window frame.
The main receiving point is the Main Studio Control Desk, ROLLS FM Tuner using supplied dipole strung vertically along one edge of a door-frame which works extremely well compared to horizontal placement which was noisy.
A secondary reciever is located at the executive desk antenna also vertical.
For the past few days since arriving at this setup no multi-pathing has been encountered.
Further observations are to be made out on the car radio driving around.
Can the antenna be connected directly to the transmitter?
Clip ...My 500 mW transmitter seemed to bring in noise at close range. But at -48dbm I was way less than part 15 and I could not even get to the porch. ...
Just to note that -48 dBm is a power level of about 15.9 microwatts -- which is more than 10 times the radiated power needed to produce the FCC §15.239 field limit of 250 µV/m at a distance of 3 meters from a 1/2-wave dipole transmit antenna.
Druid Hills asks: "Can the antenna be connected directly to the transmitter?"
I am guessing that you are suggesting having the transmitter as physically close to the antenna as possible so as to avoid a 6' transmission line (feed line), on the assumption the feed line might be radiating?
Absolutely. In fact the EDM case has wall mount provisions built in, so it could be mounted directly proximate to the antenna with a very short cable between the two.
Carl: See what your results are with the antenna directly connected and report back to us. We might need a chart or two to clarify your findings.
When I had the sainsonic ax 05 B the rubber duck antenna that came with it as reported by my engineer friend was severely mismatched. This is why the signal was so poor. The stupid antenna was actually tuned to 132 megahertz acording to the MFJ antenna analyzer.. Don't know why that transmitter it was sold with that particular antenna. Nevertheless my range was around 50 to 75 ft set at the -48 dbm level.
I'm not sure who is more prudish, the LARGE SIZED FINGER POINTER or the layed back man who let's sleeping dogs get their rest.
Earlier in this thread our neighbor Rich pointed fingerly by saying: "Just to note that -48 dBm is a power level of about 15.9 microwatts -- which is more than 10 times the radiated power needed to produce the FCC §15.239 field limit of 250 µV/m at a distance of 3 meters from a 1/2-wave dipole transmit antenna."
The case being indicted clearly showed that TheLegacy used an attenuation device to limit his signal, yet the nosy-neighbor warning.
We could have done without it.
I considered using the CONTACT tool to lodge a complaint, but decided it wasn't important enough.
50 to 75 feet certainly falls within the 200 feet the FCC seems focused on for Part 15 FM. The mismatched antenna was likely used because it WAS mismatched, and attenuated the signal to Part 15 FM levels or less.