Multipath FM

Carl,

What sort of antenna are you using at the receiver? You could try parallel connecting two of them spaced a foot or two apart. This is a poor man's spatial diversity system and it may or may ot work, but is worth a try.

Neil

"Are you telling me I must spend the next several years moving this fake tower? What, why, when, who, where?
Tell me that, and we'll shake hands."

Ok. Well, WHAT the problem is, is that some of those so called "certified" FM transmitters put out less than the allowed limit. WHY that is, well its called "cheap", WHY have expensive real WHEN you can have fake cheap. WHO to blame? WHERE do we point blame?

US. For letting things get so ridiculous..even setting up rules which prevent a small transmitter from sending a clear signal through a frigging wall of a house!

RFB

One characteristic of an FM receiver is its capture ratio, which is a measure of the difference needed between co-channel signals for essentially the audio of just one of them to be present at the receiver output. Another is the amount of signal it takes to drive the AM limiting action of the receiver into a useful operating region.

The bottom line is that the presence of FM multipath distortion can be quite dependent on the quality of the receiver used, as well as the amount of r-f signal it has to work with.

Even licensed (analog) FM broadcast stations have reception areas with a lot of multipath distortion even in rather good receivers. A good example is the "picket fencing" effect with periodic noise bursts in the audio of an automobile receiver as the auto is moving.

Neil's idea about using two receive antennas has merit, and spacings up to 1/2 wavelength (about 5 feet) and at varying azimuth angles to each other with respect to the transmit antenna may be useful.

Not to knock off the suggestions given to solve the problem, but really, one needs to throw up two or more antennas spaced up to 1/2 wavelength apart just to pick up a signal from the next room?

Heck you would end up with more antenna and cabling on the other side than the TX side.

It might help with the multipath but it will also increase the chance of interference to the "little" pathetic certified signal barely able to shove itself through it's own emitter wire.

Now nothing like this happened with the old rules, which used to be just like Canada's unlicensed FM limits are today, which CAN send a signal to the next room without such overkill on the reception end.

If nothing else, run a cable from the output of the TX to the receiver's antenna port. No multipath then, and no cumbersome clutter of antennas. The wired wireless way.

RFB

My question may have been confused by bad writing.

My question is.....

With everything the same in the vicinity of the transmit/ receive systems, at some times there is no multi-path interference.

But at other times a person moving about does cause multi-path interference.

What unseen or unknown elements make the occurrence of multi-path happen at one time but not another?

"What unseen or unknown elements make the occurrence of multi-path happen at one time but not another?"

Ever hear of temperature inversion? It's something that loves to affect VHF/UHF and especially microwave.

What happens is when there is a separation between warm air and cool air, signals can be affected by that layer, especially weak signals like that from a flea FM transmitter. This layering just doesn't happen at altitude, it also happens inside dwellings.

Ever notice during the winter why it seems warmer towards the ceiling than it is at the floor? This is an example of that temperature difference, and that can cause the issue your experiencing. During the summer, your roof is constantly hit by hot sun so you got coolers running keeping the inside cool, but there will be that temperature difference and that can mess with things, especially nanowatt signals.

Long microwave paths, especially the 7Ghz band are prone to temperature inversion problems. The beam literally becomes re-directed away from the receiving dish and the path is lost. Some installations resolve this by installing a 2nd receive dish at some height above the primary receive dish and the path is established for the lower dish. When temperature inversion happens, the upper dish is there to pick up that signal and the path is not lost.

Humidity also affects nanowatt signals. Loading on your electrical wiring affects nanowatt signals. Having a bunch of carriers running in close proximity with loose wires as antennas can blanket or desensitize a receiver, especially an FM broadcast receiver, particularly those that are cheap in their front end design..ie not well filtered or has a lot of front end gain in turn increased noise floor.

My suggestion, move the STL thing closer to the problem receiver area.

RFB

The mention of the temperature effect is a whole new enlightening for me and it is very fascinating.

The transmission antenna is a 1/4-wave four-element cage monopole hung vertically from near ceiling height on a bamboo tripod set atop a wooden stool.

Previously this antenna was in the open space of a double-wide door, but it was at risk of being knocked over by persons using the doorway.

It's new location is up against a wall four feet from an air conditioner.

The receiver is directly one floor down in the cellar, using a folded dipole that is hung vertically from ceiling to floor, the feed point being midway.

I am thinking that indoor FM antennas should probably be horizontal for reaching other floors, and for being de-tuned from vertical humans causing multi-paths. Comments invited.

Another possibility is to use a digital solution.

One configuration I had - I put a Talking Sign transmitter into a weatherproof box (I also did this with an SSTRAN 3000 to see how it did as well), and added a Roku Internet radio (hooked up to my Intranet with built-in wireless). I ran the line output from the Roku to the input of the transmitters. I had AC power distribution inside the box and pluged everything in there, running AC power to the box via a long waterproof cable (i.e., a lawnmover cable with the end cut off).

It worked like a charm. The only issue I had was some hum in the transmitted signal, and I resolved that by using very short, and well shielded audio cables.

The thing about the digital solution is that it either works (no distortion, multipath, etc.) or it doesn't. If it does, it should continue to work, particularly with relatively low bitrates in the stream.

"The mention of the temperature effect is a whole new enlightening for me and it is very fascinating."

Like heat rising off the surface and you see it as a warping effect in the distance. This is similar to what happens with a radio wave, so instead of a nice waveform or beam, it becomes all scattered and reflected.

RFB

Internet radio by wi fi is of great interest, artisan, and I look forward to exploring that technology. It evidently has built in defenses against the random occurrences of drop out that would occur with multi-path.

Some additional facts about the current FM transmitter tower being used here to send STL to two separate AM transmitter locations:

The audio cable from the computer to the Wholehouse transmitter is limited to 6-feet, which keeps the tower near the work station.

The tower is set near a wall to keep it away from walking traffic through the room.

I am going to continue pursuing the problem because the subject of indoor antennas interests me. I think that most of the antenna theories normally presented are based on open-space outdoor setups, and I'd like to see more attention given to the unique problems of indoor antennas for AM & FM.

The idea of two receiving antennas, a diversity method, is not being ignored. I will look into that fully as time goes by.

The next thing I want to do is figure out how to build either a circular antenna or a horizontally-vertically polarized antenna for FM transmission.

The AM transmitter in the basement would probably receive (STL) best in the horizontal plane, whereas the AM transmitter's STL in the kitchen is best fit into the space on the vertical plane, so I need both.

It is 5:30 PM with very overcast skies and a 40% chance of storms.

All of a sudden there is no multipath.

I happened to be trying something, having noticed a stack of CDs stacked right underneath the FM transmitter/antenna, and I thought maybe the CDs were messing up the signal, so I moved them.

No multipath.

So I rolled an office chair into a location that caused multipath around an hour ago.

No multipath.

I put the stack of CDs back under the tower.

No multipath.

Here's the problem as I see it....

If there are times when there is no multipath, there is nothing to fix.

When the multipath comes, I can try to fix it, but I will never know whether it's fixed, or just "past".

Per RFB's idea about direct RF cable, we in fact did have a house cable when our AV business was alive. We had 3 TV modulators on the cable for our in-house production use, and had the channel 3 output from a VCR so we could watch television during dinner.

Those cables are still installed, 75-ohms I think, and could be put to use for FM.

But, the purpose of the wireless FM is to avoid running audio cables, and right now our current work station does not have a cable-ready "hole" in the baseboard.

It has to be wireless for now.

I doubt that temperature or humidity effects within a room are going to cause any observable changes in the way 100 MHz signals bounce around or get from the transmitter to the receiver. As Carl observed, physical objects are the likely cause of his "mulltipath". Here, seated 3 feet from my FM whip, sometimes I hear a strong buzz on my Walkman in my pocket and moving just a few inches restores the signal. The noise I hear sounds like the digital crap on HD and there is a strong station just a few hundred KHz down the dial from my frequency. The change I and Carl observed from time to time may be a change in the interfering signal's multipath and not ours..

Carl, be aware there can be problems with wires running around in an environment with part 15 transmitters operating. I am testing my new outdoor system and have found that the interference and buzzing heard on the indoor receivers changes as I move wires around. My hypothesis is that the wires are acting as antennas and are coupling the AM signal from the transmitter to the wall warts and other things in my listening area. The hum is not in the transmitted signal, it originates near the receivers. For example, the hum was greatly reduced when I placed a powdered iron toroid over the lead wire feeding power and audio to the outdoor transmitter probably because it chokes off the RF coming down this line before it can get into my computer or near the bank of wall warts here.

I have also noticed that the hum depends strongly on what is grounded and where and this story is not yet written except to say that wiring in an RF environment can cause counter intuitive things to happen.

Neil

Something changed a few hours earlier and there has been no multipathing ever since. Perhaps somebodies SUV parked nearby might have driven away.

In addition to all the wires and metals located near an antenna, as Neil pointed out, I wonder about all solid objects, from walls to books.

I compare this to a sentence from Radio Handbook, p.27.2, "In areas of poor surface conductivity, the actual reflection surface may seem to be several feet below the actual surface and the layer of earth near the surface acts as a lossy dielectric to the radio wave."

I find this to be comparable to the indoor surfaces, which also act as lossy dielectrics and to some degree as reflective agents, at various angles.

I suspect that for indoor antennas neither vertical nor horizontal is recommended. I believe antennas should be at angles to the surfaces, perhaps 67.5-degrees, or 22.5-degrees, depending on how you look at it.

The transmit and receive antennas should be at this same angle, with respect to each other, for maximum polarity match.

I'm guessing, but it's an attempt to escape the intensity of a constant reflective relationship when an antenna is parallel with a nearby surface.

With an antenna actually on a surface, as when I used a glue stick to run a white wire up the wall, the wall and antenna would be in phase, in that there would be no distance between them, although some reflections come back from signal that penetrates the surface of the wall and reflects from other materials within the wall.

Perhaps wall materials could be found that were minimally absorbent at the frequencies of interest, thus returning less reflections from within.