The black helicopters are taken for granted. They seem to have a lot of time to spend watching me.
You'd think they'd have something better to do.
The expression "All things equal" comes to mind in the FM signal field here inside the Internet Building where a C.Crane Transmitter/antenna has been in the exact same place for many days, the tabletop radio out in the Upper Management Lounge has remained in its location for months, the rooms and their contents remain more or less the same, maybe different chairs get used to mix it up a bit... but the multipath, the loud buzz caused by physically "being" in certain spots does NOT remain "equal."
At times there is no multipath, at other times it's caused by being near the receiver, but that changes when multipath occurs walking in a different room. It is literally a moving target.
Some unknown factor causes the mutipathic behavior of the FM field to vary over time.
What I suspect is that "something is going on."
What about EMI? Can something like a refrigurator or AC unit start a magnatic field strong enough to cause a multipath like effect on the receiver? Also we talk about ground alot and the very patch cords you use to plug into the FM Transmitter can act as a ground reflector or like a CB antenna you have the positive element which is the top of the antenna and then the reflector radials. So the longer the patch cord the longer ground wire you have. If you connect a 10 foot patch cord to the C.Crane transmitter you'd have a better ground than that little cord that comes with it. I know the telescoping antenna is the actual antenna, but the patch cord is the actual ground. So even moving the power cord around on your computer could effect the signal. You'd want to keep the patch cords as straight as possible (Not bunched up). It may not make the signal go further, but it will give a more even reflector and may prove to have better performance. I know part 15 FM has lots of little things that can cause hum and interference as I have found with my SainSonic FM transmitter. If I have the boom box too close to it (sometimes) I hear a hum. But when I straighten out the power cord and keep the power supply box as far away from the transmitter as I can the hum is gone. I don't hear it as loud it barely is audible. But if the cord is not straight it does effect my unit (low or high powe. More so on low). Also I see that sometimes other electronics running in the house seems to effect the SainSonic at times as I've experimented. Don't get me wrong 99% of the time the signal is Rock solid and clean. But there are a few things that seem to make this transmitter act a little weird. Even if I get too close to the transmitter or touch the aluminum case of the transmitter on low power will make it hum and also lowers the signal. So the case acts as a reflector I see. Wonder what would happen if I were to tape a 4 ft piece of wire to that? I bet on low power I'd get to my porch. I would not want to increase the signal on high power as it is well and good there and we don't know if that is slightly over already. But low power when I just need my property I could use the extended ground to reach that field strength of part 15 and where it don't go too far out of my property. Has some good uses when I mess around playing stuff from my computer and don't want my neighbors to be confused lol. The reason I mentioned this was not to go over part 15, but to note how ground can effect a signal.
The audio ground of the C.Crane FM Transmitter is not RF ground.
The audio ground and the power jack, positive and negative, pass through inductance coils which filter RF from passing.
The design of both the C.Crane and Wholehouse 2.0 completely isolate the external world from being able to connect to the transmitters true RF grounds.
No reflector means varible floating ground. When there is no ground electricity will try its best to find a ground. So what you get is a signal that will wavor up and down. The SWR will also be extremely high. Hence another reason most of these part 15 transmitters can't even cover a few feet because there is no reflector for the signal to push itself off. There should have been an allowance for a little reflective radiation for part 15 when they done the 250 uV/m at 3 meters. OK best thing you can do is put the transmitter on top of a metal cabinet. Even though it doesn't touch ground directly it will make for a slight reflector. Now the Schoche (in stock condition) I gaurantee if you move the cord around will make a difference. Plus if you add length to the cord (this is where I wish we had a way to measure the field strength to see if it actually went over). Hell just plugging it onto something with a long power cord could actually act as a ground and make the signal go further. In essance it could be why these transmitters are set lower than the max limit because they allow for these variables because all things are not equal. Even a metal table could make a difference. You know this in itself could explain why some people are getting further with stock part 15 FM transmitters tan other folks just because how they have the transmitter placed and things around it. Even if you place it outside. You still have to allow for surroundings. Makes for some interesting results and things to experiment with. Since its such low power more variables can fall into play that may not effect say 500mW or even 2 mW because of the higher power you would not notice it as much. Fun to play with though. We can learn how different things effect the quality of the signal. Now I know I'm gonna hear 15:239 sometime soon in a reply lol.
Previously the indoor tests done with the C. Crane FM Transmitter utilized a vertical antenna, and as reported there was a recurring problem with multipath disturbances at one or more radios in different rooms.
It seems to my thinking that a horizontal FM transmitting antenna would be able to express itself better inside a building because rooms are wider than they are high, possibly resulting in fewer standing-waves.
Now running on a horizontal wire antenna we get the same signal coverage but in several walk-arounds have not been able to cause any multipath interference at all.
We will observe this setup all weekend.
As reported earlier in this thread I tried an End-Fed L Antenna, which caused a 10 dBu drop in the signal level from either a vertical or horizontal antenna alone.
I've always wondered how a Mid-Fed L would work (this is FM by the way) and I'm doing it now.
Spectrum Analyzer readings:
Vertical wire antenna alone: 53 dBu
Horizontal wire alone: 55 dBu
Mid-Fed L antenna: V = 56 dBu
H = 60 dBu
There seems to be a signal gain and I am guessing that perhaps now I have vertical/horizontal polarization.
I walked around the rooms and detected no multipath on any of four radios.
You should walk around outside and see how far you go now. Interesting indeed.
Using a Whole House 2.0 in it's as supplied certified state isn't really a standard. Certainly with a spectrum analyzer you can compare relative output quite well, but my guess is going to be (and this is only a guess) that most of these available certified transmitters are not operating at exactly 250uV/m. And I bet a lot of the little metal box imports are way over. Just a guess. To compare outputs, swell, but to actually know what the output is, nope.
My Decade should be here next week. I'm anxious to play with it as I've never messed with FM. But I can easily get the exact field intensity and see where it falls on the chart right out of the box. I am not putting my money on 250uV/m at 3 meters. I could be surprised.
To actually measure field intensity (not field strength, two different things) there are basically two devices to use, the Potomac FIM-71 or the Ztechnology R-506 or R-507. I have both a FIM-71 and an R-506. The Ztech units are the current device of choice at the FCC mostly because they can easily be tied to a GPS and output data to a computer for printing and saving showing location, computing FI and distance, etc. But to measure field intensity at 3 meters a FIM-71 is just as good. Not only do you need the unit but you MUST use a calibrated antenna, and take into account antenna factors and frequency -- which are computed automatically in the R-506 if you have the antenna calibration data loaded into the machines internal computer (which I do). Clearly these units are out of the grasp of a hobbiest with the FIM-71 only available used and generally selling for $3000-$4000, although you can find them cheaper when you're lucky. The Ztech units are available new. The R-506 is about $11,000 and a calibrated antenna is about $1150. I stumbled into mine being sold by an engineers estate who had no idea what it was. Then I spent a small fortune for the calibrated antenna and to have the factory load the calibration data into the machine so I can take corrected direct readings.
So, it's pretty easy for me to grab exact field intensity and see how things stack up at 3 meters.
As it is today the FI limit of 250uV/m is just that. That is omnidirectional, of course. If you whip up some sort of experimental antenna and you get, say 400 uV/m in one direction, you're not legal.
Tim in Bovey
About that Wholehouse 2.0. (remember, I know nothing about the FM transmitter market and have yet to see any of them in real life). I see them on Amazon and some of the reviewers have mentioned the "included antenna extender wire" that gives more range and also the "range boosting switch" which located under the ID sticker in the battery compartment? Is this true? I'm betting adding the wire and flipping the switch brings it to the Canadian standard? I can't imagine they'd sell it operating at less than legal power and then letting you flip a switch to boost it to maximum legal power.
These FM transmitters are quite befuddling.
Tim in Bovey
Tim, we are very fortunate that you have chosen to take a genuinely professional study of these "near toy" small transmitters.
I realize that all the comparative readings I've been taking with a spectrum analyzer are not true values, merely relative amounts without the "relative" of a base of known comparison (excuse my attempt at sounding scientific).
Based on Rich's guideline of "using certified transmitters in their certified state" I believed that it might mean something that my Wholehouse 2.0 and C.Crane FM1 both yield exactly the same reading at the same distance, so I dubbed the moment as "probably compliant."
Other than that single observation my experiments since then have been aimed at coming up with an indoor antenna config that works solidly through the house with no multi-path caused by walking about.
Unlike most other FM enthusiasts I am not shooting for maximum range on FM, just reliable in-house service.
Yes, the Wholehouse 2.0 (and from what I have heard also the 3.0) has a secret switch which boosts the field strength. Is it the Canadian setting? The supplied instructions say nothing about it.
On the Whole House 3.0 you would hold down Power and Mute at the same time for about 10 seconds till you see the lightning bolt. And Mark tested that and we found this to be at the BETS level for Canada. So tht in mind he was able to get around 1,000 feet at high power if I remember his tests. That is close to what I'm shooting for but he didn't have a Patomac FIM-71 to know the field strength at both low and high power. He did however compare the BETS test to the Decade CM-10 set to the BETS level. We will have to see the data here. I'm sure the same high power mode for the SainSonic AX-05B works on the Decade CM-10 as this transmitter has the same circuit board as found online. So if you hold power and plug it in and get a Lo or Hi we'll know too that it is a clone of SainSonic sold at a robbery price tag as I suspect with the Decade. If its not 100% the same only a few mods were made. For one the Decade has a telescoping antenna Made Into The Unit. Other than that it is a SainSonic AX-05B transmitter with a different name on it. But again we'll see. I'm so curious to know rather or not this is true. Mark himself has said it was the same transmitter. If this is the case I'd like to test it with a SainSonic. I wonder too if Decade used a better power supply than SainSonic? Mark reported no hum even right on top of it. I sometimes get some but I found straightening the cords did help. SainSonic claims it is using 75uS pre emphasis and I'd like to know if this is true as well in their statements.
Regarding Carl's post #22
Carl, to my understanding, most of the Decade MS100 FM transmitters that carried my former station's signal used a dipole antenna commonly used as an indoor television antenna for VHF LOW/HIGH reception. The rods were kept almost horizontal and internal penetration of a large building was very possible. At least 4 floors in a concrete building was covered quite well. The MS100's were ran at USA specs with the exception of the addition of the folded dipole. Of course, the included one element telescopic antenna of the MS100 was totally collapsed for 87.9MHz, the two dipoles were measured out to the total length required for 87.9MHz and there was no multipath issues noted.
I'm sure people would say those MS100 were no longer certified because the antenna was altered with the addition of a dipole, but it was truly the only real attempt to get a signal to home stereo systems that came with a 300 Ohm dipole wall mount FM antenna. As for boomboxes with a single telescopic antenna, I think once the signal penetrated the walls and floors, it was altered to work both horizontal and vertically.
You might want to try a few experiments using such an antenna on the test transmitter.
Here is a photo of such the antenna we used, but this one is vertical 
Bruce.
I have always wondered what the variable power control inside the C.Crane FM Transmitter actually looks like on the spectrum analyzer. Today I found out.
I feel confident in saying that VR2 (as per the schematic) is a fine-factory adlustment for output to compensate for differences between manufactured units.
Turned all the down counter-clockwise the output disappears into the noise-floor.
The RF output range on the pot is all in the upper region between about 2 o'clock and fully clockwise.
As I have done before, I placed the C.Crane in its original certified state at the same location with the Wholehouse 2.0 in its certified state. When I first did this a few days ago I got exactly the same readings over on the spectrum analyzer, 25-feet away, but today there were slight differences.
Today the Wholehouse 2.0 has ranged between 33 dBu and 39 dBu, and I can only guess that changing placement of things in the transmitter/antenna field caused the changes.
The comparative reading on the C.Crane is 40 dBu with VR2 fully clockwise.
When Tim does the professional measurements I suggest you (he) watch for changes in results over time with the question: "What is causing the readings to change?"
MrBruce the dipole you showed is a real great work horse for transmitting and receiving.
As of this time I am only analyzing the in-house stability of the FM signals, but intend to take measurements outdoors around the campus and off in the car.
My Mid-Fed L Antenna still has me believing that I have invented an improvement over plain vertical, but fitting an antenna into an indoor environment so it doesn't poke you in the face is a tricky problem.
Johny C. in Ragland told us the rules in New Zealand ban horizontal antennas for the 1 Watt stations. Makes you wonder. Johny explained why but I'll need to ask again.
Carl, you should try feeding a mono audio signal into your transmitter. That very often eliminates FM multipath problems. Wide stereo separation in the audio (L-R) can wreak havoc on multipath. Grab yourself a good ol' stereo to mono adapter plug, and see what happens!