So, I was cleaning in the basement this weekend and happened upon an old CB mag mount antenna stuck on the side of a refridgerator. I got to thinking --
When you use a magmount CB or ham radio antenna, there is no direct connection between the ground side of the antenna and the car body -- there's a plastic layer on the mag mount and several layers of paint and primer. Yet the car becomes the ground plane for the antenna through capacitance coupling. Or so I've read. I never really thought about it before.
Additionally, I recall years ago in the days of the big ol' cell "bag phones" -- you know, those big three watt jobs from Motorola in the leather carrying cases -- that we had external antennas mounted on a window of the vehicle that stuck on the outside, and a matching coupler that stuck on the glass on the inside of the car -- the signal would pass through the glass to the antenna, again, referred to as a capacitive coupling, I believe.
Naturally this brought the questions to mind. Why wouldn't this concept work for Part 15? Would it work at all, and would it be legal?
Example. Get a base magnet from an old mag mount antenna, connect to your transmitter ground, make sure the layer of plastic is still in place on the bottom of the mount, get a matching size disc of metal, stick it on the bottom of the magnet, run a lead from that to your chosen ground. In my case, an elevated installation, this would give me a way to ground my transmitter with no direct connection between the transmitter and ground lead. Clearly, running a plain ol' ground lead would clearly be a violation as a "long ground lead". But if you could demonstrate with an ohmeter that there was NO connection between the transmmitter ground and the ground lead, would it be legal?
Further, say you had the standard 3 meter antenna, as I do, as provided by Procaster. If I were to put a plastic sleeve over, say, the top 3 feet of that antenna, and then slid another 10 foot section over that would it in effect create a longer, more effective antenna that would be legal, since it was not actually connected to the legal antenna? Again, an ohmeter test would show the extended section was not connected to the actual legal antenna.
Perhaps this has all been discussed before. But see what finding junk in the basement leads to? Answers to the effectiveness and legality need to be investigated! Now, I don't do ANY sort of experimenting with my installation, as quite simply, it's been on the air for 18 months, I actually have listeners, and I have what I like to refer to as a "blatantly legal" installation. So, I don't want to screw with it while it's on the air and negatively impact my listenership. But it DOES make me think about purchasing another transmitter simply to experiment with. I have the equipment necessary to take actual field strength readings at, say, 30 meters. And can readily deterine if any of these schemes actually work. Hmmm.
Tim in Bovey
... an ohmeter test would show the extended section was not connected to the actual legal antenna. ...
A meter reading only d-c values will show no connection to the extended section, but not a meter capable of measuring r-f signals in the spectrum of interest.
Capacitors don't provide a conducting path for d-c, but they do for a-c.
Tim's idea musn't be dismissed based on the fact that AC is able to travel capacitively.
Our 3-meter antennas already couple passively in a capacitive manner to surrounding metals in the near-environment; this same phenomenon is well known with full power stations which become reflected and or canceled by capacitive reaction with fences, metal structures, nearby vehicles and so on.
The kind of un-intended capacitive couplings that occur naturally are not expressly counted in the FCCs part 15 rules, which simply refer to length of solid wire without regard to AC or DC voltage.
I would argue that adding INTENDED capacitances is not expressly disallowed by FCC rules... so it's probably yet another situation where inspectors may vary as to their findings.
Still, adding an actual capacitor, whose "air-space" is concealed inside the body of the part, has the look of a solid connection... so Tim's plan should best be kept in a category of "demonstrable air-space" coupling.
But my platform would be that no ground lead would be connected. In this case I'm using the ohmeter to demonstrate continuity. No extra antenna would be connected, a continuity test would verify that. As Carl mentioned, tons of metallic objects can affect signal, re-radiating, reflecting, etc.
What would be the effect of say, a dozen radials, say, 6 feet long, mounted 6 inches below an elevated transmitter but in no way connected to it? Or a giant teepee of radials from 6 inches below the transmmitter mount running 30 feet to the ground from an elevated transmitter? But never touching or being connected to the transmittter? Would there be any effect of, say, a ground lead from a cold water pipe in the basement, up 30 feet to a transmitter, but not connected to it in any way? With, say a 1/8 inch air space between the top of the lead and the ground connection on the transmitter? Would there be any coupling with any effect? I'm sure there are modeling programs and electronic formulas that woujld tell us these sorts of things -- but with another transmitter I'd be testing these things "in the field".
I'm quite sure any of these ideas would come in under a gray area subject to interpretation by an inspector, but if appealed I wonder if they would hold up to the letter of FCC rules.
"Give a man a FIM and you give a man the urge to find a better way"
Tim in Bovey
Tim wrote "I'm quite sure any of these ideas would come in under a gray area subject to interpretation by an inspector..."
I believe you have answered your own question.
The reactance presented by a capacitive coupling is inverse to the frequency. Those window glass antennas and mag mounts designed for cell use or for VHF amateur radio use operate at frequencies where even a small capacitance gives a small series reactance. Even at 27 MHz for the CB this reactance is rather small but not so at AM broadcast band frequencies.
To raise the capacitance and lower the reactance the area can be made larger and the distance between the conductors can be made smaller.
As Rich pointed out, such a connection which is open to DC is not open to AC and the conductors are therefore connected. The degree of this connection depends on the capacitance and the frequency and if something were constructed in a way to maximize this capacitive connection such as the coaxial antenna setup or the radial system you mentioned it could be argued that there is a connection. I would not buy that there is no connection but I am not a FCC inspector.
Having said that, the incidental capacitive coupling to nearby objects or power lines is very small in comparison and probably would not be considered as a "connection". This is how I would interpret it.
Neil
If a part 15 transmitter is mounted atop a metal pole which is stuck in the ground but not metalically grounded to the transmitter it seems like an equivalency to having a non-connected ground wire up the pole... except the pole itself would be the long-ground.
Happens all the time. Not illegal.
My transmitter (a Procaster) tested continuity between the ground terminal on the borrom of the transmitter to the bracket mounts on the back of the case. So, near as I could tell, the pole would indeed be connected to the transmitter ground, at least in this case. This is why my transmitter is mounted to a 3 foot piece of gray PVC electrical conduit, which is in turn bolted to the side of the wooden attic window frame with two metal pole mounting brackets. The transmitter is completely insulated from the house, the brackets, etc. As I said, I want to be "blantantly legal." I certainly don't keep my station a secret, and should anyone evil start nosing around I don't want any question as to legality.
I pose these technical questions just because I think of stuff and wonder about it, so I toss it out for discussion. And I wouldn't be beyond experimenting to see what the actual results might be in practice if I had a second transmitter up. But for my purposes I don't really need to try to increase my coverage. if I get out any further it will just go into the wilderness where no one is living, or if it went, say, another mile, it would mean people passing on the highway would hear me for less than an additional minute.
But it's still fun and educational to wonder....
Tim in Bovey
Tim I hope you do establish a second station for experimentation. I believe experimentation is "the other" great use for part 15 radio... with programming being the most common use.
By the time my entire system is built I'll have numerous program channels with backup transmitters for each one, as well as a set of experimental sites for antenna/ground combinations.
Might keep an inspector busy for several days.
... This is why my transmitter is mounted to a 3 foot piece of gray PVC electrical conduit, which is in turn bolted to the side of the wooden attic window frame with two metal pole mounting brackets. The transmitter is completely insulated from the house, the brackets, etc. As I said, I want to be "blantantly legal." ...
No matter if the "ground lead" conducting path is hard-wired, capacitively coupled between the r-f ground terminal of an elevated transmit system and a true r-f ground, or even absent -- such a system still may be non-compliant with FCC §15.219.
This also can be the result of the added antenna system length provided by the d-c power and/or audio program lines connected to the elevated transmit system, unless the radiation from such conductors (provably) is suppressed to the satisfaction of the FCC.
... I believe experimentation is "the other" great use for part 15 radio ...
However "experimentation" probably will not be a winning rebuttal for unlicensed operators receiving FCC NOUOs for non-compliance either with §15.209 or §15.219.
Would a police officer accept/pass the explanation of a driver clocked doing 100 mph in a 45 mph zone who reported that his speed was only "experimentation?"
Member Rich made these two points:
However "experimentation" probably will not be a winning rebuttal for unlicensed operators receiving FCC NOUOs for non-compliance either with §15.209 or §15.219.
Would a police officer accept/pass the explanation of a driver clocked doing 100 mph in a 45 mph zone who reported that his speed was only "experimentation?"
Now my Two Replies:
Experimentation with part 15 transmission would naturally be a temporary test period during which to make measurements and observations. The likelihood of an FCC response during such a limited test is nil. No persons are put at risk.
In the case of doing experimental speed testing with a vehicle on a public street we hope the law would crack down on the violater for endangering the public saftey. It's not a symetrical analog to the transmitter test.
"This also can be the result of the added antenna system length provided by the d-c power and/or audio program lines connected to the elevated transmit system, unless the radiation from such conductors (provably) is suppressed to the satisfaction of the FCC."
This is where I rely on the certification of the transmitter. My unit is installed with the cable and power supply provided by Procaster. I would guess their certification process, involved as it is, would include the process of getting power and sound into the transmitter. I don't think the FCC would have certified a unit without providing power and audio to it in the certification process. I also use no outboard processing, but that which is included in the transmitter itself. Pretty sure documentation for a certified unit would include to not use more than 2 inches of wire to connect power and audio if this was a concern. Specifically since their documentation outlines an elevated installation in the first place.
Tim in Bovey
the audio and power line radiation is provably suppressed to fcc satisfaction. it part of the EMC Testing @ the FCC Testing Lab. it has to be under a certain threshhold to pass is my understanding. look up 15.107 Conducted limits. i'm not sure if kit transmitters such as the sstran conform to this but certified transmitters have to conform to this rule part.
Audio and power lines to radio transmitters are configured according to standard practice and no material difference exists between the way "certified" transmitters are wired compared to how any other transmitters are wired.
§ 15.107 Conducted limits.
| Frequency of emission (MHz) | Conducted limit (dBμV) | |
|---|---|---|
| Quasi-peak | Average | |
| *Decreases with the logarithm of the frequency. | ||
| 0.15-0.5 | 66 to 56* | 56 to 46* |
| 0.5-5 | 56 | 46 |
| 5-30 | 60 | 50 |
| Frequency of emission (MHz) | Conducted limit (dBμV) | |
|---|---|---|
| Quasi-peak | Average | |
| 0.15-0.5 | 79 | 66 |
| 0.5-30 | 73 | 60 |
It seems unlikely that a part 15 radio transmitter drawing its power from the AC line is capable of back-hauling enough energy into the line to come anywhere near violating 15.107.