I've rediscovered a web site that I'd seen some time ago, containing ideas for hobby transmitters said to be suitable for Part 15 operation.
Of particular interest to me right now is one of the author's designs that deal with the "10 foot limit" for antennae on AM, and, the issue of grounding & counterpoise.
His approach was to construct a high - Q loop, often referred to as a magnetic loop. He stated that he built the loop to comply with a strict interpretation of the 10 foot limit, by building the loop as a square with 30" sides, for a total of 120", or 10' total.
There were technical challenges involving bandwidth, but, he stated he got it working for speech, and, suggested mods that might allow music.
I've used similar loops on ham radio, and, were impressed with them. I'm considering a Part 15 version. It would solve several issues related to being in an apartment with limited access to earth ground. It would also allow me to focus the signal out along the open space between buildings, instead of trying to radiate through them.
the web site is: http://www.techlib.com/electronics/amxmit.htm#Unfair Radio Transmitter and http://www.techlib.com/electronics/amxmit.htm#improved .
I thought I would bring it up here, to subject the idea to some scrutiny as to legality: does the 10 foot rule allow for a loop, as opposed to a vertical, etc?
Don't forget that the 10 foot rules includes not only the antenna, but the feedline as well - you'd have to install the transmitter right at the base of the loop to get around that (perhaps the article talks about it - I will be looking at it later).
And while only an FCC inspector can determine the legality of an installation, if the radiating element of your antenna + ground (in this case, none) + feedline is 10 feet or less (actually 3 meters or less), then I can't see where they would have a problem with it.
For now, I envision the transmitter to be in a small non metal box at the base of the loop. The box will be built ruggedly enough to also serve as the sole support of the loop. There will be powerful magnets on the bottom of the box, to allow me to slap it on top of one of the window air conditioners. The magnet will not provide a ground, in fact, there will be a signifigant air gap. There will be a cable to feed audio, and, DC, but, I will be able to install RF chokes on each and every wire, completly eliminating any RF from passing to the air conditioner.
Hey, Signal Guy;
I came across that same blog about the "Unfair" transmitter; quite intriguing. And a friend in the UK has had very good success with a high-Q AM-band transmitting loop, albeit running some tens of watts into a loop of 30 feet in diameter!
It would seem that the Unfair transmitter does indeed conform to Part15 rules, as the total length of the copper pipe in the guy's loop falls under the limit. A ground is of no material consequence in a magnetic loop, and any 'feedline' to the transmitter mounted at the base of the loop is only audio and power.
I don't suppose a transmitting loop of those same dimensions, except made of plastic sprinkler pipe, and with several turns of heavy wire inside would still meet Part 15 restrictions, as the "total length" of the radiating element might be 50 feet with five turns inside the plastic pipe. Several turns would be easier to drive than a single turn, where lots of milliamps have to flow, and with more inductance and less capacitance to tune it, the Q might not be as restrictive.
I have seen articles on magnetic transmitting loops with far smaller dimension using ferrite bars, which claimed good results in 160-meter ham applications, just a bit above the broadcast band. I wonder if anyone has tried this? I've got a PDF of the original article, which I'd be happy to email if interested. Drop a private note.
Please keep us updated with any experiements, successful or otherwise.
Thanks for response, I need to stay tight to the regs, so, will build slowly & carefully. I'm going with the copper loop. Wondering what would happen to bandwith if the loop were a very shallow cone?
Interesting ideas on large and small loops as magnetic antennas. I hadn't heard of reports of anyone running serious power into them like tens of watts.
The loop's possibilities are intriguing, going by the Unfair Transmitter page, the properties of transmission by the Magnetic portion of the Electro-Magnetic wave.
It seems that the signal from a transmitting loop would be nice and strong within a tighter area, and die off faster. Vertical antennas like we use in Part-15 are mostly sending out Electro signals, with a strong signal nearby, but a far field that goes on and on.
Electro signals are also fragile in the environment, absorbed by many things, like trees, building materials especially in the area of the antenna's near field. That explains why the usual Talking House has disappointing results with the antenna inside and you get 200 feet, but outside it might go a mile. I think the only reason my Talkinghouse gets down the street from inside, is because the signal hitches a ride on the various wires coming into my building!
That the guy's Unfair Transmitter can be heard in the bomb shelter next door gives a clue that the Magnetic signal from them can cut through building materials in a way that a Talking House antenna station can't.
A loop will also likely couple signal to any nearby loops like power lines, which might be useful in some situations. A loop also shouldn't have much of an Electro signal, so the transmission will be strong and drop out after a certain distance and fade to the background much faster than for example, Talking House.
Why have vast amounts of area where there's a really weak signal and you're sure no one's listening there? If the signal is concentrated in one area, it would be like a small cell and less likely to be detected or worried about by hostile engineers if it's only in a little area.
There are two regions to consider when dealing with antennas, the near field and the far field. The near field is within a distance of wavelength/2pi from the antenna which is about 0.16 wavelength and the far field is distances beyond this.
In the near field capacitive and inductive coupling can influence the coupling of the signal from the antenna to a receiver whereas in the far field the coupling is by means of the propagating electromagnetic signal alone.
If an antenna is producing primarily an electric near field (such as a vertical whip) then the signal will have a stronger voltage component relative to the magnetic component of the field. Conversely, a loop antenna which has a low voltage but high current will produce a field dominated by the magnetic component. Given these differences, it is to be expected that differences in near field performance will be seen between the two types of antennas depending on the receive antenna and intervening structures and obstructions and could lead one to conclude that one type antenna is better than the other.
However, beyond the near field into the far field the relative strengths of the electric and magnetic components of the radiated field will become a constant determined by the intrinsic impedance of free space and it will no longer matter whether the antenna is a whip or a loop and the far fields produced by both types will be the same providing the polarization and directivity are the same.
Measurements and observations made in the near field are not valid to extend to predict the far field performance of the antennas. There may be advantages of one type antenna over the other in the near field depending on the environment and receiving antenna but there should be no difference in the far field. The best comparison of antenna performance is that which is done in the far field.
For Part15 AM applications, the loop may have an advantage due to not needing a ground plane, but my point is that the measurements should be done in the far field.
Neil
Can a loop antenna be combined with a whip antenna?
Whip antennas and loop antennas both generate electric and magnetic fields, which always have the same relation to each other in the far field. The E field is physically aligned with the longitudinal axis of a linear radiator, and the H (magnetic) field always is 90° rotated in space from the E field, traveling in the same direction.
Neither field can exist without the other, and neither field can be generated/radiated independently.
For more info on this, see Reply 3 at:
http://www.part15.us/forum/part15-forums/transmitter-loop-antenna
i just made a loop today with a couple of 5600pf caps and oddly used a carrier current coupler to load it which oddly gave me a 1.2:1 SWR with 1.3 on either adjacent channel. had the coupler's inductor set to 10 ohm, and the series capacitance set to 1600pf. basically it was a partially-radiating dummyload but it sounded great with a wide swr curve.
Eventually somebody's going to invent something and when it happens I for one will put the new thing to work, whatever it does.
Something new always gets discovered no matter how old everything is.
The secret, in my mind, is to find undiscovered parts of the law of physics. We haven't reached the end of that story.
... The secret, in my mind, is to find undiscovered parts of the law of physics. We haven't reached the end of that story. ...
Those who promote/experiment with such in hopes of finding the "undiscovered parts of the law of physics" first need fully to study and accurately understand the existing, proven principles of physics upon which they hope to advance.
Mr. Rich says: "Those who promote/experiment with such in hopes of finding the "undiscovered parts of the law of physics" first need fully to study and accurately understand the existing, proven principles of physics upon which they hope to advance."
Obviously.
The occasional exception being The Fluke.
Rich said "need fully to study and accurately understand the existing, proven principles of physics"
The thing is, in science, you can never prove anything. The best you can do is NOT disprove something. Then, according to the scientific method, you can assume that your theory is correct...at least for the moment until something else comes along.
Newtonian mechanics were thought for ages to be "proven", until Einstein demonstrated that they were at best an approximation until you took the properties of light into consideration.
So who really knows what could be found by experimentation?
And as a major character in the Codex Alera series by Jim Butcher says, ""In the course of my life, I have more than once been too ignorant to know that something was impossible before I did it anyway."
There are things known and there are things unknown, and in between are the doors of perception.