It would be cool to build one for a part 15 AM Transmitter.
I don't completely get it, I'll probably have to read closer, but it looks like a center loaded whip, and there's a slant feed wire to the top, or it's fed at the base. If fed at the top it seems like a loop with a vertical radiator, not likely a small loop, the inductance loading makes it a big one.
These Part 15 FM transmitters that we've been messing with recently have short stubby antennas, so I wonder if we might wind little coils to make them act like slightly longer antennas.
When answering my question, first answer the question itself about what little coils might add to the performance of short antennas.
Second, if you are driven to do so, give me the lecture about how little coils would invalidate certification and defy 15.239, which I am not asking.
Carl said "Second, if you are driven to do so, give me the lecture about how little coils would invalidate certification and defy 15.239, which I am not asking."
I said: That would be Rich's department.
Loading an electrically short Part 15 FM antenna using a coil doesn't make it electrically longer, except for the end-end length of the added coil.
But if that coil offsets some/all of the capacitive reactance at the feedpoint of the short antenna, then more of the available transmitter power will be radiated by that antenna system.
This is a direct analogy with operation under §15.219 when using a base loading coil with a ~9.5 foot whip.
Something to add to our FM transmitter knowledge.
Thank you Rich.
In cases where a small coil would manage to radiate more of a transmitter's available power we could make use of those internal pots to turn back the output power, so as to come out 15.239 compliant out there at 3-meters.
For those who like to tweak all the time.
Those wanting to be compliant with §15.239 would need an accurately-calibrated field intensity meter (including its supplied antenna), and some knowledge of its use.
But increasing the radiation efficiency of the antenna system used by a Part 15 FM transmitter seldom is needed, as many of them radiate fields in excess of 15.239 even without a loading coil.
Alternate methods have been found to match a modified Part 15 FM transmitter to a known compliant transmitter by use of a spectrum analyzer, S-meter, or the dBu reading on select FM radios such as the TECSUN PL-310.
how does one know that the "reference" system actually meets §15.239?
Proving that requires an accurately-calibrated field intensity meter.
An FCC-certified Part 15 FM transmitter doesn't always comply, as shown by Tim's FI measurements.
I've got good news.
Tim's field test of the C.Crane FM2 Part 15 Transmitter found the device at only half of the legal output per 15.239.
Therefore using a C.Crane FM2 as reference is more than safe.
Tim's test also showed that the C.Crane FM2 greatly exceeded the §15.239 field limit when its "secret" internal power setting pot was cranked up.
Don't turn it up.
It can be turned down, which is useful when it comes to antenna improvements.
Why bother adding a loading coil to the antenna of a Part 15 FM transmitter to increase radiation efficiency, and then reducing transmitter output power so as to comply with §15.239 (assuming you had accurate means of measuring such compliance)?
Using an accurate FI meter could permit setting the unmodified transmitter power output to meet §15.239 wihout the need for a loading coil -- at least at the location of the measuring antenna, and in that propagation environment.
Happy to tell you that you don't need to worry about it because you aren't doing it.