Since 20MHz is such a long wave compared to a tiny PCB, is impedance matching between the transmitter and antenna required if the antenna is a short loop (2"-4" in diameter)? It's really intended as more of a transformer for a very short range radio link. About 6" is the expected range. I'm not terribly worried about transmitting at 100% efficiency either.
Once RF (radio frequencies) are created and launched into free space
by any means, deliberately with an "antenna" or accidentally through inadequate shielding of the generating source, they fall under the jurisdiction of the Federal Communications Commission (FCC). Except for certain "special cases" outlined in
Code of Federal Regulations, Title 47, Part 15 (47 CFR 15) a license is required. If unlicensed operation is allowed, then it must not interfere with licensed operations on the same frequency. The use of 20 MHz is not advised because this is reserved for broadcasting time information by National Institute of Standards and Technology (NIST) station WWV.
There is no such thing as "a very short range radio link." Once an EM (electromagnetic) wave is created it continues to propagate with diminishing intensity as a function of distance, but unless completely absorbed by some object it never just "stops" at any particular range.
There is always a "noise floor" present when trying to detect deliberate radio frequency emissions, but this is just background noise that can be filtered out, discarded, and otherwise "worked around" to recover deliberate emissions buried many decibels below the noise floor. So, you may think your little loop antenna has only a 6" expected range, but it does not. That may be the range your application needs, and you may adjust your power output to reliably receive a signal at that range, but the RF emissions travel far beyond six inches at any power level.
If your RF emissions cause interference, and someone complains about it to the FCC, they can come to your facility, shut you down, seize your equipment, levy heavy fines, and even (through a court of law) impose prison sentences for flagrant disregard of their rules and regulations. Does this sound Draconian? You bet it is! But it does protect our valuable electromagnetic spectrum from anarchy while providing appropriate, regulated, access to everyone. Suggest you become very familiar with 47 CFR 15 before proceeding.
