There is a seemingly endless series of articles on small transmitting loops on the cheap.
(eHam 2014) is another, it describes a so-called magnetic loop for transmitting on 14.2Mhz using 4.57m of 2.6mm copper wire for the main loop. The author reports the bandwidth of the finished antenna as 100kHz. One of the claimed benefits is that with such wide bandwidth, a variable tuning capacitor is not required.
There is enough information in the last paragraph to make a good estimate of the antenna’s efficiency, the percentage of RF power input that is actually radiated.
If it has a bandwidth of 100kHz, it has a Q of 14.2/0.1=142.
We can calculate the radiation resistance of the loop as around 0.432Ω, the is the component of loop impedance that results in radiation due to the loop current.
A loop of those dimensions has an inductance of 5.86µH, a reactance of 523Ω. Given that Q had been calculated as 142, then the total loop resistance must simply be 523/142=3.68Ω.
Radiation efficiency then is simply Rr/Rt=0.432/3.68=11.7%. The author’s claim of 40% efficiency is not supported by his own data.
Average gain is -9.3dB.
Twice as much wire formed into a dipole would radiate nearly 10 times the power!
Using the loop well away from its centre frequency and using an ATU as advised to mask the problems will degrade efficiency even further.
- eHam. 2014. Magnetic loop observations. http://www.eham.net/ehamforum/smf/index.php/topic,95147.0.html (accessed 20/02/14).