A reader referred me to a video of a presentation to a radio club, the subject being “The Truth About Magnetic Loop Antennas – MYTH BUSTING!”
The presentation includes prediction and measurement of a small transmitting loop. This article tries to reconcile the claimed radiation efficiency between prediction and measurement.
Radiation Efficiency
The presentation liberally uses the term “efficiency”, let us take that to mean Radiation Efficiency:
the ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter.
Note that Radiation Sphere requires that radiated power must be measured / determined / summed in the far field.
Radiation fields decay inversely proportional to distance, other fields immediately around an antenna decay more quickly and are insignificant for the purpose of radio communications at great distances. Hence, Radiation is the usual objective of radio communications antennas.
The calculation tool used in the presentation defines:
Efficiency (%): The percentage of input energy that is actually radiated and not lost as heat.
The prototype loop antenna
The prototype loop antenna is described:
A table is given which appears to be predicted performance based on https://miguelvaca.github.io/vk3cpu/magloop.html .
The table above contains quite high predicted efficiency.
I have attempted to duplicate the tool model at 7.0MHz.
Above, focusing just on three key results, we get:
- BW: 2.05kHz;
- efficiency: 46%; and
- Q:3409.
The results here are not quite exactly those in the table given in the presentation, it may be due to different inputs and / or revision of the calculator over time. Nevertheless, the results are close enough.
On the surface of it, the efficiency figure would look very attractive though the bandwidth might be impractical (it might be very difficult to tune for full efficiency, and perhaps not stable over time and temperature).
But, the numbers are simply unbelievable.
Measurement
The presentation offered a VSWR curve for the antenna at 7MHz.
Above, the VSWR curve dips to 1.0, so we can infer the half power bandwidth from the VSWR curve. Half power bandwidth for this type of antenna is well approximated by the VSWR=2.618 bandwidth (Extrapolating VSWR of a simple series resonant antenna).
I measure bandwidth at VSWR=5 to be 40kHz, which implies a half power bandwidth of 22.4kHz, more than 10 times the predicted value.
I did not see description of the measurement environment (height above ground, soil type, proximity to structures etc) in the presentation, they are all relevant if the experiment was to be replicated.
Let’s estimate radiation efficiency from the above information.
Radiation efficiency estimated from the measured VSWR curve and loop geometry suggests radiation efficiency is 4.2%, less than one tenth that shown in the table from the presentation.
Conclusions
The following conclusions are drawn:
- the prediction tool used in the presentation gives a grossly optimistic estimate of radiation efficiency; and
- the measured VSWR curve and loop geometry can be used to calculate a more realistic radiation efficiency figure.