Hi Alan,
72% sounds a bit high. Is this number based on loop size alone ("in theory")?
Or are they taking conductor geometry and other losses into account?
Wayne
N6KR
> On Jan 18, 2021, at 2:05 PM, Alan Bloom <n...@sonic.net> wrote:
>
> MFJ makes a pair of small, remotely-tuned loop antennas, the MFJ-1786 that
> covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+ MHz. As far as I can
> tell, the two antennas are identical except for the size of the tuning
> capacitor. Each consists of a 3 foot (91 cm) diameter loop made of aluminum
> tubing and a plastic housing that contains the tuning capacitor, motor, and
> coupling loop. No control cable is required since the control voltage is
> sent from the control box in the shack to the motor in the antenna via the
> coaxial cable.
>
> Before I purchase one of these I wanted to get an idea of the efficiency of
> such a small loop. MFJ is silent on the subject so I did my own
> calculations. The calculations and results are on a 1-page document that I
> uploaded to Dropbox and can be downloaded here:
>
> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>
> My calculations are based on the assumption that the efficiency of the MFJ
> antennas is similar to the (no longer manufactured) AEA Isoloop (my reasoning
> for that is in the document) and that AEA's specification of 72% efficiency
> at 14 MHz is correct. From that number I can calculate the efficiency and
> gain on all the other bands.
>
> If you don't want to download the document, here is a summary of the results:
>
> Freq Eff Gain with respect to a half-wave dipole
> MHz dB dBd
> 7.0 -7.3 -7.7
> 10.1 -3.5 -3.9
> 14.0 -1.4 -1.8
> 18.068 -0.6 -1.0
> 21.0 -0.4 -0.8
> 24.89 -0.2 -0.6
> 28.0 -0.15 -0.5
>
> I'd be interested in any comments people may have on the accuracy of
> my assumptions and calculations in the document.
>
> Alan N1AL
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