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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