[Elecraft] Efficiency of MFJ remotely-tuned loop antennas

[Dave Cole]

Thank you for sharing that...  I have saved it to my system.

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 1/18/21 2:05 PM, Alan Bloom 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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