[Elecraft] Efficiency of MFJ remotely-tuned loop antennas

[David Gilbert]

You are neglecting the losses in various connections in the system ... 
including possibly the construction of the capacitor itself.  I don't 
believe that they are insignificant.  There is a reason why top quality 
variable capacitors often use welded plates.

I would also guess that contact resistance is worse for dissimilar 
materials, such as a copper wire to an aluminum tube.

Yours is a limited theoretical analysis ... not a practical one.

Dave   AB7E



On 1/18/2021 5:38 PM, Alan Bloom wrote:
> Well let's see...
>
> Radiation resistance of a small loop is 31,171 * (Area / wavelength^2)^2
>
> For a loop with a 91cm diameter at 14 MHz, I believe that comes out to 
> 0.064 ohms.
>
> Assuming the loss is due to the RF resistance of the loop:
>
> From the internet I get the volume resistivity and skin depth for 6063 
> aluminum is 0.03 microohms-meter and 23.3 micrometers respectively, so 
> the surface resistivity is 0.03/23.3 = 0.0013 ohms per square.  The 
> outside circumference of the tubing is PI * 1.05" = 3.3" and the loop 
> length is PI * 36" = 113" so the loss resistance is .0013 * 113/3.3 = 
> 0.045 ohms.
>
> So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%
>
> So worse than AEA claimed, but in the ballpark.
>
> Alan N1AL
>
>
>
>
> On 1/18/2021 3:39 PM, Wayne Burdick wrote:
>> 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 <n1al at 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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>>
>
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