[Elecraft] Efficiency of MFJ remotely-tuned loop antennas

Tue Jan 19 16:35:17 EST 2021

The plates in the two air variables in the [now ancient] ARC-5 command 
TX from WW2 were very securely welded in place to the rotor axle.  We'd 
remove many of the rotor plates and re-pad it with a fixed capacitor to 
spread out the ham band on the dial.  It took a lot of bending back and 
forth to break the welds.  Lower power loops often use butterfly caps 
since there is no resistive loss through the rotor connection.  Higher 
powered loops often use vacuum variables because of the high voltages, 
although I saw a design not long ago that used two coaxial copper pipes 
with a PVC pipe as the insulator.  The inside conductor was mounted to a 
long threaded rod moving it in and out of the outer conductor.

In your list of pros, you might note that while the bi-directional 
primary lobes of the loop when mounted vertically are very broad, the 
null perpendicular to the plane of the loop is extremely narrow and 
deep.  You can use it to null out noise or even another signal without 
sacrificing much of anything in the forward direction.

My Alexloop works ok on 30, poorly on 40, and really seems to come into 
its own on 20 and up.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 1/18/2021 8:54 PM, Alan Bloom wrote:
> > There is a reason why top quality variable capacitors often use 
> welded plates.
>
> I believe they do weld the capacitor plates and also weld the loop to 
> the capacitor.  (I don't have one, but that's what I've read.)
>
> > Yours is a limited theoretical analysis ... not a practical one.
>
> A number of reviews I have read (including the QST review of August 
> 1994) have reported comparable performance to full-sized wire antennas 
> located on the same site.  If the loop is down by, say, 3 dB, that's 
> only half an S unit, which would hardly  be noticeable in the QSB of a 
> typical amateur band.
>
>
> As I see it, the advantages of the MFJ-1786 10-30 MHz loop are:
>
> - Continuous coverage on 6 amateur bands.  A convenient way to cover 
> all the WARC bands.
> - Small and light.
> - Omni-directional (when mounted horizontally)  so does not need a rotor.
> - No control cable required - control voltage is fed through the coax.
> - Narrow bandwidth provides excellent RF selectivity.  Might be good 
> on Field Day to reduce inter-station QRM.
> - Users have reported lower receiver noise compared to wire antennas.  
> No doubt that is because the isolated pickup loop prevents feedline 
> radiation/pickup.
>
> And the disadvantages:
>
> - Expensive ($500 list price)
> - Less gain than a simple dipole (although you would theoretically 
> need 6 of them).
> - Fiddly to tune.  If you QSY too far you have to re-tune.
> - MFJ quality control leaves something to be desired.  (You may have 
> to open it up when you get it and  make minor repairs.)
> - You have to pay attention to the problem of entry of water and/or 
> bugs into the housing.
> - The controller can be damaged by a DC short in the coax e.g. from an 
> shorting-type antenna switch.  (I don't understand why MFJ didn't 
> include a fuse or some other way to protect the controller.)
>
> I probably wouldn't buy the 7-21 MHz MFJ-1788 because of the poor 
> efficiency at 7 MHz.  I think you'd have a better signal just using 
> the coax as a random end-fed wire (with a tuner).
>
> Alan N1AL
>
>
> On 1/18/2021 8:17 PM, David Gilbert wrote:
>>
>> 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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>>
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>
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