[Elecraft] Efficiency of MFJ remotely-tuned loop antennas
Walter Underwood
wunder at wunderwood.org
Tue Jan 19 00:23:56 EST 2021
MFJ sells the butterfly capacitors they use. You can see them here.
https://mfjenterprises.com/products/mfj-19
https://mfjenterprises.com/products/mfj-23
wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)
> On Jan 18, 2021, at 8:54 PM, Alan Bloom <n1al at sonic.net> 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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