[Elecraft] Efficiency of MFJ remotely-tuned loop antennas
Alan Bloom
n1al at sonic.net
Mon Jan 18 23:54:51 EST 2021
> 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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