[Elecraft] Efficiency of MFJ remotely-tuned loop antennas
Alan Bloom
n1al at sonic.net
Tue Jan 19 01:58:10 EST 2021
> That doesn't sound like they are welded, and given the cost
difference for welded air variables I doubt MFJ used them.
As I said, I don't have one so I can't say for sure. I got my
information from the MFJ web site: "All welded construction, no
mechanical joints, welded butterfly capacitor with no rotating contacts
... Each plate in MFJ's tuning capacitor is welded for low loss and
polished to prevent high voltage arcing, welded to the radiator ...".
https://mfjenterprises.com/products/mfj-1786
Also, for what it's worth, some of the reviews on eham.com and qrz.com
mention that it has a welded tuning capacitor.
I got the impression that one reason people often receive units with
bent capacitor plates is that they got bent in the welding process.
It would be interesting to look at one and see what they actually mean
by "welded".
Alan N1AL
On 1/18/2021 10:10 PM, David Gilbert wrote:
>
> According to another ham who recently posted here, he had to "tighten"
> the plates on the MFJ capacitor to get it to work properly. That
> doesn't sound like they are welded, and given the cost difference for
> welded air variables I doubt MFJ used them.
>
> I sincerely doubt that an actual practical small loop is only down 3
> dB from a full size antenna. That makes no sense to me at all. If
> that were the case everyone would be using one, because they are not
> that difficult to make ... at least for manually tuned ones.
>
> But you seem determined to believe differently, and it's not my place
> to convince you otherwise. You asked for inputs and I have made
> mine. Hopefully you are right and I am wrong.
>
> 73,
> Dave AB7E
>
>
>
> On 1/18/2021 9:54 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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