http://elecraft.85.s1.nabble.com/Efficiency-of-MFJ-remotely-tuned-loop-antennas-tp7667578p7667641.html
> > 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 <
[hidden email]> 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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