Here's an interior shot of the tuning capacitor https://imgur.com/a/sYdvgzF
-Eric KI7LTT On Mon, Jan 18, 2021 at 10:59 PM Alan Bloom <n...@sonic.net> wrote: > > 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 <n...@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 > >>>>>> ______________________________________________________________ > >>>>>> Elecraft mailing list > >>>>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft > >>>>>> Help: http://mailman.qth.net/mmfaq.htm > >>>>>> Post: mailto:Elecraft@mailman.qth.net > >>>>>> > >>>>>> This list hosted by: http://www.qsl.net > >>>>>> Please help support this email list: http://www.qsl.net/donate.html > >>>>>> Message delivered to n...@elecraft.com > >>>>> > >>>> > >>>> ______________________________________________________________ > >>>> Elecraft mailing list > >>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft > >>>> Help: http://mailman.qth.net/mmfaq.htm > >>>> Post: mailto:Elecraft@mailman.qth.net > >>>> > >>>> This list hosted by: http://www.qsl.net > >>>> Please help support this email list: http://www.qsl.net/donate.html > >>>> Message delivered to ab7e...@gmail.com > >>> > >>> ______________________________________________________________ > >>> Elecraft mailing list > >>> Home: http://mailman.qth.net/mailman/listinfo/elecraft > >>> Help: http://mailman.qth.net/mmfaq.htm > >>> Post: mailto:Elecraft@mailman.qth.net > >>> > >>> This list hosted by: http://www.qsl.net > >>> Please help support this email list: http://www.qsl.net/donate.html > >>> Message delivered to a...@elecraft.com > >> > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to garn...@gmail.com -- --Eric _________________________________________ Eric Garner ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
