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 <[email protected]> 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 <[email protected]> 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:[email protected] >>>>> >>>>> This list hosted by: http://www.qsl.net >>>>> Please help support this email list: http://www.qsl.net/donate.html >>>>> Message delivered to [email protected] >>>> >>> >>> ______________________________________________________________ >>> Elecraft mailing list >>> Home: http://mailman.qth.net/mailman/listinfo/elecraft >>> Help: http://mailman.qth.net/mmfaq.htm >>> Post: mailto:[email protected] >>> >>> This list hosted by: http://www.qsl.net >>> Please help support this email list: http://www.qsl.net/donate.html >>> Message delivered to [email protected] >> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> Message delivered to [email protected] > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
