Magnetic Loop Antenna, cross sectional shape of radiating element

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 2E0CIT, May 9, 2017.

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  1. K2CAJ

    K2CAJ XML Subscriber QRZ Page

    Hi,

    I got the MFJ-935B in the mail, and hooked it up with 16' of 3/8 copper tubing for 20m:

    [​IMG]

    I would have lost my mind without an antenna analyzer, or maybe I'm just a wimp, but it tunes up like so:

    [​IMG]

    The resonant frequency is so obscenely sensitive to the dial setting that I can move this point by just applying pressure to the knob, and getting the sense that the knob has responded to the pressure---if I feel the knob actually turning, I've gone off the chart.

    The loop is a single piece of tubing that I just sawed at 16', then squeezing the ends flat in a small bench vice and drilling holes. I've only had time to listen a bit, and to experiment with the analyzer.
     
  2. NH7RO

    NH7RO Ham Member QRZ Page

    Looks good, Scott and judging by your readout it has fairly high Q. Since the capacitance in the system is subject to your hand being on the tuning knob I'd suggest finding a piece of PVC or wood dowel to glue or otherwise affix to the knob for "slightly remote tuning" :D that should help in that regard.

    I also use a RigExpert for manually tuning my 7-footer on 40M; I set the desired frequency, hook it up to the loop and turn the reduction drive shaft until the SWR suddenly drops to the magic low number.

    Once propagation is decent on 20M I'd give it a try with some RF. Thanks for reporting back on your choice and installation; follow-ups are far and few in between around here as a rule.

    73, Jeff

    PS: In case you will be creating more "optimum circumference loops" for other bands here is what I've determined so far:
    6M 4.75' (don't think the MFJ-936 will tune there but in case it does...)
    10M 8.5'
    12M @ 9.75' (IIRC)
    15M 11.33'
    17M 13.5'
    20M 16.5'
    40M 31-33' (but that equates to around 10 feet in diameter and so only practical if one has a tree limb or high support to suspend the loop from)
    80M would also be decent with this same size but less from optimum.

    3/8th is fine but larger O.D is always better. Weight can become an issue, though but the HVAC soft copper is both easy to shape and not nearly as heavy as water pipe.

    Hope this helps you and others considering a plug-and-play STL like the MFJ and home-brew loop option offers.
     
    Last edited: May 19, 2017
  3. K2CAJ

    K2CAJ XML Subscriber QRZ Page

    Hi,

    I might shoot for 40m by making a slightly longer tube, like 22-24' instead of 32'. Then I can try to get by with a stand like the one I have, in exchange for the reduced efficiency.

    With the 16-footer, 40m is just barely outside the tuning range of the MFJ---and 20m is just inside the tuning range for the other end! I have enough tubing left over to make something big, with enough left over after that to make something small, like a loop for 12-15-17.
     
  4. NH7RO

    NH7RO Ham Member QRZ Page

    That's what length I chose for my 7-foot 40M loop; 22 feet of 7/8th" tubing---just fits under my patio roof when not in use and is still light enough to lift to place atop a 5-foot wooden stand for proper heoght above ground.

    Your 3/8th" version should perform almost as well on 40M. Using KI6GD calc the difference is about 20% in terms of efficiency---but efficiency is not the be all and end all that some claim.

    Have fun!

    73, Jeff
     
  5. MM6IXH

    MM6IXH Ham Member QRZ Page

    The loop on the left was rolled with a set of manual rollers from e.bay originating from Poland at a cost of £135 .
    They can roll 1/2" 3/4" and 1" copper no bother the only slight alteration was to widen the 1" dye a bit to stop it gripping a bit as the diameter of loop got smaller. TIG welding copper requires helium gas as a shield and is not very easy unless you are experienced as the copper tends to be there one minute then falls in on itself very very quickly if overheated.
    73 Robbie
     
    2E0CIT likes this.
  6. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Your MFJ photos don't work for me. I really want to hear more about the 935b, and wish the photos worked!

    By the way, I am suddenly seeing all of these posts about loop antennas in various Facebook forums... AlexLoop, LNR Precision, Alpha Antenna, etc... seems like ready-made loops are becoming hot sellers.
     
    Last edited: May 21, 2017
  7. KG7SWP

    KG7SWP Ham Member QRZ Page

    They should be hot sellers, for the people that either do not have the ability or facilities to build a loop antenna. In your case, I would strongly reccomend for you to build one. You just need to get the right capacitor & then build the loop around it.

    Ed
     
    KK5JY and NH7RO like this.
  8. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    I want to, but my main issue is that I mostly need a RX antenna for 40m, at home. It's hard for me to crawl up on the roof to set things, like a capacitor, and I'm not good when it comes to motors and mechanical such things, to turn dials.

    My doublet with ladder line is already averaging about 8 meters above the roof of my 5 story apartment building. It's hitting the US west coast pretty good, for 100w, at the right time, on decent days. Transmitting is not my problem, but receiving 40m can be. I am actually putting off buying an amp, because I already transmit further than I can receive, on 40m. I know this, because I'm hearing myself on websdr, and others calling me back, sometimes, but not hearing them locally, over the noise.

    When working portable, with the KX2, it is a different story. I have height here, pretty much wherever I work, and seem to do best with a vertical dipole, when portable. I have a 12m high Spiderbeam pole that performs really well, with various vertical dipoles on it, for different bands. That's really pretty simple to put up, as a vertical dipole When I'm up on the mountain, I don't have any city noise to worry about, and the vertical dipole seems to transmit, and listen, really well.

    What I'm really becoming interested in is one of the new MFJ receive loop antennas (or maybe I'd be better off building that flag design, I was given). I might give that a go, this summer, when I get a break. I'm winding down with the school year, and heading back to the USA in 3 weeks, for 2 weeks of stateside family vacation time. I did try KK's single RX loop idea, but I didn't have a proper balun (very important), and receive was nowhere near where I could hear much of anything, compared to my dipole... even using 2 preamps. Apparently, this is just a small loop with 10 feet of wire, and no faraday loop inside. Big fail, how I did it -- no good.

    I now know how an AlexLoop performs at my QTH, on RX, on 40m. I have found where to get the parts I need to build the small flag antenna, and that's going to be my next build, I think.

    I still think loops are cool, and those vacuum capacitors are really neat looking. Kind of afraid I'll break the glass on one, while experimenting, but I do want to try one!
     
  9. 2E0CIT

    2E0CIT Ham Member QRZ Page

    Hi Robbie, -any chance of a link to the supply company for your pipe bender? (If it's still on sale) I found a manual ring-rolling machine on Ebay, also from Poland, but the price was well over £200.
    73, Jeff (not the Texas one)
     
  10. K2CAJ

    K2CAJ XML Subscriber QRZ Page

    That's odd, they don't work for me either---they're links to images I uploaded to my QRZ page. I just examined them again on the QRZ page and they started appearing again in the post.
     

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