Mag loop

Discussion in 'General Technical Questions and Answers' started by KG5WKO, Jan 7, 2020.

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  1. KG5WKO

    KG5WKO Ham Member QRZ Page

    I am going to build or buy mag loop 20-10 or 6 I have nice 36 inch coax loop nice center loop but what range capacitance do I use and what kv rating for xmitting at 65 watts ssb?
    Or Ready made on e like alex loop chameleon or alpha?
    Sadly am headed into limited circumstances my 35 foot vee will not do the trick on balcony hidden by flowers and like!!!!!!
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    This calculator is handy for determining things like capacitance values and peak voltage requirements at different power levels based on loop size and frequency range:
    KK4NSF likes this.
  3. KK4NSF

    KK4NSF Ham Member QRZ Page

    if you google "magnetic loop calculator" you will find quite a few resources that will allow you to input the basic parameters, and will calculate the capacitor needed. What type of capacitor you use will depend on the amount of power you plan to use. For 100w or less you can use a good air-variable. MFJ sells several that work great. For higher powers you may want to look into a vacuum variable.
  4. KG5WKO

    KG5WKO Ham Member QRZ Page

    I laid out a small loop and inner coil. Basic stuff but i ran into confusion on two concepts I am 20-10 so I calculated 3-300pf. I also calculated to run 65 watts ssb I would need 4.7kv. variable cap. MFJ I do not think has one but another company advertises variable air caps but shows ratings in Vrms. I got lost here due to inexperience. I tried converting the 4.7Kv tp V rms but oh boy did it come out a mess. help?
    I am
  5. K6CLS

    K6CLS Ham Member QRZ Page

    Don't run 65 watts. 25 is a lot.

    Please post pix of whatever you make!!

    I like the loops I've made. Trombone capacitor, 20fp through 90pf. And MFJ air variable.

    My Alexloop works great, I think it's rated to 20w? Lightweight, portable one, sturdy, packs down compactly.
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    The mag loop calculator linked above gives necessary capacitor voltage ratings in Vrms. So if the capacitor you are thinking of buying is also rated in Vrms it's no problem and there's no conversion.

    If you are thinking of purchasing a variable capacitor rated in terms of peak voltage to breakdown then it's easy. Vpk = Vrms * 1.414

    So for instance if your design calls for a 4.7kV RMS capacitor and the caps you're looking at are rated in peak voltage you'd want a capacitor with a 6.64 kV peak voltage rating or greater.

    But the bottom line is really stick to lower power if you want to use typical commercially available air variable capacitors as in power in the 10 to 25 watt range max. If you want to build a loop that can handle more power than look into vacuum variable capacitors or homebrew sliding vane or trombone variable capacitors as it's hard to find air variables that can handle the kind of voltage associated with small loops and even 100 watts of drive power.
  7. KF5LJW

    KF5LJW Ham Member QRZ Page

    I have built one, and you can cover all three bands you want. The challenge and I do mean a challenge is Remote Tuning. Especially on the lower frequencies the bandwidth gets 1real narrow and the cap adjustment has to be done with precision and no gear lash. It gets so tight you can look at it wrong and become untuned. It is touchy. Of course you can work around it by not having a Remote Tune Mechanism, but will soon discover only realistic operation is going to be Remote.

    Second tip do not use the antenna for RX, especially if you have any kind of Band Scope or Panadaptor. Makes them completely worthless and extremely difficult finding contacts.

    They work good, they have their place in a limited space location and a great attic antenna, but a PIA to use.

    Right now I can tell you will need around a 15 to 450 pfd capacitor. To cover all three bands will likely take vacuum variable capacitor which are very expensive.
    KH6AQ and W7UUU like this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Actually, STLs are great for RX because they're more immune to noise than most antenna designs, and they can be rotated to further reduce noise and improve S/N unless the noise source happens to be in the same direction as the desired signal.

    But I sure agree a "band scope" with a very high-Q antenna does seem a bit of a waste of time. The bandwidth of the antenna can be so narrow it's almost like a crystal filter, and the wide sweep of a band will only show where the antenna is tuned to and not the rest of the band.

    I've owned one of the MFJ "Super Hi-Q Loop" antennas for about 20 years, it's a model that covers 40m through 15m and is remotely tuned by a motor in the tuning housing of the antenna (which also contains the coupling loop). It's weather-resistant and can be used outdoors in the rain and snow and actually works better than I thought it could.

    I don't use that at home, ever, since I have much better antennas, a tower, beams, etc. there. But I've used it portable/camping/hotels and such dozens of times and it's pretty impressive. Just don't expect to "hear" anything much outside its tuned frequency.

    The MFJ uses a differential capacitor for tuning and handles 100W.
    KK4NSF likes this.
  9. KF5LJW

    KF5LJW Ham Member QRZ Page

    I agree, but they are killjoys. I am proud of the one I built, it works great but a PIA to use and it just takes all the fun out of it for me. Bandscope I can find a signal in 5 seconds and be tuned in. It can take a long time to find a signal and get everything tuned in before it is gone.

    Now as a Transmitter, things are a bit different. If you cannot get the height and have the space for a full size antenna,. I cannot think of a better antenna under those conditions as the STL does not need the height to get the directional and low takeoff angles for DX.

    Like any antenna have gives and takes. STL has a place in ham radio. Hard to beat for a TX antenna with Remote Tune. Find your target on RX Bandscope, aim, and tune TX antenna, fire away on TX.
  10. KG5WKO

    KG5WKO Ham Member QRZ Page

    There is a bottom line here. I have a balcony about 5 foot by 4 foot. I can hide my 2/440 as a flag and you do not object to a flag pole in a facility with 90% vets??
    However, the llop I built so far is made of coax and is based on a model. I do not know what pf range to get for 40-10 or 40-15. The power I can turn down to 40 watts ssb easily and a vacum variable would make greater sense if one was local Radio Shack but i live in no actual city, The loop suits them and me as we can always deck ito put in seasona flowers and put other ines by it. The Pf range is last issue and I found a usa manufacturer who calculated size for 40 watts and is under 100.00.
    I did not plan in falling off roof and damaging myself so I have to make the compromises to keep ham Radio. Why am building all star node and acquiring D star. Gotta do what can.

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