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Author Topic: A magnetic loop antenna for 40/30/20m.  (Read 27517 times)

White Tiger

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #15 on: December 25, 2012, 06:30:20 PM »
Hey Gil, would any of these work if I were to attempt to build my own magnetic loop?

http://www.ebay.com/sch/i.html?_trksid=p5197.m570.l1311&_nkw=air+variable+capacitor&_sacat=0&_from=R40
« Last Edit: December 26, 2012, 12:27:04 PM by White Tiger »
If you're looking for me, you're probably looking in the wrong place.

gil

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #16 on: December 25, 2012, 08:21:22 PM »
This one:

http://www.ebay.com/itm/E-F-Johnson-split-air-variable-capacitor-large-2-section-part-number-152-504-3-/251202802177?pt=LH_DefaultDomain_0&hash=item3a7cda9201

For air variable capacitors, since they are only one turn, I wouldn't use one with a range of more than 100 or 150pF. Otherwise, tuning will be difficult. Mine is a 9-110pF and is hard to tune already. the slightest movement of the knob throws the SWR way off. Also get one that has at least a 2KV rating, and that probably limits you to 10W (to be calculated), prefer a 4KV model. If you go too low, it will arc as you increase power.
Look at the link I posted previously, Surplus Sales of Omaha, if I recall.. They have good ones, though a bit expensive. Snatch that one above, it's a good one.. You can also put in in parallel for more capacitance or use the two gangs separately.. Gives you more options..

Gil.

gil

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #17 on: January 07, 2013, 01:57:50 AM »
I experimented with a 330pf high voltage capacitor in parallel with the air variable. I can tune the loop on 80m with a 1.1:1 SWR! Good news, but it is very inefficient. Though, Ray heard me, barely, 800 miles away. With 12W in, I am sure the amount radiated is measured in milliWatts... Reception isn't that great either, but hey, it works.

The loop will be painted in the next few days, bright red!

Gil.

gil

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #18 on: January 20, 2013, 07:05:17 PM »
It's red!

Ray suggested yellow flames as well, but I am lacking the artistic talent  ::)

Now it needs some kind of a stand and a fiberglass rod to extend the capacitor knob, as to avoid touching the loop while tuning, which could be fatal!

Gil.
« Last Edit: January 20, 2013, 07:07:36 PM by gil »

Randy

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #19 on: January 21, 2013, 09:51:52 AM »
Hey Gil --

Great article. I'd love to have a mag loop for my QRP radios. However -- since I have an implanted pacemaker/defibrillator -- as you pointed out ---  could be an error in judgement on my part.  Still -- I may soon build one if only for receiving.  Take care --- it's nice seeing you on the Rock-Mite pages.

Randy  K4LJA

gil

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #20 on: January 21, 2013, 11:16:22 AM »
Hello Randy,

I would definitely be careful about that. Maybe the company that makes it can give you some information...

Gil.
« Last Edit: January 28, 2013, 11:28:02 AM by gil »

cockpitbob

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #21 on: January 28, 2013, 11:06:29 AM »
I knew the voltages would be high, but over 1KV for 14W input is astonishing.  No wonder you need such an open capacitor.

Really nice work Gil!  Thanks for posting this.

gil

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #22 on: January 28, 2013, 11:37:19 AM »
Thanks Bob,

The antenna has been great. It's unbelievable what you can do with a bunch of stuff from Home Depot and end-up with a good antenna that's only 6 feet tall and works on 40m! I even tried it on 80m with an extra capacitor in parallel and it works, though efficiency is pretty dismal. The downside of course is the lethal voltage and narrow bandwidth. However, since 95% of my QSOs are in Morse, I don't need to retune that often. One advantage of the loop is that it cuts down on noise considerably. It's the only antenna I can use with my Rock-Mite 40 without receiving AM broadcast stations on top of the CW. It filters out everything outside the band. Though I don't plan on transporting it, it can be taken apart in two pieces. Cost was somewhat less than $200. It's pretty easy to make..

Gil.

cockpitbob

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #23 on: April 03, 2013, 10:03:02 PM »
Gil, do you collect antiques?  I noticed in your first picture a wood tennis racket in a storage frame.  I still have mine from 40 years ago, but that's the only other one I've seen in a long time.

gil

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #24 on: April 03, 2013, 10:34:58 PM »
Well, no  :o .. I helped a friend put a bunch of them on Ebay for sale. He is a big time collector and player. So, to thank me he gave me one. It's from the 30s, a Davega... I really don't know anything about it, but if someone ever asks me to go play tennis, well, Ihave a racket!

Gil.

w2db

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #25 on: May 16, 2013, 02:42:28 AM »
Built one in 1986 from a QST article, June 1986 by W5QJR. Had the biggest longest giggle fit of my life when, with the antenna hanging on my bookshelf, manually tuned (hadn't added the remote stepper control yet) I was around 7.007 and tuned it by hand...ground floor...100W from FT-757...I called a JA who was calling CQ. He comes back giving me a 579. I put it in the attic. I even pumped my SB-220 into it! The capacitor I used was MASSIVE. Sold by W5QJR.

These loops work, and work very well. This one was 3' diameter and made of 1" copper tubing, 8 1' lengths, with 120 ? elbows fed with a short length of #14 wire as a matcher...It was slick!

GeorgeC
W2DB
Cedar Park, TX
Allstar 2360

K0DEN

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #26 on: August 06, 2013, 05:36:45 PM »
Can someone verify I'm heading down the right path with my rjeloop1 calculations? I want to build a magloop for 20m and 40m (if possible, but definitely 20m) in my patio. Due to HOA restrictions, magloop is probably the best antenna for me, I can't have an antenna above 12' AGL. If I've figured right, at 3m (~9' around, or ~3' tall) loop circumference and 1" copper pipe I need 53pf (20m, 14.090) - 223pf (40m, 7.090) and up to 4kv if I pump in 10-15w with a significant safety margin.

20m


40m


Does this sound right? If so, from the linked page http://www.surplussales.com/Variables/AirVariables/AirVar1.html it sounds like a (CAV) 12-53 would work, yes?

I appreciate any direction you guys can give, I'm definitely not the worlds electronics expert, I'm more of a software guy.

gil

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #27 on: August 06, 2013, 08:47:22 PM »
Hello,

A 3m perimeter would work great for 20m, but will not be cery efficient on 40m. If I had to redo mine to work on 20m as well, I would reduce it a bit from 21ft to maybe 17ft. Note that the 21ft loop was supposed to work on 20m per rjeloop1 calculations but it did not. The loop needs to be slightly smaller than calculated. I encountered the same problem with the loop I made for 6m, which will have to be reduced a bit. The CAV 12-53 would work fine, but with a bigger loop and maybe the CAV 12-23, you would get better performance. In any case, I believe that the size of the loop should be about 20% smaller than what rjeloop1 says...

Gil.

K0DEN

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #28 on: August 07, 2013, 03:31:19 PM »
With the CAV 12-23, it doesn't look like it would have a high enough capacitance (110pf max) for 40m (which, according to rjeloop1 calculations, would require 222pf). Also, as I play with reducing the circumference of the loop, the more capacitance I would need. Would it make sense to have two of them, and have a switch to put one in parallel or bypass based on which band?

gil

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Re: A magnetic loop antenna for 40/30/20m.
« Reply #29 on: August 07, 2013, 04:50:36 PM »
Quote
With the CAV 12-23, it doesn't look like it would have a high enough capacitance (110pf max) for 40m (which, according to rjeloop1 calculations, would require 222pf). Also, as I play with reducing the circumference of the loop, the more capacitance I would need. Would it make sense to have two of them, and have a switch to put one in parallel or bypass based on which band?

I wouldn't do that. If you want to keep it small, you'll need the 12-53. If you make it around 17ft. perimeter, then you might be able to use the 12-23.

My concern with the 3m loop and the 12-53 is that the minimum of 36pf is close to the 53.6pf calculation... I made the mistake twice about trusting the calculation and ended up having two capacitors that did not go low enough. I would have needed to make my loops smaller... As I said, maybe 20% of the perimeter length.

Since your loop is round in shape, it's easy to cut to size... Mine is an octagon and that is a bigger problem...
You might get lucky. No harm in trying and again, easy to cut.

A 3m loop will work great for 20m, but don't expect miracles on 40...
If you want it to be portable, I understand... Otherwise, try a 17ft. loop or so.

Gil.