Radio Preppers

General Category => Antennas => Topic started by: gil on September 17, 2012, 02:38:11 AM

Title: A magnetic loop antenna for 40/30/20m.
Post by: gil on September 17, 2012, 02:38:11 AM
Sometimes trying to make an HF antenna fit into your backyard is like trying to make a size 14 woman fit into a size 8 dress... Some things are just not meant to be. Enters the magnetic loop. It's fairly small, decently efficient (I hope), and fairly easy to build. There is no free lunch, especially when it comes to the laws of physics, so something has to give. For one, they produce voltages and RF that will fry your pacemaker in a heartbeat (your last..). Bandwidth is as thin as an airplane sandwich and they require expensive high voltage variable capacitors, air type (if you're lucky), vacuum (if you're rich).

I decided to build one out of 1" copper tubing. It worked very well for my Slim Jim 2m antenna, so I have high hopes for the magnetic loop. Most of the information came from Frank (N4SPP), and I thank him profusely here for writing his excellent article: http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm (http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm). He even sent me screen shots of the RJELOOP1 software used to calculate all the loop parameters. I later was able to run it on my Mac using DOSBOX, a DOS emulator. The software was designed by the late Reg Edwards, UK Amateur Radio Station G4FGQ and can be found here: http://www.zerobeat.net/G4FGQ/page3.html (http://www.zerobeat.net/G4FGQ/page3.html).

I wondered how big I should make it. My requirements were only that it fits through a regular door frame and covers 40m. It just happens that a 6'3" tall loop works perfectly for 40/30/20m!

Here are the results:
40m:
(http://radiopreppers.com/images/loop40m.png)
30m:
(http://radiopreppers.com/images/loop30m.png)
20m:
(http://radiopreppers.com/images/loop20m.png)
Notice how efficiency improves when the loop is raised only a few feet (second floor..) 40m:
(http://radiopreppers.com/images/loop40mhigh.png)

I will be feeding the loop through a FT-240-31 toroid, which I already have: Fair-Rite part number 2631803802: http://www.fair-rite.com/cgibin/catalog.pgm?THEONEPART=2631803802 (http://www.fair-rite.com/cgibin/catalog.pgm?THEONEPART=2631803802). The capacitor is an air type, 9-110pf 4KV (CAV 12-23): http://www.surplussales.com/Variables/AirVariables/AirVar1.html (http://www.surplussales.com/Variables/AirVariables/AirVar1.html). It won't be as easy to tune as a multi-turn vacuum capacitor or take as much power, but I am not spending $250! It should arrive on Wednesday. Now I need to go to the hardware store!

I will be posting the whole build here, so stay tuned  :)

Gil.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on September 23, 2012, 02:20:03 PM
It's a beauty:

(http://radiopreppers.com/images/aircap.jpg)
Title: A magnetic loop antenna for 40/30/20m.
Post by: White Tiger on November 07, 2012, 10:29:06 AM
Have you any performance report on the magnetic loop?

And while $250 is expensive - $460 for the MFJ Super Hi Q version is unbelievable for a stealth antenna!

Also, it seems that it would EAT electrical power? If so, that would make it stealthy, but not practical for a shtf event!

So, I will be interested in following your progress reports on this project Gil!
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on November 07, 2012, 03:39:03 PM
I haven't finished it yet, no money for the copper...

Gil.
Title: A magnetic loop antenna for 40/30/20m.
Post by: White Tiger on November 07, 2012, 04:23:31 PM
I haven't finished it yet, no money for the copper...

Gil.

How much copper do you need? Maybe we'll make two...?
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on November 07, 2012, 04:52:23 PM
Not sure, it's 21' of 1" tubing plus 45deg elbows. I have a torch, solder is pricey too, but I don't think it would be too much. Much much less than the MFJ antenna in any case.
What's needed for the antenna is four 5' 1" tubes. You cut them in half to make the eight 2.5' segments. I have two ferrites for feeding the loops.

Gil.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on December 13, 2012, 10:43:44 PM
The magnetic loop is up! I went to Home Depot today and bought four 5' lengths of 3.4" copper tubing, as well as eight 45deg elbows. It took me a few hours but it works! One thousand mile QSO with a RST of 559 tonight! The toroid core takes four turns of 14ga wire for best SWR, which varies between 1.2:1 to 2:1. the tunig is extremely touchy. I wish I could have afforded a multi-turn variable capacitor.. The loop has a 6ft. diameter. I put a coupling on the bottom, so it can be disassembled in two pieces. The capacitor comes off too. I can change polarization simply by putting it on it's side. It does seem less noisy than my verticals. I haven't tried it on 20m, only 40 and 30. The band isn't great tonight, but the first test is a success!

Gil.
Title: A magnetic loop antenna for 40/30/20m.
Post by: White Tiger on December 13, 2012, 11:24:10 PM
Excellent! Have you tried it on 40m SSB?
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on December 13, 2012, 11:39:14 PM
Not yet, but it radiates at all angles from the horizon to vertical, so good for low angle DX AND NVIS! Without changing anything! We can test it anytime you'd like :-)

Some numbers (14W in):

14.06MHz:
Antenna efficiency: 94% (-0.3 dB below 100%)
Antenna bandwidth: 85.9 kHz
Tuning Capacitance: 24 pF
Capacitor voltage: 1,045 volts RMS
Resonant circulating current: 2.19 A

10.106MHz:
Antenna efficiency: 83% (-0.8 dB below 100%)
Antenna bandwidth: 26.0 kHz
Tuning Capacitance: 46 pF
Capacitor voltage: 1,367 volts RMS
Resonant circulating current: 3.99 A

7.03MHz:
Antenna efficiency: 58% (-2.4 dB below 100%)
Antenna bandwidth: 8.74 kHz
Tuning Capacitance: 95 pF
Capacitor voltage: 1,639 volts RMS
Resonant circulating current: 6.87 A

I still have one capacitor and a a FT-240-31 toroid... Hum... What should I do? A 3ft diameter 15/17/20m loop!

Gil.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: RadioRay on December 14, 2012, 09:24:25 AM
This is REALLY excellent news!

Congratulations


559 on forty into Kanses aith 14 Watts max, using an indoor, homebrew magnetic loop. . .   That is really excellent proof that it's doing it's job and radiating well. Any change to prop it up outside on the balcony?



>de RadioRay ..._ ._
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on December 14, 2012, 12:31:20 PM
I could definitely put it on the deck outside, second floor, which runs North/South. Unfortunately, it does not tune on 20m  :( The capacitor I am using has a minimum capacitance of 9pf (9-110fp) and it might not be low enough. I get 5:1 SWR minimum. Fortunately, I have another toroid core and a 5-37pf air variable capacitor, so all I need now is two 5' lengths of tube and eight elbows  ;)

Gil.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on December 16, 2012, 12:37:04 AM
Here it is with a size reference. I'm 6'2"  ;)
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on December 16, 2012, 12:43:28 AM
Tonight, Russia on 6W: 5900 miles!

Feeding:
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: RadioRay on December 16, 2012, 11:26:43 AM
You know - a two way contact with Russia from the southern united States would be good enough with 100 Watts and a full sized dipole up 60 feet, but doing it with a home made 6 Watt transceiver into a home made magnetic loop, located inside of your home - that's excellent!

It also seems that the loop might have dramitically improved your signal to noise ratio on receive and that's critical - especially in an urban environment.


73 de RadioRay ..._ ._
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on December 22, 2012, 04:09:54 PM
The big loop works so well, I have decided to build two more! One for 15/17/20m (10' perimeter) and one for 6/10/11m (4.5' perimeter). They will be made of soft copper tubing and thus have a nice round shape. Feeding will be through FT-140-43 toroids, though FT-140-31 would be better.. For the medium loop, I want to use the same capacitor as my big loop, 9-110pf. For the small loop, ideally, 5-70pf or something like that.. I will post the calculations later. I plan on using PVC water pipes for the handle/frame. The feeding will be at the top and capacitor at the bottom. This way the loops can be hand-held and the capacitor turned with the thumb. At the top I will have a 90deg elbow with maybe 6" of tubing ended by a BNC plug to extend the feeding line away from the loop. Not sure if that's the best way to go, and I might feed at the bottom.. More on that later.. The small loop will probably be fed at the bottom and made to plug directly into my Gonset Communicator case...

These two antennas will be cheaper than the big loop. No elbows to buy, little soldering, and for the small loop, a smaller capacitor. I just ordered the toroids..

I am also looking for a 300pf 2500V capacitor to use in parallel with the air capacitor on my big loop and tune 80m. Performance will be low, but at least it will work!

Gil.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: White Tiger 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
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil 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 (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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil 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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil 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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: Randy 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
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil 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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: cockpitbob 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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil 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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: cockpitbob 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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil 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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: w2db 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
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: Kayden 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
(https://farm4.staticflickr.com/3800/9454055922_33403eee1b_b.jpg)

40m
(https://farm4.staticflickr.com/3793/9454055886_ca8a7a5a23_b.jpg)

Does this sound right? If so, from the linked page http://www.surplussales.com/Variables/AirVariables/AirVar1.html (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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil 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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: Kayden 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?
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil 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.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: Kayden on August 07, 2013, 10:51:40 PM
Okay, now I know what to get, and it's not for portable operation, my TS-820 really does earn the name boatanchor. I will try to get that capacitor in the next month or so and start building the rest of the loop now. Thank you gil, for the help!
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on August 07, 2013, 11:10:48 PM
Let me know how it goes!

Gil.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: cockpitbob on October 01, 2013, 10:00:18 PM
OK, I got off my butt and started tinkering with a loop.  I want something portable that I can set up in my upstairs bedroom or bring on a trip and set up in a hotel.  Something like the Alexloop.  And I want 20M & 40M.  Here's what I've got so far.

(http://i120.photobucket.com/albums/o170/cockpitbob/SAM_0295_zps3a40f1ab.jpg)
 
The loop is 9' of RG8 with center and shield shorted at the boxe's connectors..  In the box is a 10pF - 110pF cap that's good for at least 1KV, so 5W only.  At the bottom of the loop is an FT-114A-43 toroid.  The toroid is so small that I have to slip it over the PL-259 connectors, then wind the wires.  When it's tuned right it is perfectly resonant with my antenna analyzer showing only resistive impedance and no reactive component.
 
On 20M it works well with 6turns on the toroid.  2:1 SWR bandwidth is about 70KHz.  Yes, it's very touchy to tune.
 
On 40M I add 150pF across the cap and have to have 12 turns on the toroid to get 50ohms.  2:1 SWR bandwidth is about 35KHz :o :o .  Tuning is real touchy.  I know with a longer cable the BW and efficiency would go up, but the goal is to have something that fits in a suitcase.
 
I expected to have to add capacitance at 40M, but I wasn't expecting to have to put a switch on the toroid to change the number of turns.  I probably should try replacing the toroid with a loop coupler.  the Alexloop uses that and he gets a very broad tuning range.  To finish this project I'll also have to make a mount for it and probably put a gear motor on the cap (I have a 3rpm motor that will run on a 9V battery).
I may decide to shelve this project for a while. Gil has me all distracted thinking about 80M end fed antennas >:(    ;) .  My next project may be a broadband coupler that would allow a 135' wire to operate 80M 40M and maybe 20M.  I already have ferrites coming from Amidon.
 
Hey Gil,
Don't you have to change the number of turns on your toroid when going between bands?
 
 
 
 
 
 
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: gil on October 01, 2013, 10:39:33 PM
Quote
Hey Gil,
Don't you have to change the number of turns on your toroid when going between bands?

Hello Bob, great project! I want to do the same.. No, I do not have to change the turns.. Four turns on mine. The SWR might be better if I do so, who knows.. I do get about 1.5:1 on either 30 or 40m though, so no problem. Tuning is indeed very touchy and hand capacitance does detune it, so it's a little tricky to get it right. If you can't make it work with the same winding, you might want to try two torroids, one for each band...

What frequency range do you get with that 10 to 110pF capacitor?

Gil.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: cockpitbob on October 02, 2013, 07:50:36 AM
With just the variable cap, it will resonate from 9.5MHz - 15MHz, but the impedance varies from 20ohms to 90ohms over that range, with it being 50ohms at 14MHz. 

Adding 150pF in parallel, it will tune from 6.5MHz - 8.3MHz and the impedance stays very close to 50ohms over that range.

I have a feeling if I had a loop the same size as yours the impedance wouldn't change as much with frequency.  But as it is, the change is dramatic.  Going from 6 turns to 12 turns at 40M means the impedance changed 4:1 from 20M to 40M. 
Title: A magnetic loop antenna for 40/30/20m.
Post by: gil on October 02, 2013, 12:16:14 PM
Efficiency is probably quite low on 40m due to the loop's small size.. I would use it for 30m max and make a bigger one for 40m...

Gil.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: cockpitbob on October 02, 2013, 02:49:50 PM
Agreed.  The small diameter of the RG8 doesn't help either. 

I think I'll try replacing the toroid with a wire loop coupling along with going to a longer loop.  I want to see if I can get it down to just having to plug in a different cap to change bands.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: Lamewolf on October 15, 2013, 11:42:28 AM
If you guys want a portable loop for qrp use, try the "Army Loop Tuner" in the attached schematic.  No feedloop needed and you can attach different sizes of loop depending on what bands you want to cover.  I built one for use from motel rooms and with up to 10 watts from an Icom 703 I have worked all over the world using a simple wire loop of about 4' square.
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: cockpitbob on October 15, 2013, 01:14:02 PM
That design looks similar to the mag loop tuner MFJ sells. 

If it's less fussy to tune between bands that has a lot of appeal since everything is at the bottom of the loop. 

Where did you find the dual gang cap?
Title: Re: A magnetic loop antenna for 40/30/20m.
Post by: Lamewolf on October 15, 2013, 02:58:06 PM
That design looks similar to the mag loop tuner MFJ sells. 

If it's less fussy to tune between bands that has a lot of appeal since everything is at the bottom of the loop. 

Where did you find the dual gang cap?

I collect radio junk, so the caps in my loop tuner came from my junkbox and they originally came from an old receiver.  But the tuner does use the same layout as the MFJ does but mine doesn't seems to be "fussy".  All I do during tuneup is to tune the dual gang cap for maximum signal or noise level then use the single gang cap to fine tune the swr, then I tweak each one for best match.  The thing I like about the tuner is that the loop doesn't need a seperate feedloop, all the controls are at the bottom, and you can change the loop size depending on what bands you want to work.  Mine is mounted in a small aluminum box from Radio Shack and with a few different lengths of wire I can work all bands 10 thru 80 meters.  Also, I might add that you can make a nice support for the loops from normal PVC pipe.  I have built and used magloops from soft copper tubing, but the thing I like about the Army Loop Tuner is that it will all break down and fit in a brief case along with my Icom 703 and other related gear.