A magnetic loop antenna for 40/30/20m.

[Kayden]

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!

[gil]

Let me know how it goes!

Gil.

[cockpitbob]

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.



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?

[gil]

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.

[cockpitbob]

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. 

[gil]

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.

[cockpitbob]

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.

[Lamewolf]

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.

[cockpitbob]

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?

[Lamewolf]

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.