Magnetic loop help

[KG4ULT]

OK finally got all the pieces to make this thing. I was super excited now i am at my witts end HIHI.

Here is what i have:

3.5kv air variable cap from MFJ
5' 1" diameter 1/2 copper pipe
pvc mast
3'1" diameter 12 gauge copper wire for coupling loop
6' coax to rig



Place the coupling loop in place with main loop and no receive and infinite swr reading. Despite me changing the shape of the loop or raising and lowering up the pvc mast no change. Receive is low and swr is through the roof

NOTE: No analyzer as i had to sell mine a while back.

Capacitor - this one i dont understand at all - no matter how i connect the capacitor to the ends of the main loop - when rotating the capacitors arm receive sound does not change at all.

Now back to the coupling loop - i discovered that if i place this guy where the center conductor touches the main loop swr drops to 3.1 and the noise level climbs - capacitor still does nothing.

I have been back and forth on the yahoo groups forum for mag loops with no luck.

Any ideas???

1. Why does the coupling loop behave this way?
I made the coupling loop of 1 solide wire going from center of so239 to the ground portion of the same 239 connector.
2. Why doesnt the capacitor seem to be responding?


Please help

Frank

[N3OX]

Capacitor - this one i dont understand at all - no matter how i connect the capacitor to the ends of the main loop - when rotating the capacitors arm receive sound does not change at all.

We need more details about the capacitor. What's the maximum and minimum capacitance of the tuning capacitor you have? What's the model? Is it a butterfly cap? Have you tested at many different frequencies? Are you 100% sure the loop should resonate at the frequencies where you've tested if you use that capacitor? If you're outside the frequency range where the capacitor will resonate the loop, the sound will not change.

If you're sure it should resonate at the test frequency, are you touching the capacitor with your hand when you try to tune it? If so you might try using a long insulated stick. Dry wood is probably fine for a temporary test; something like fiberglass would be better if it's to be a permanent part.

Also I think your coupling loop is too big if it's really 3 feet in diameter while the main loop is 5 feet in diameter. Should be quite a bit smaller than that; closer to 1 foot in diameter. But that probably wouldn't make it so you didn't hear ANY increase in sound when you got the loop to resonate. It would make the SWR bad though.

[KG4ULT]

We need more details about the capacitor. What's the maximum and minimum capacitance of the tuning capacitor you have? What's the model? Is it a butterfly cap? Have you tested at many different frequencies? Are you 100% sure the loop should resonate at the frequencies where you've tested if you use that capacitor? If you're outside the frequency range where the capacitor will resonate the loop, the sound will not change.

If you're sure it should resonate at the test frequency, are you touching the capacitor with your hand when you try to tune it? If so you might try using a long insulated stick. Dry wood is probably fine for a temporary test; something like fiberglass would be better if it's to be a permanent part.

Also I think your coupling loop is too big if it's really 3 feet in diameter while the main loop is 5 feet in diameter. Should be quite a bit smaller than that; closer to 1 foot in diameter. But that probably wouldn't make it so you didn't hear ANY increase in sound when you got the loop to resonate. It would make the SWR bad though.

Interesting comment on the capacitor being outside the freq range.

Ok first lets start off with model of the cap; the same is an MFJ air variable cap model number 282-2015 10-300 pf 3.5kv capacitor. It also reads something about being a differential capacitor. However it does not say anything about it being a butterfly capacitor.

I made the main loop to resonate at 14.125. I am not touching cap when tuning; i am using a plastic tube that came with the cap in order to rotate.

The coupling loop is 3ft long but 1' in diameter when made into a circle.

This is all very confusing i mean i followed the diagrams on the net to the "T" minus size of course. Mine turns out to be a but larger than most as i wanted to take advantage of having plenty of space in my bedroom with upsetting YL..HIHI.

Would you all recommend i start from scratch and downsize the loop to mimic all others i have seen on the net where they are 3ft in diameter and work from 30-10mtrs?

Would suck though as i would have purchased 20ft of copper for nothing.

Frank

[N3OX]

Ok first lets start off with model of the cap; the same is an MFJ air variable cap model number 282-2015 10-300 pf 3.5kv capacitor. It also reads something about being a differential capacitor. However it does not say anything about it being a butterfly capacitor.

MFJ's site lists the 282-2015 as a 40-300pF, 2.5kV capacitor. http://www.mfjenterprises.com/Produc...uctid=282-2015

A 10-300pF should be fine to resonate a five foot diameter main loop, but a 40-300pF wouldn't be.. If you're using it as a differential capacitor hooking one stator to each side of the loop and leaving the rotor floating, that is probably better but I would guess that your problem right now is that the minimum capacitance doesn't go down far enough to resonate on 20m or higher bands, unless MFJ's site is a typo or they changed designs for that model number.

You need about 16pF to resonate a 5 foot 1 inch diameter loop on 20m and if MFJ's site is accurate I doubt your cap goes down to that in any configuration, but I don't know why your materials would say one thing and MFJ's site would say another.

To test without re-building anything try 40m or 30m if you have't already... maybe it will resonate there even if it won't on 20m. If you haven't tried a wide range of frequencies, try it, and if this is your first magloop you might be surprised how sharp the tuning is.. tune SLOWLY.

The coupling loop is 3ft long but 1' in diameter when made into a circle.

OK, got it. That should be about right if the loop is made of about 16 feet of tubing for a 5 foot diameter circle. It certainly should be in the ballpark and show a good noise peak... the way you described the two loops I thought at first you might be running a 5 foot diameter main loop and a 3 foot diameter coupling one

Mine turns out to be a but larger than most as i wanted to take advantage of having plenty of space in my bedroom with upsetting YL..HIHI.

Indoor magloop might pose even more of a challenge...but if the indoor environment is messing it up it probably won't mess it up on all frequencies...

I made the main loop to resonate at 14.125. I am not touching cap when tuning; i am using a plastic tube that came with the cap in order to rotate.

OK good.

Would you all recommend i start from scratch and downsize the loop to mimic all others i have seen on the net where they are 3ft in diameter and work from 30-10mtrs?

Not necessarily, though if there IS too much capacitance when the capacitor is rotated to minimum, you'll either have to shrink the loop or modify the capacitor (or get a new cap).

If the picture of your cap on MFJ's site is accurate, you could potentially modify the cap in a reversible way by unbolting it and taking out some plates or turning the plates so they face out instead of in to the body.

But if you haven't tried the loop on other frequencies, I would first go lower in frequency and see if you can find a received noise peak at least. That will help sort out if it's just not possible to tune it on 20m or if there's some other problem.

73,
Dan

[KG4ULT]

OK here is something odd...

I placed one end to lug 1 and the other to the center plank - noise level went way up and i can actually discern a difference in noise level when rotating the capacitor. I was able to get the swr down to 3.5 more or less (im using the swr meter on rig - i know not the best thing to use ) and cannot get it any lower. Would you say that now its just a matter of playing with the coupling loop?

Also i cannot get the antenna to tune on any other band so far. It appears to have the highes noise levels on 7.200 , 24.500 mhz and that is it no higher or lower.

Some progress is better than no progress huh..

Frank

[KO6WB]

I have a magnetic loop for the 80-40 meter bands. It works well. Mine is made from 1" copper pipe and it has a total of 40 feet with a diameter of 10 feet.
The coupling I use is the gamma match. From the description your coupling loop isn't constructed properly. Some have had success with that method but usually the most popular coupling is to use coax as the coupling loop.
Theres tons of data out there that will help.
Take a look at this site; http://www.aa5tb.com/loop.html.

This one is good; http://www.nonstopsystems.com/radio/...na_magloop.htm.

73
Gary

[ZASRANETS]

how do you do?