I have an old MFJ 935B loop tuner.  They have a newer model which has SO-239 connectors on either side of the housing for attaching coax.  

What I want to do is build an adapter for the back of my older tuner that will allow me to connect coax.

I plan to use some brass L brackets I made, and solder the outside of the SO-239 to the bracket along with the center conductor.  Then use a piece of kydex as a stiffener to keep the brackets from rotating on the terminals on the back of the tuner.

The question is, is brass a good enough conductor?  It's made of copper, nickel and zinc, right?  I think it should be good enough, particularly if I tin the holes with solder where they will contact the tuners output terminals.

I also worry that if my brackets are too close together it will add too much stray capacitance.

Why not get a couple of chassis mount SO-239's and replace the ones on the device?

Two different styles are shown Here.

In order for the loop to mount in line, I would need to mount them to the sides of the chassis.  And, the sides would need to be replaced with a non conductive material like ABS sheets.  It looks like this is what MFJ did for their newer model, the 935C.  But, they also spaced the big tuning cap farther from the side to give clearance for the connector.

It would be almost a total rebuild in a different enclosure if I wanted to do that.

This would be a 100% non-intrusive adapter.  I know that with loops you have high RF currents and need low resistance connections, that's why I asked about the brass being good enough.

Might sound silly, but can you get your brass pieces silver plated? I had a friend that built amplifiers and got the roller inductors silver plated if they needed it.
73,
Rob

Not easily.  I could maybe find some silver solder and tin them.  I was just planning to tin them with 60/40.

Here's what I have so far.  I don't have a mill so I cut these out with snips and bent them in a vise. Trimmed them with files, etc.

This is just a test fit before any soldering.  Obviously it will add a couple of inches to the loop length but I don't think it will matter too much for these HF bands.

Attached File

Interesting project, the names in the game are "Surface area" and "Conductivity". I'm wondering if the internal connections could take the weight and torque of the heavy coax. Then again, the match box could probably be hung from the coax loop. My next doubt would fall to the coax connectors, those would have to be really really tight. Any misalignment twist from the cable would be transferred to the hardware in the box.

I'm now thinking heavy diameter Litz wire which will carry lots of current and is very flexible. Such could be soldered, crimped and attached to the box hardware with heavy duty ( think surface area) ring tongues. The litz wire is not stiff at all so it would need to be hung from some supports.

I know, more questions than answers, search around for reviews on how the loop tuners are being used for some more ideas. HTH
73,
Rob

The tuner itself comes with a PVC end cap screwed to the top.  You are supposed to buy or build a PVC "cross" that fits into the end cap and supports the larger loops.  In practice, when I made a 4 foot circumference loop using 1/4" copper refrigerator line tubing, it was not necessary to use a support.  The bigger 5/8" diameter 9 foot circumference tubing is heavy enough to need the PVC.

MFJ says that 9 feet is a good length for 20 and 30 meters, with 13 feet being optimum for 20 meters.  I ordered a 9 foot length of RG213 coax to test it out.  There are several designs out there besides the MFJ that use coax as the antenna element for portability and ease of set up and take down, and changing loop length.  They all use off the shelf SO-239 connectors.  Probably not the best connection, but you are making compromises for convenience, and these aren't intended to be QRO antennas.

I may be wrong , but it looks like a metal enclosure and the shield of the coax is connected to the enclosure

I think the loops that us coax use both the inner conductor and the shield shorted to each other  so the shield radiates because the larger the diameter of the conductor, the more efficient the loop.

So I think  the shields can not be connected to the case

Correct.  The newer MFJ model uses plastic side panels for their case so the shield is isolated.

In my case, the rear terminal posts are spaced off the back of the enclosure about a half inch, and the brass brackets mount to those posts with wing nuts.  There is a kydex stiffener behind them. There is no contact between the shield and the case.

Hopefully this picture is a little better.  MFJ has brass threaded terminals with wire jumpers coming out alongside them with ring terminals on them to give the best possible conductivity without actually soldering.  I am considering soldering my own jumpers from the adapter to the capacitor inside, but then I would be committing to only using the coax connectors for my antenna elements and not using copper tubing or other wires anymore.

Attached File

The cable arrived today so I took some time after work to solder it all up and mount the cable to the PVC cross.  The RG213 isn't as stiff as I expected, but it works, and coils up compact for transport.

I'll have to test it tomorrow before it gets too late for 20 meters.

Attached File


Attached File

Looks good, obviously 20M is a lot easier to deploy than 40M .
73,
Rob

Thanks!

Yeah 40 meters is a big wire.  The hard part is you are supposed to get it at least one loop diameter above ground.  If I had a second floor it would be easy.  

I don't see a small  'driven' loop.

Is that tuner the  so called
' military' way where the loop in the driven loop and it has a capacitive network in the box, not just a single capacitor.

I think I want one, that looks nice and portable.

What is the length of the RG213 that you are using? Have you computed the efficiency, can it tune to 40m?

Yes, the loop is the driven element.

There is a big butterfly cap in the box with a reduction drive, and then a smaller cap to match it to the feedline.  There is also a loop current meter as well.  You tune for maximum loop current then match it to your feedline, according to the instructions.  You can buy the capacitor separately from MFJ if you want to build your own.

I wouldn't know where to begin to compute the efficiency.  

You can get 2 or three bands with one loop.  This 9 foot circumference loop should tune 20 and 30 meters.  If you want 40 meters, MFJ says a 20 foot loop will do it, with 28 feet being the most efficient according to them.

But with a loop size that big, you need to get the bottom of it up higher off the ground on a second floor or something.

This antenna would be ideal for operating from a second floor apartment or hotel room.  If you do get the current model, it comes with the coax connectors from the factory, in addition to the regular screw terminal posts on the back.

For higher frequencies you go with an even smaller loop.  10 meters only requires a 4 foot loop!  There is a youtube video of a guy making contacts with it using a bicycle rim as the element.

I have made 2 loops.

One was a 13 foot loop out of 1/2 inch copper tubing with an air variable cap.

I talked SSB to a guy in TX from MA on 15 watts, that was my first contact with it. It was also at the bottom of the solar cycle.

Then I made a 20 foot one for 40m and  put a big ass air variable cap on it in my basement and could here signals and made a QSO on SSB 40 meters at 100 watts.

I think I friend my nuts with that one.

I used to take a small one with me on the road for 20m road and made FT8 contacts with the loop facing out my hotel window. at 5 watts, that was with the QRP loop tuner from MFJ, just like the one you are using but at 5 watts


ETA: also made one with a hula hoop

Thats interesting about the 20 foot loop.  You say the loop was partly below ground level?  If that's the case it might be worthwhile for me to try to string up a big loop for 40 meters just to see what I can get out of it.  I haven't messed with it because they recommend you get it a loop diameter above ground.

The hula hoop is a neat idea.

I think this is the ideal antenna for digital where you know you will be on a fixed frequency for a long time and the high Q rejects everything else so it's low noise.  I might take the 9 foot copper tube I have and try to build a permanent trombone capacitor or something that is tuned for one of the popular digital model and just mount it up in the rafters somewhere and just set up an old computer and one of my radios and see how many contacts it can rack up on autopilot.  Maybe turn it 90 degrees after a few weeks and see if it can pick up stations that were lost in the nulls.

yes, the entire loop was below ground in my basement and I made contacts on it at 100 watts.

I have entertained the idea of making a 40 foot square loop ( 4 x 10 foot sections ) made of aluminum gutter drain ‘pipe’ because it is 4 inches in diameter, square not round. The Calculator says it should be very efficient. Putting 2x2 lumber on the inside as a frame should work, but it would have to sit on the ground or close too it. Maybe some day I will try it.

I wonder if you'd see a significant increase in usable bandwidth with the larger diameter gutter over a wire.  Probably be a good idea to use low temp aluminum brazing rods to connect the parts instead of screwing them together.  Neat idea for readily available off the shelf parts!

Makes me wonder if you could build a larger diameter element for a loop antenna similar to a cage dipole.  Just a wire frame element to get the bandwidth benefits without the weight and expense of solid tubing.  Or would it introduce weird capacitance issues between the individual wires?

What about using the metal tape that is used in duct work?

I have heard of people using the accordion style expandable aluminum dryer ducts.  MFJ suggests if you can find a really long strip of copper clad PCB it works well.

Lots of things to try.

This antenna looks like it's made of foil tape.

https://www.eham.net/article/26572

Aluminum drier ducts, i am going to try that. great idea.

I was thinking more along the lines of the aluminum tape, and doing a "cigarette paper" wrap around 3/4 or 1" pipe, instead of the way he wrapped it.
I'm a moron with a ham ticket, so who knows if something like that would work.
I should start a utube channel, "Moron with a ham ticket"

I think that aluminum tape has a coating on that would have to be removed for connections, and since the other side is adhesive, any seam in the tape application would have to be connected together electrically.

i have seen pics of people wrapping things in it, but don't know how well it works. If you could get it to work, it could be a very light weight antenna, depending on what you wrap.

I have some handy, i will put a meter on it and report back