Sunday, July 03, 2016

A good day for buildng

Even though we're several hundred miles apart, my good friend Bob W3BBO and I seem to find ourselves on the same frequency - and this time I don't mean the radio.  I spent the day working on my mag loop, he spent the day working on a regen receiver project.  Bob's handiwork is phenomenal. He just finished building, painting and drilling out the chassis for his latest receiver:

I wish I could make the stuff I build look this good.  If I built that chassis, I'm sure it would be more of a trapezoid than a rectangle!  

While my project came out looking more "hacked" than professional, I AM pleased to say that for once, something I built actually worked.

Behold the W2LJ Magnetic Loop antenna:

The frame is standard 1/2" Schedule 40 PVC purchased from Home Depot.

The main loop is 10 feet of LMR-400, which is different than any coax I've worked with before.  I tried putting on a PL259 in the way I am used to, and soon discovered, "This ain't gonna work!". So remembering that "Google can be your friend",  I found this tip from Steve WB2WIK on e-Ham:

"Any standard PL-259 fits LMR-400 exactly, and perfectly without any modification to the cable or the connectors.  I've installed hundreds of these on LMR-400s and use ordinary Amphenol 83-1SP PL-259s.

You *don't* peel back the braid of LMR-400 for this operation, where'd you hear that?  That won't work at all.  The correct procedure is the same as installing a PL-259 on regular RG-213/U.

The braid must remain in place exactly as it was originally, and the only thing you strip is the black vinyl jacket.  Leave the braid right where it was, under the jacket and tightly braided over the foil.  The best way to prepare the LMR-400 cable end is with a sharp (new) single-edged razor blade, cutting through the vinyl jacket, braid, foil and dielectric all in one single slice and leaving only the center conductor, stripping all else (with a single cut) back about 3/4" from the end of the cable.

Now, you have a copper plated aluminum center conductor sticking out and the rest of the cable fully intact.

Now, measure back 1/2" from the edge of the vinyl jacket and use much less pressure to strip only the jacket, and leave the braid, foil and dielectric intact.  This only takes gentle pressure, not the several pounds the first "strip" requires.

Pull off the jacket.

Push the PL-259 over the end of the cable so the center conductor protrudes through the end of the center pin and when you hit an obstruction, that will be the cable jacket hitting the internal threads in the PL-259 body.

Rotate the PL-259 body clockwise while applying gentle pressure to the connector, and it will screw itself on to the cable jacket.  About four full rotations are required to fully assemble the connector on to the cable, and when you're done, it won't twist on any more, and you'll see the braid showing through the PL-259 body solder holes."

I followed his instructions and had no problem putting PL259s on the LMR-400. One thing I found out is that when you make that initial scoring on the jacket to expose the center conductor - don't worry about getting the knife blade anywhere near as deep as the center conductor.  Just score all the way around and "break' the dielectric.  After that, just take some linesman's pliers and rotate the outer jacket and it will break cleanly off, leaving just the center conductor pristine and nick-free.

At this point, the capacitor box is more a prototype than anything else:

This is not the good capacitor that I purchased from RF Parts. This is a junk box cap that I purchased at the W2WQ Hamfest a couple of weekends ago for $2.  The box itself is an old lunch meat container. It's flimsy and fragile, but works well enough for the prototype. It's currently also held in place using a Velcro cable wrap.

The smaller coupling loop is a 2 foot piece of some 14 gauge wire I had hanging around.  I soldered it to a length of RG-8X, and it's held in place with some Velcro cable wraps. I also fastened the RG-8X to the frame with some Velcro cable wraps, as I read that some people were experiencing fluctuating SWRs if the feedline was able to move around, freely.

I had intended to run the coupling loop through the PVC, but the PL-259s are a tight fit and I can't feed the main loop through the PVC Tee piece with the coupling wire already in place, so I had to make a last minute design change.

Once the construction was finished, I brought the loop up to the dining room and set up my KX3 on the dining room table,  I tuned around 20 Meters and heard IQ2WJ calling CQ around 14.030 MHz.  I tuned the capacitor for max noise and then hit the "Tune" button on the KX3.  Much to my surprise, I had hit the sweet spot easily and the KX3 brought the SWR down to 1.1:1 from 1.3:1.  I hadn't been that far off!  I was impressed when IQ2WJ answered my call on the first shot!  I'm sure he probably has a monster antenna system; but I was surprised none-the-less that my RF made it all the way to Italy!

The antenna tuned pretty easily on all bands, 20 through 10 Meters.  On 40 Meters, I wasn't able to get a "max noise" setting.  I think the minimum capacitance of this junk box cap is too much.  The RF Parts capacitor has more range at both ends.  I think it will work better, when I finally get around to finding the proper enclosure for it.

I also found that tuning the capacitor was a bit touchy, but not THAT touchy once you get the hang of it.  There is a definite sweet spot of maximum receive noise and if you miss it, you'll get a higher SWR as a result.  The final capacitor box will not only have the good RF Parts capacitor with broader range, it will also have the 6:1 ball bearing reduction drive that I purchased.  The final box should be real nice, once I get it done. But even with the prototype enclosure and capacitor, I did not find hand capacitance to be a problem.

The question is, will the mag loop replace my Butternut HF9V and my W3EDP?  Definitely not! However, if I ever develop another case of tendinitis in my ankle that makes going down to the basement shack a pain filled nightmare - I'll have a useful alternative that I can set up and use in the living room. Plus, it can also be a useful alternative for portable ops.

The best part is that this will break apart pretty easily and will fit nicely in one of the old gym bags we have hanging around. So in the end, the W2LJ Mag Loop is nowhere near as elegant as the Alex Loop; but is sure was a heckuva a lot cheaper; and I got the satisfaction of building something from scratch that actually worked the fist time I tried it!

72 de Larry W2LJ
QRP - When you care to send the very least!

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