W2LJ's Blog - QRP - Do More With Less.

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A good day for buildng

Posted: 03 Jul 2016 07:25 PM PDT
http://feedproxy.google.com/~r/feedb...medium=emai l

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!