[N2HTT] In the loop.
 

73, de N2HTT

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In the loop.

Posted: 30 May 2016 11:07 PM PDT
https://n2htt.net/2016/05/31/in-the-loop/


Its Memorial Day weekend, and the weather in the Northeast is finally
summer-like. We were at the upstate QTH over the weekend expecting to be
doing some work on the place, and so combined the holiday with a few
vacation daysÂ*Â*only to discover that the delivery of some construction
materials had beenÂ*delayed. We had no recourse but to relax and enjoy
ourselves instead.

Making the best of it, we sought other recreation, and I of course turned
to ham radio for diversion.Â*With Â*the nice weather back, I love toÂ*operate
out-of-doors. We have a nice deck and a big back yard, so my operating
expeditions rarely take me more than a few feet from the house, but are
adventures nevertheless.

The only problem for me on this particular weekend was the CQ WW WPX CW
Contest, held 0000Z, May 28 to 2359Z, May 29.Â*I regularly participate in
sprints and weekend CW events, but of the sort where 15 WPM hand-sent CW is
typical (take a look at the SKCC and NAQCC Â*sprints for example.) Im not
that comfortable with the rapid fire, high-speed CW of real contesting. I
will listen to a call about 10 times to get it, send an exchange in my
plodding fist at QRP power, and if the other op is patient, he will reply.
Or ignore me altogether, which is often theÂ*outcome. So I decided that for
all intents and purposes 80, 40, 20 and 15 meters were not available over
the weekend, and I should probably play on the WARC bands. Daytime
conditions for the high bands were actually not that great, so it was
pretty much going to be that 30 meters was the band of choice.

My normal setup for QRP operation is to used an end-fed halfwave wire cut
for 40 meters, with quarter wave counterpoise. I use a 4:1 balun directly
at the termination of the wires, and use a tuner connected to the balun by
about 6 feet of coax. This arrangement works well, and with the tuner
covers 80 10 meters easily. But, with the bands uncooperative, and some
time on my hands, I decided to try something new, and build a full wave
loop for 30 meters.

Using the classic formula 1005/(frequency in MHz), and shooting for 10.125,
the middle of the 30 meter band, I came up with loop length of 99.26 feet.
I have two 20 foot fishing poles that I use as portable masts, so I figured
that a long rectangle, 10 feet high, would be easy to support with the
poles, with the horizontal bottom wire well off the ground. I just had to
take up 79.26 feet with horizontal legs, which meant a horizontal run of
39.63 (top and bottom wires.) Putting up the poles about 40 feet apart
seemed very do-able, this idea could work!
Hard to see 30 meter loop

I have tried to build full loops in the past, with no success, and have
discovered some gotchas to avoid.Â*The first is dont try to build a portable
loop out of one continuous length of wire. I have tried this, using all
kinds of clever plastic spacers made of poly containers to hold the corners
of the loop, and I have never gotten it to work. The damn things slide
around no matter what you do. Even duct tape doesnt help, not to mention
that it adds a kind of down-in-the-heels look to the resulting antenna.

Nope, this time I constructed the antenna usingÂ*five lengths of wire,
constructed to come apart into two pieces. I cut the two 10 foot verticals,
and the 39.6 foot top horizontal wire, and joined the corners with crimped
ring tongue terminals. The ring provides a place to tie a short length of
twine and fishing tackle clip, which fits over the top section of the mast.
This construction takes care of the top and two sides, which are stored as
one piece when the loop comes down.
Top corner of the loop showing ring terminal crimped in place.

The I cut the bottom horizontal wire in two, and made a 4:1 balun for the
center attachment of coax. I crimped a ring tongue to the ends of the two
verticals, and the ends of the bottom horizontal. These terminals
accommodate #6 hardware, and a 1/2 inch #6-32 bolt and wing nut attach the
horizontal wire to verticals, completing the loop.

I made the 4:1 balun using a pill bottle, and 12 turns of 24 gauge speaker
wire forming the bifilar winding. Recipes for 4:1 air core baluns abound on
the web, so I am not going to link any here a web search will result in
dozens to choose from. I wrapped the windings and connections of my balun
in very attractive violet 3M electrical tape which I got at the local home
improvement store. They make this tape in a variety of colors to go with
any decor.
Pill bottle 4:1 balun, wrapped in attractive violet electrical tape

The second thing I learned from prior loop-building experience: do not use
twisted-pair wire as the radiating element of a loop antenna. I know, when
you say it like that it seems kind of obvious, but at the time I tried this
it was a surprise when the resulting loop didnt behave well at all.

I had the luck a while back to obtain a 1000 foot spool of twisted pair
telephone wire for $2 at a flea market. This is really nice 24 gauge solid
insulated wire, but to use it for antenna purposes you have to separate the
strands. The good news is I got 2000 feet of antenna wire for $2. The bad
news is that it took over an hour of excruciating untwisting to separate a
hundred feet of the stuff. I use the Method of Two Cardboard Bobbins as you
can see in the photo below; once you get more than 4 feet separated the
stuff goes all over the place and tangles maddeningly unless you wrap it on
something.
A large quantity of cheap twisted-pair, and the Method of Two Cardboard
Bobbins

I started building the loop late Saturday, and it wasnt until the middle of
the day Sunday that everything was set up. I checked the resonance of the
loop with an antenna analyzer, and found that it was perfect at 9.200 MHz!
I guess the 1000/frequency formula is intended to be a starting point, and
you adjust from there. My loop was resonating almost exactly 10% low.
Looking at the formula, you can see that changes in frequency are
proportional to changes in length, which meant that the loop was 10% too
long, which was just about 10 feet. This is where the
construction-in-pieces approach paid off. I decided to leave the verticals
10 feet long, and remove the excess from the horizontal wires. This worked
out to be 2.5 feet at each end of both horizontal wires. After a few cuts
and crimps, the loop was put back together with the poles 5 feet closer
together. This time, the measured point of resonance was spot on at 10.125,
with about 100 kHz of bandwidth 2:1 SWR, more than enough to cover the
entire 30 meter band, no tuner required. Now on to the real test an actual
QSO.

I have lots of small QRP radios fit for the job, one of the bestÂ*being my
kit-built KX1. I have this rig all tricked out for portable operation,
complete with a homebrew clipboard to hold everything, and Li-ion
rechargeable batteries for a little extra kick.
KX1 portable operating station, on homebrewed custom clipboard

This clipboard stationÂ*works really well; I shamelessly stole the ideaÂ*from
a very nice commercial product from SOTABEAMS; mine is an inexpensive fiber
clipboard with some notches and rubber bands.

Unfortunately, the HF bands were pretty moribund during the day, Sunday.
Although 40 and 30 meters perked up aroundÂ*sunset, it was very late before
I had a chance to try operating, and I decided that I did not want to go
sit under a tree in the yard in pitch blackness testing my new antenna.

Monday, Memorial Day, was very nice weather, and although we had a brief
thunder-storm in the afternoon, things cleared up beautifully after that,
with cooler temperatures and less humidity than Sunday. I kept my eye on
Band Conditions, and just around sunset, about 8:30 PM local, 30 meters
indicated as wide open. I soaked myself in bug repellant and ran down to
the tree where the coax was parked with my KX1 kit, hooked up and took a
listen. I didnt want to spend the time to drag a lawn chair down there, so
I just stood under the tree.

Almost immediately I heard Carl, WB0CFF calling CQ from Belle Plaine,
Minnesota. At first there was a little QSB, but then he came in very solid.
I was standing there under the tree with the board in my left arm, keying
with my right hand. My keying was a little shaky, as the it was hard to
hold the board still while standing up; it took a little time before I got
used to the arrangement. We had a nice, but short QSO; Carl gave me a 559
report for my 3 watts into the loop. As it turns out, Carl is an SKCC
member, so we exchanged numbers as well. It all worked perfectly.

It was quite dark when we were done, and I decided to not hang out in the
yard. Earlier in the day, we had seen either a badger or porcupine run
across the yard
North American Porcupine

(a porcupine would be more likely, but it really looked like a badger.)
Badger By Yathin S Krishnappa Own work, CC BY-SA 3.0,
https://commons.wikimedia.org/w/inde...curid=24504952

In either case, I didnt wish to run the risk of meeting our visitor in
person, especially connected by a run of coax to a fragile construction of
poles and wires. As far as Im concerned, the project was a success.

So it was a really nice weekend, beautiful weather, a little sun, some bug
bites, one QSO, and a new portable antenna. And, come to think of it, my
first and only CW QSO to date completed standing up.

73,

de N2HTT