In the loop.

It’s 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 days – only to discover that the delivery of some construction materials had been 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. With  the nice weather back, I love to 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. 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  sprints for example.) I’m 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 the 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

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. The first is don’t 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 doesn’t 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 using 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.

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

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 didn’t 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

A large quantity of cheap twisted-pair, and the Method of Two Cardboard Bobbins

I started building the loop late Saturday, and it wasn’t 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 best 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.

KX1OnClipboard

KX1 portable operating station, on homebrewed custom clipboard

This clipboard station works really well; I shamelessly stole the idea 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 around 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 didn’t 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

North American Porcupine

(a porcupine would be more likely, but it really looked like a badger.)

By Yathin S Krishnappa - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24504952

Badger By Yathin S Krishnappa – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24504952

In either case, I didn’t 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 I’m 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

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