Wednesday, July 08, 2009

Handiham World for 8 July 2009

Welcome to Handiham World!

Pat sitting on bench, wearing stacked straw summer hats

Did you miss us last week? Well, after all, it is summertime here in Minnesota and we have to take a few days off to enjoy the fine weather. Next week will also be a "vacation week" for your weekly e-letter and audio lectures, but this week it's business as usual. In case you suspect that we are inordinately lazy in the summer, I think I should point out in our defense that this is typically the slowest time of year in the office, so it is the best time for staff to enjoy some time off without affecting our service to members all that much. Other organizations, including our friends at TIPSNET, take a summer break, as do many radio clubs. Once ARRL Field Day is over, the pace of ham radio slows considerably.

Not, mind you, that there are not things to do on the bands and around the ham shack!

I've been slowly collecting the materials for a new 160 meter wire antenna. This will be a random-wire job, and I'm going to tune it right at the feedpoint with an LDG autotuner. I've always wanted to try this kind of antenna arrangement, and the little LDG tuner is just the ticket. A random-wire antenna is really just an end-fed wire of more or less whatever length fits in a given space, and I have a nice, deep back yard, so there is no problem at all running the wire out at least 125 feet. If I cared to put a bend in it, I could easily make it 200. The problem with 160 meters, and the reason so few hams actually use that band, is that the antennas needed to operate on a frequency like 1.9 MHz are very long and thus very difficult to fit into a typical urban property. Let's take a moment to calculate the length of a half-wave dipole at 1.9 MHz, shall we?

Start with the formula: 468 divided by the frequency in MHz = the length of a half-wave in feet.

468 divided by 1.9 = 246.3 feet. That's a long dipole antenna! In fact, many operators barely have space for a 40 meter dipole that requires only about 65 feet. A vertical antenna might be a consideration, but for 160 meters, it could easily top 120 feet for a quarter-wave. I think folks in my neighborhood might notice something that tall in the back yard!

So consider the beauty of the end-fed Marconi wire. Like the vertical antenna, it is fed at the base, with the antenna's radiator near the ground and a counterpoise of radials or other conducting material serving to cut current losses in the ground near the feedpoint. The wire radiator goes up for whatever distance is practical, near the roofline of my house in this case, and then the wire continues out into the long back yard to make up the rest of the required length. If I use an antenna tuner right near the feedpoint, the tuner can decide if the radiating length is too long, in which case it will add a bit of capacitance to electrically shorten the antenna, and if the radiating length is too short for a given frequency, the tuner will add a bit of inductance to electrically lengthen the wire. I love letting an antenna tuner do the work - it's so much easier and more practical than cutting the antenna wire to exactly the right length, which is always time-consuming and problematic. If you cut off too much wire, the antenna tunes too high in frequency, and then you are stuck. There is an old joke about "reaching into your toolkit and pulling out the wire-stretcher", but that mythical tool has never been in any toolkit I've ever owned.

A Marconi antenna works best with an extensive radial system, but I'm not going to worry too much about that. I'll use the house's copper water pipe system, which is also near the antenna's feed point, as well as a galvanized metal window well that is conveniently located at the basement egress window near the feedpoint. I figure I can always add a radial or two if that isn't enough. Even shorter radials will act to reduce ground losses near the feedpoint, which is where most of the current flows anyway.

The 160 meter band is more useful than you might think. Even during a sunspot minimum, 160 meters remains reliably "open" during evening and nighttime hours. Although it can be difficult to use in the summer thunderstorm season when static levels rise, the 160 meter band always has some nighttime activity. My local radio club hosts an evening net on or around 1.9 MHz at 20:00 United States Central Time. You can find the SARA net nightly except Tuesday & Thursday. SARA, the Stillwater Amateur Radio Association, is a Handiham affiliate.

My goal is to get my wire antenna up & running before winter - antenna work is one ham radio activity that is best done in the summertime! For anyone out there looking for "bonus points", convert the length of an end-fed wire 1/4-wave at 1.9 MHz to meters.