Wednesday, December 5, 2012

160m Inverted L

Back in 2001, when Gary Breed, K9AY, helped me to put up my tower, he suggested that I operate the ARRL 160m contest. My excuse at the time was that I had no antenna for 160m. His suggestion was simply to load up whatever I had, that this contest was an opportunity to make a lot of contacts without much of an antenna -- and, besides, it was a lot of fun.

That year, I threw together an inverted L. It wasn't much more than 140 feet of THHN house wire, mounted to a simple particle board frame that could house a series variable capacitor. I ended up removing the series capacitor and just feeding it straight through.

I only had wire and room for two 100 foot elevated radials that were about 40 degrees to each other.

The inverted L never did work right that year, and I ended up using an 80m doublet about 10m off the ground for over 200 contacts.

I never forgot that experience, and I have enjoyed the ARRL 160m contest many, many times since then. I eventually shunt-fed the tower. Over the years, I managed to work all states and over 25 countries on 160m.

Out in Floyd county, I figured I had plenty of space to try out the inverted L again. It's taken a year, but I finally put it up, as you can see from the picture above. Or not. You see, it's about 200 feet up the hill going up about 60 feet into a red oak tree. The remaining 80 or so feet goes east through the woods to another tree. Even if you know where to look, it's hard to spot. Impossible, really.

Putting down radials is more an exercise in finding wire. Copper has gotten very expensive. I bought a spool of 500 feet of 14 gauge THHN wire and cut it into quarters. ($45 at Home Depot! ouch) Then I drug the four wires into the woods. Bad idea. They ended up looking hopelessly tangled.

No matter. I've been untangling wire and rope since I was 10 years old -- I have decades of experience. It took a while, but I rolled out the four radials roughly ninety degrees to each other. I fed the antenna with over 200 feet of the Wireman's best RG-8X, Certified 118. With only four radials, I did not expect much.

I was pleasantly surprised.

The antenna isn't quite resonant in-band. My initial readings showed an SWR valley around 1860 kHz. A little high, but useable. Subsequent measurements from the shack show the low SWR is below 1750 kHz. All the coax makes the antenna broad -- just because of the line losses. (This stuff is rated at 1.5 dB loss per 100 feet at 30 MHz -- losses at 2 MHz can't be much more than 1 dB) For this years ARRL 160m, I just ignored the losses and used a tuner.

My schedule only permitted me to operate for about four hours on the second night. This antenna was extremely effective. I could work just about anyone I could year, and calling CQ was reasonably effective.

The most amazing thing about this antenna is how quiet it is. At first, I thought it might be due to all the coax and ground losses. But, since the antenna was so effective -- at least as effective as the shunt-fed tower, I've changed my mind. It's just really quiet out here in the country in Floyd county.

My plan is to add more radials -- four more 125 footers ($45!) and eight of half that length ($45!) -- for a total of sixteen radials. I also want to add a trap for 80m, and I'll worry about trimming the antenna to resonance at that time.

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