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| Can you spot what is wrong in this picture? |
Last winter, I wrote about erecting three Beverage antennas to the NE, NW and SE on my property. Each is 500 feet of wire. For each I built small plastic boxes containing terminators and transformers for the beverages.
I was impressed by the Beverages I erected. Noise levels were easily 1-2 S-units below the transmitting antenna, and I could hear and copy stations that were copiable on the transmitting antenna. I managed to add four new countries to my DXCC totals on 160m.
They were working great. Or so I thought.
Regular Maintenance
Since I've been sick with my liver infection, it's been hard to keep up with the required antenna maintenance. I noticed a month ago that the NW beverage feedpoint and first 30 or so feet of wire was lying on the ground. My friend Renee noticed that the NE beverage was touching the ground as it headed down the crest of the ravine. We didn't manage to fix these until this week.
The NW Beverage had a failure of the insulator at the feedpoint. I replaced the single insulator with two, which should give better support. Also, I changed the angle of the tensioning rope so it was more in-line with the antenna. We walked the first 100 feet of the antenna, and was able to spot the next 75 or so feet, and everything looked copacetic. I'll save walking the entire Beverage for another day.
The NE Beverage had a similar problem -- the insulator right at the crest of the ridge had failed. Replacing it was easy. Working about 75 feet back toward the feedpoint, we also added a rope catenary to support the Beverage where it passed too far away from trees to mount an insulator. This largely solved the problem with the pull we saw at the feedpoint, and re-adjusted the tension rope.
At some point, Renee had asked "What's in the little black boxes?" When we got back into the shack, I showed her. Although I erected only three Beverage antennas, I built enough termination and transformer boxes for five Beverages. The transformer box was easy enough to explain. I had to pry open the terminator box to get a view of what was inside.
The Huge Error
When planing my beverages, I had gathered the best possible information and had built everything accordingly. Tom Rauch, W8JI recommended the use of metal-ceramic composition resistors for the terminators -- specifically the Ohmite OY series devices. These guys were not cheap, costing over $2 apiece. But they had the best qualities -- non-inductive, low-noise and durable in the presence of overloads due to lightning strikes. So, that's what I ordered about a year ago.
Take a close look at the picture at the top of this article. Notice anything? OK, I've known how to read the resistor color codes since I was about 9 years old. You'd think that 55 years later, I would have noticed these resistors are Yellow-Purple-Yellow-Silver. That's nominally 470,000 ohms. Not the 470 ohms they are supposed to be.
Once I noticed this, I opened the other termination box I had, and it held the same resistor. I went back and reviewed what I had ordered from Mouser -- sure enough, it was the Ohmite OY474KE resistors - 2 Watt, metal-ceramic composition resistors of 470,000 ohms, with a 10% tolerance.
Once I noticed this, I opened the other termination box I had, and it held the same resistor. I went back and reviewed what I had ordered from Mouser -- sure enough, it was the Ohmite OY474KE resistors - 2 Watt, metal-ceramic composition resistors of 470,000 ohms, with a 10% tolerance.
My terminator boxes weren't terminating at all -- the resistors were a thousand times too large.
Why Did They Work At All?
My next question was easy -- if the resistors are wrong, then how did the Beverages work at all? Why did they show clearly 1-2 S-units lower than the transmitting antenna? And how did they fool me into thinking they were working great?
The answer is simple. With such a high resistor value, my Beverages were basically unterminated. This means they were bi-directional -- receiving equally well along the line of the antenna, and less so at right angles. Therefore, there was some reduction in noise, but not nearly as much as there should be with proper termination.
The answer is simple. With such a high resistor value, my Beverages were basically unterminated. This means they were bi-directional -- receiving equally well along the line of the antenna, and less so at right angles. Therefore, there was some reduction in noise, but not nearly as much as there should be with proper termination.
I have ordered proper OY471KE resistors. Let's hope they get here soon.
