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Fall 2006 Issue |
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ANTENNAS Connecting the Radio to the Sky The Big Opportunity By Kent Britain, WA5VJB |
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Photo 1. The 1840-foot broadcast tower. |
 Photo 2. The other end of the 1750 feet of transmission line. |
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It was one of those rare opportunities. The local TV Channel 2 was building a new transmitter site. Yes, analog TV is supposed to go away, but they could economically justify a new transmitter and a new antenna even over just a two-year operating period.
The antenna went up two weeks behind schedule.
However, the transmitter was running three months behind schedule. Oh, how
horrible. Here was a 12-panel array for 56 MHz at 1631 feet, 1750 feet of
41/2-inch feedline with .875 dB of loss at 55.125 MHz, and nothing
connected to it. It also didn’t hurt that the Director of Engineering for
the network is N5UUE. He was kind enough to supply the 41/2-inch hardline
to Type N adapter. I don’t have many of those in my junk box. Somehow I
feel like I should do penance. Adapting hardline like that to a PL-259
connector for 6 meters is just plain wrong. Rule 1: Never connect your rig directly to the feedline! You’ll fry it! At one location we measured nearly 20 watts coming back down that coax. In our case there are over 50 TV transmitters and over 30 FM stations within a mile or so. We’re talking a couple of megawatts here, and the antenna was picking up 20 of those watts! Connect a rig to the coax without a good filter, and say goodbye to those front-end parts.
Rule 2: Good filters. For 6 meters I
was able to use a Drake 6-meter low-pass filter. It was marginal, but kept
the front end of my IC-706 Mark II from smoking. The old TV Channel 2 is
still putting out full power about a mile away, and signal levels were
incredible. Even with the Drake filter, the noise floor was S5 to S7 on 6
meters. As I tuned up the band, the noise level increased. With the help
of WB5KGL, we’ve whipped up some new filters. At TV Channel 2, the skirts
are now over 70 dB down, the limits of my test equipment. I hope it’s
enough, and we will be trying them out shortly, but I had to finish this
column first!
On 144.2 MHz and 432.1 MHz, I used 144- and
450-MHz FM passband cavity filters from repeater diplexers. They worked
great and the noise was way down compared to 6 meters. On 2 meters the
noise floor was S1 to S2; on 432 MHz the noise floor was S0. The quest
continues. I have a lead on a 50-MHz passband cavity filter and hope to
give one a try soon. Even so, I am still scrounging. We also plan to test
222.1 MHz in the near future. |
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