John Beckman - W4BTX HAM RADIO "A PAST TIME FOR A LIFETIME"
I'm always encouraged when I see a ham solve a problem by using his knowledge of radio. I do think the ARRL's and FCC "dumbing down" the licensing requirements to become a ham has garnered us a lot of (let's be kind) "less knowledgeable" group of hams. Thee is no question that there is more ignorance of radio theory evident in conversations on the air than here once was. Of course there wee a lot of hams coming out of WW-II who had gained their radio experience in the service. So, they were well prepared to build or service the gear we had "back in the day."
So I was quite pleased when I read in a recent ARRL letter of a ham who used his brain to fix a problem, instead of running to the the "repair man." The article was called "Taking Advantage of Coaxial Cable Capacitance,"
Jeff Blaine, AC0C, had a bad capacitor in the 15-meter filter section of his 5B4AGN multiband band-pass filter, because as he puts it, "they have been treated terribly over the years." One way to damage a filter is to use power levels at the limit of the filter into a load that presents a high SWR -- especially a high duty cycle mode like RTTY. He didn't have the required small 14 pF capacitor, so he trimmed a piece of RG-58 to the required value - coaxial cable exhibits a certain amount of capacitance per unit length.
"The repaired 15-meter filter runs 1.10:1 or better SWR across the band with an insertion loss of about 0.65 dB," he reports. Blaine advises that the RG-58 "can be coiled up and secured with some tie wraps" and that it should be kept away from the toroid to minimize interaction -- a half-inch is probably sufficient. This approach may work well for other applications where a low-value capacitor is difficult to obtain and space available.
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Blaine uses an L/C meter to measure the capacitance as the piece of cable is trimmed to approach the target value, and he leaves the cable about a quarter of an inch, then trims the braid back to increase the distance between it and the center conductor. He puts shrink wrap over the end, and measures the passband of the filter section with the chassis cover in place. -- Thanks to the ARRL Contest Update
Before my eyesight faded due to age, one of my most pleasant chores was repairing or rebuilding older gear to make it serviceable again. We do have a lot of "appliance operators' now but we always have. There are two types of Amateurs out there - those who just talk, and those who embrace all aspects of our hobby. There are those who look at a piece of gear that is not functioning properly and say, "It's broke" and those who see it as a challenge to repair it and get it working again Given that there are more of the first type than the second these days, it is nice to see some who put brain power over talking power.
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