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The following article was written by Ross VK4JRO |
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FIXING A 2 METRE - TVI PROBLEM using a 1/4 WAVE STUB FILTER |
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First some history. I had a Television Interference problem at my current QTH some years ago, on all bands (HF and VHF) although the worst was from 2 Metres. I Could not run more than 5 watts into a ¼ wave aerial, without the XYL telling me of the PROBLEM. My set up was; 3 TV sets connected to 2 different antenna systems. 1 TV to 1 aerial and 2 TV's via an amp/splitter to the other antenna. Moving the TV antenna and amp/splitter as far from the shack as possible helped with the HF problems but not the 2 Metre. Then a chance conversation with Keith VK4YET, about a TV Point installation where he had been lazy and just extended from the point on the other side of the wall, some 6 inch away. When he plugged in the already tuned up television he found that he had completely lost 1 of the UHF TV Stations. He found that the point on the end of the run was all-good but the point with a short open-ended stub i.e. the feed to the other point, would notch out the TV station. Armed with the idea of an open-ended stub being able to notch out a frequency or section of a band, I started to experiment. These were my results: I found that a ¼ wave open stub Tee’d into the feed line before the amp/splitter fixed the problem. (i.e. between the Antenna and the Amplifier) A ¾ wave open stub also worked but had the effect of taking out one of the UHF TV channels. I then set up some tests: A ¼ wave open-ended piece of RG58 coax with 45W in - I then saw 200mW out when the stub was fitted – that’s about 21 to 24 dB down A ¾ wave open-ended piece of RG58 coax with 25W in, I then saw 600mW out when the stub was fitted – that’s about 12 to 15 dB down Both showed a 10Mhz Notch i.e. 5 Mhz +/- of the center frequency. ¼ wave RG58 Coax Stub was 290mm long plus about 15 to 20 mm for the ‘T’ ¾ wave RG58 Coax Stub was 960mm long plus about 15 to 20 mm for the ‘T’ I now use a ¼ wave stub before my amp and can now transmit 100watts into a 9-element beam on 2 Metres with no complaints from the XYL...... Happy Wife = Happy Life
UPDATE... UPDATE... UPDATE... In the above article, the stubs where based on RG-58 coax, as this was the only cable on hand at the time and something hams always have a lot of. After recently offering to make some stubs using RG-6 (75 ohm) and “F” connectors I promptly found that the lengths as per RG-58 did not work. So back to the start, with the aid of a Motorola Communication Test Set, which has a spectrum analysers and tracking generator, the following results where obtained.
Also found that at 445.2 Mhz, gave a 24db Notch | |
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