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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.
The system with the amp shown the biggest problem.

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.

¼ wave stub of RG-6 cut at 410 mm gave a notch filter with results in the 2 meter band of :
144 Mhz          24db
145.6 Mhz       30 db
146.6 Mhz       32 db
147.6 Mhz       29 db
148 Mhz          27 db

Also found that at 445.2 Mhz,  gave a 24db Notch

What are these goats all about?

Wise old billygoat teaching the young ones

 

 

Grandpa Billygoat has become the symbol for Club-Member Help pages.

The young 'kid' at the top? - He's rushing about, looking for answers - which we hope he'll find here.

If you have any updates or link info please contact: email

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