Fix 2 Metre TVI Problem

Using 1/4 Wave Stub Filter


Television Interference caused by my 2M transmissions

XYL say... We have a problem

First, some history

I had a Television Interference problem at my QTH some years ago, on all bands (HF and VHF) although the interference was much worse when I was using 2 Metres. I couldn't run more than 5 watts into a ¼ wave aerial, without the XYL mentioning 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 caused the biggest problem.

Moving the TV antenna and amp/splitter as far from the shack as possible helped with the HF problems but did not reduce the interference I was causing on 2 Metres.

I had a chance conversation with amateur 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, extending about 6 inches away.
When he plugged in the already tuned up television he found that he'd completely lost 1 of the UHF TV Stations. Testing backwards in the line, 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 began experimenting.

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. placed 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:

  • ¼ wave open-ended RG58 coax with 45W in - result: 200mW out when the stub was fitted, that's a drop of 21 to 24 dB

  • ¾ wave open-ended RG58 coax with 25W in - result: 600mW out when the stub was fitted, that's a drop of about 12 to 15 dB

  • Both showed a 10Mhz Notch i.e. 5 Mhz +/- of the center frequency.

  • ¼ wave RG58 Coax Stub was 290 mm (plus about 15 to 20 mm for the "T" piece)

  • ¾ wave RG58 Coax Stub was 960 mm long (plus about 15 to 20 mm for the "T" piece)

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


In the previous notes, the stubs used were 50ohm RG-58 coax, as this was the only cable on hand at the time testing. Amateurs use this for transmission line so we always have plenty of short off-cuts laying around.

Once I tried the experiment using TV coax RG-6 (75 ohm) and “F” connectors I promptly found that the lengths above using RG-58 did not work!

It was back to the start to find the appropriate filter lengths. 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

We trust the above information assists others in tracking and rectifying a TV interference problem.


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Wise old Grandpa billygoat is just passing on his knowledge on these Club help pages.

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