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.
What are these goats all about?
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.
