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TVI filter for 144 Mhz

PostPosted: Sat Jun 13, 2009 7:42 pm
by VK4DD
Hi Gents,

In some occasions we can have TVI interference ...
This can happen with a big linear.. but some times even with 10W.

But that is no reason to give up. I used to have problems transmitting in one direction and 10W was enough to wipe out the TV.
As a result I started looking around and came up with this filter.
I even added a 50 Mhz trap, but that is not needed.

It is simple to make and cost you $15 on components.

Tuning ... I did not have a spectrum analyzer... just give it a go, and make sure you can receive TV. Test it first on your own TV before you give it to the neighbours...
If you know a Ham with a spectrum analyzer... even better.

All the info is in the pictures. If you have any questions. Feel free to email them to me.

The filter notches out 2m with 70dB... so if you can run 100mW with out interference than this filter will be more than enough for legal limit +

73 Ron

Re: TVI filter for 144 Mhz

PostPosted: Sat Jun 13, 2009 8:03 pm
by VK4DD
Hi Gents,

If you look at the schematic (picture) than there is actually no trimmer to tune the 432 Mhz notch.
The two trimmers in series with the 5W inductors tune the 144 Mhz notch.
The 3th trimmer (15pf) parallel on the 22pF makes the filter wider or narrower...
Depending if you have a TV channel close to 144 Mhz you need to tune it carefully ... to give minimum attenuation in the TV band.

Now there is also a "hidden" notch which came sort of un expected. This is the notch which I tuned to 432 Mhz.
But you can also tune it else where ...
To cut a long story short.. you need to push down the 50 Mhz coil close to the PCB and tune like this the 432 Mhz notch.
Make sure that it is out side the TV band any way.
Stretching the 50 Mhz coil will tune the 50 Mhz notch.... thats how that one is done.
If your not interested in 50Mhz.. just leave it out.

This will give you trouble free operation any time of the day, preamp or not. weak reception area or not.
In insertion loss is about 1dB over this filter (depending how you tune it.
My filter had 4dB less insertion loss than a commercial one, the ACMA had.

The commercial filter gave a poor picture with snow. Not acceptable.
This filter gave no difference in the picture.

Tuning trick... (no spectrum analyzer).
Insert it between your 2m base rig and your antenna. DO NOT TRANSMIT WITH THIS RIG while the filter is in between.
Now use an HT to call a repeater... and now notch out the signal, tune it to minium reception of the repeater.
Even better use a local to put up a signal on 144.200 and notch it out.

Insert it also between the TV and make sure you get good reception.

Good luck with this well tested design.

73 Ron

Re: TVI filter for 144 Mhz

PostPosted: Sun Jun 14, 2009 8:26 pm
by VK4DD
Hi Gents,

Just got feed back from Karl VK2GKA.

I have build the filter yesterday. I measured the notch with my Spec an. The notch on 144 is 65dB down.
Quite sharp and needs precise tuning.. I got some de-tuning when I put the lid on of my metal box.
So you need access to your trimmers from outside.

This filter has at least 30dB extra attenuation compared with my old 1/4 wave stub.
Sorry no pictures from the spectrum Analyzer... the filter is already on the roof...
TV is now good/ TVI problem solved.

73 Karl

Re: TVI filter for 144 Mhz

PostPosted: Wed Oct 06, 2010 3:37 pm
Hi All,
Just a quicky to tell of my efforts in using Ron's filter to help out with C10 Digital.
Firstly, looks great, but I did shrink mine down a bit, but it has yet to be properly set-up and fitted in line to TV.
BTW my thanks to Rob 2GOM for the info passed on.
03102010141 sml.jpg
03102010141 sml.jpg (23.89 KiB) Viewed 1218 times

The box is the smallest one from Jaycar 60mm X 60mm X 40mm.
Now off to try it out....

Re: TVI filter for 144 Mhz

PostPosted: Wed Oct 06, 2010 5:20 pm
The results are in...
Without any adjustments to any coils or trimmer caps the result is 95% better than before.
Previously Channel 10Dig would totally drop out, incurring the wrath of others upon humble operator.
Now, sitting just behind the telly, a minor pixelation when tx on the higher 2m freq,
Will try to get hands on a spec-an to "tune" a little better.
Then just need to insert at the antenna for maximum results!