Notch traps in HPFs

[VK3LU]

This is a hypothetical question.
If I was to build a HPF to install at the input to a digital tv and wanted to insert notch tuned ccts to eliminate interference from 432mHz or 144mHz transmissions, should I use series resonant ccts across the input of the filter or parallel resonant ccts in series with the input?

Cheers
Nev

[VK2EM]

Nev
Probably the easiest way would be two pieces of RG-58 coax, a quarter wavelength long x the velocity factor of the coax, at the desired frequencies across the input (filter or TV). If the coax is open circuit at the open end, then a quarter wavelength represents a short circuit where it is connected to the filter, thus reducing or eliminating the Amateur signals into the TV. There are articles about this on the 'Net.

73 Bruce VK2EM

[GM3SEK]

This is a hypothetical question.
If I was to build a HPF to install at the input to a digital tv and wanted to insert notch tuned ccts to eliminate interference from 432mHz or 144mHz transmissions, should I use series resonant ccts across the input of the filter or parallel resonant ccts in series with the input?

Cheers
Nev

Not sure exactly where your DTV band is in VK, but this design by G4SWX should give you some ideas: http://www.ifwtech.co.uk/g3sek/swxfiltr/swxfiltr.htm#4

The stub notches out 432MHz (with 1296MHz as a bonus) and the HPF part gives a steep roll-off at 144MHz and the FM band.

To complete the all-bands package, add a ferrite choke for HF on the coax.




73 from Ian GM3SEK

[VK3LU]

Thank you Ian and Bruce.
I fully appreciate the benefits of coax stubs but I don't want lengths of coax dangling out of the small filter case.
I just want to know if there is any difference between the trap methods I described in my original post.

Cheers
Nev

[VK2AVR]

Hi Nev,

As I'm sure you know, a series tuned circuit will be low impedance at resonance and parallel will be high impedance at resonance. If the driving source cared about this, then you would have to choose one type over the other (eg transistor circuits generally do not like driving into a short, so you would use a parallel notch in series).

If your antenna comes directly in (no masthead) then the impedance presented to the antenna doesn't matter because it's passive. If there is a masthead, it may not like driving into a short at particular frequencies so presenting a high impedance would be safer (although mastheads may be robust enough that they don't care?).

The main other consideration is that filter circuits are imperfect due to the DC resistance of the inductor. I'm not sure whether series and parallel circuits are affected differently by this.

[VK3LU]

Thanks everyone fo your input.
I have done a bit of trawling and have decided to build a filter using the SVCfilter program and install a quarter wave coax trap at the input for 144 mHz. This will also perform at 432 mHz.
Now to gather the bits.

Cheers

Nev
ps the Fc of the filter is 160 mHz.