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Author Topic: High Pass Filter Question  (Read 778 times)
VA3GUY
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Posts: 177




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« on: January 08, 2013, 08:34:21 PM »

I'll make a long story short.  I have been generating some RFI and in the process of trying to eliminate it, I was loaned an Industrial Communication Engineers Ltd, Model 430, Wide Spectrum TVI Filter.  Specs are from 54-800MHz, 75 Ohm, non-DC passive, polarized.  I put it in-line with my Cablevision and it seemed to work well.  Problem is, there are certain channels that do not come in anymore.  I am using a Scientific Atlanta 8300HD DVR.  It appears that the filter is blocking those channels out.  Is it possible that the filter IS doing that and if so, how/why would it do it only to certain channels?

Also, does anyone have any information on where I could buy more of this company's filters?  Or recommend a good high pass filter?  I checked around and Radio Shack and MFJ seem to be the only place I can find one and the reviews were not favorable.  I thought about making my own so I 'Googled' high pass filters and found what seems to be a nice little calculator (http://www.wa4dsy.net/filter/hp_lp_filter.html).  Might it be better to build my own?

73,

- Guy
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VA3GUY
Member

Posts: 177




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« Reply #1 on: January 08, 2013, 08:35:26 PM »

Here is the link:

http://www.wa4dsy.net/filter/hp_lp_filter.html
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WB6BYU
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Posts: 13149




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« Reply #2 on: January 08, 2013, 08:50:53 PM »

Many cable systems use all available frequencies - including in the middle of the HF band.
Your filter will cut out those channels below about 50 MHz, as well as any they are using
that are above 800 MHz.  (I don't know exactly what bands are in use in your area - you
might be able to find that out from the cable company.)

If your interference only occurs on the lower HF bands, you might be able to build a
filter with a cut-off of, say, 10 or 20 MHz, and pick up a couple extra channels.

On the other hand, your RF shouldn't be getting INTO the cable to cause problems.
That might mean that there are some open connections in the system:  the same
bad joints that let your signal into the cable also will let TV signals leak out, so you
may be able to track the open joints that way.  (Often a corroded ground
connection or an improperly terminated cable.)

Another thing you can try would be a ferrite choke on the outside of the coax,
made by winding several turns of the coax around a snap-on ferrite core.  Most
of the modern problems with RFI are due to fundamental overload from the
transmitted signal, and the outside of the coax makes a much more effective
antenna for the equipment to pick up such signals than the inside of a shielded
cable.  You can use a short jumper of coax to wind a choke on a ferrite toroid
core with mating connectors so it is easy to put connect in series with the
cable - that has the advantage that there should be no attenuation of the
cable channels.
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KE3WD
Member

Posts: 5694




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« Reply #3 on: January 09, 2013, 08:46:53 AM »

Remove the filter and see if the missing channels come back. 
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