[NLRS] VHF low pass filters?

[Dr. Gerald N. Johnson]

The same filters that worked for analog TV will work for digital TV. And 
the high pass performance may be improvable by having a higher cutoff 
frequency than 54 MHz in most locations.

Initially all TV station went to UHF channels for digital while they 
reworked their VHF antennas and transmitters. Some stayed on VHF, some 
moved back to their original VHF channels, though the low band (ch 2 -6) 
were the least likely to return to VHF (though Ch 5 at Des Moines did) 
and it seems like most of the UHF channels moved RF frequency while 
retaining their old channel number just as a name. The TV set and 
converter display the channel name, not the real RF channel and the RF 
channel assignments remain the same though there are now none above 700 
MHz. UHF TV at its beginning extended to 890 MHz. Its been nibbled away 
for other services and digital has allowed getting 3 to 6 video channels 
in one 6 MHz RF channel by judicious use of compression and sprites over 
a nearly unchanging background.

The FCC has a page somewhere of local TV stations, and their real RF 
channels plus signal strength and direction data and can show it by zip 
code. Its more ambitious here than what I actually see in some 
directions and yet I get one Cedar Rapids station at about 100 miles 
occasionally. I've seen a link to it somewhere in the past few days, but 
I wasn't successful searching the headers at www.fcc.gov.

As for transmitting filters, I'm sure there have been CSVHF proceedings 
as well a QST and QEX articles on high power low pass filters for 6m, 
usually accompanying articles on amplifiers. Typically they will use PC 
board for the capacitors and coils wound of #12 copper wire air spaced, 
or cylindrical transmitting ceramics (sold by RF Parts) for the shunt 
capacitors. Often 3 coils and two capacitors, or 4 coils and 3 
capacitors. There are many tables in the ARRL handbook of filter 
designs, though at VHF the stray inductance of shunt capacitors usually 
makes them work differently than calculated neglecting that capacitance. 
At power and 6m the capacitors have to be substantial size to carry the 
RF current that will be circulating in the filter network.

Besides QEX ant QST most any ARRL handbook since TV became popular will 
have a chapter on TVI. But good 6m filters won't be in vintage 
handbooks, I'm looking in a 2006 right now. Chapter 13 is on filters and 
it refers to the ARRL RFI Book for transmitting filters. Chapter 14 is 
on transmitters but no low pass filters.

ARRL 2011 Handbook, beginning on page 11.49 has a 6m low pass filter 
good for a KW RF. And its TVI chapter refers to the ARRL RFI Book 
mostly. What Jon has said is all important. Also its generally a bad 
idea for you to work on your neighbor's TV because if it fails for any 
other reason he may try to make you cover the repairs because you were 
inside it or changed its connections. Its best to get rid of the antenna 
mounted preamp that usually has no selectivity or shielding and unless a 
long ways from the TV stations is not needed at all.

While chasing K5N and recording through my converter box I damaged some 
pixels too, will have to catch one of those programs again to see the 
details of some electrical work, but it will show up again. My 6m 
antenna is only a couple feet from the antenna I switch between 2m and 
432 horizontal and TV use so the coupling is close. Need to improve on that.

On 1/2/2013 7:03 AM, kp hpjr wrote:
>
>
>
> Group,
>
> Happy New year.
>
> A neighbor approached me asking if I was operating the other night. He said that I had been interfering
> with his over the air TV signal. He placed the time frames during the MS rally and during this weekend Es opening.
>
> I have only been on 6m as of late at times running up to 600w. His home is about 1000' away from me. His setup is a new digital TV with outdoor antenna fed with the typical radio shack type coax. His antenna is basically looking my way.
>
> I'm needing some help with this. Are there filters I/he can use?
>
> I'm well grounded and use very good hardware...Andrews connectors and feedlines etc.
>
> Your input welcome.
>
> Kirk, N0KK
>
> BTW: 1st RFI complaint in 18 years.
>
>