[RFI] TV Converter Boxes & RFI

Jim Brown jim at audiosystemsgroup.com
Sat Apr 19 18:52:49 EDT 2008

There are two VERY different issues being addressed here. 

First, you should define what a "Bright House" cable is so that we 
understand YOUR question. Is it coax, or is it some other kind of cable?  
What is the spectrum and power level?  

Second, let's look at Dick's comment re: trash from DTV stations in 
Portland. I don't have experience with DIGITAL TV stations and spurs, 
but I DO have experience with spurs and trash from analog TV (and two-
way systems) in downtown Chicago. There, ALL TV and FM transmitters are 
on either Sears or Hancock, two very tall buildings within about 1.5 
miles of each other. Some transmitters are multiplexed into a common 
broadband stick (mostly FM), but most have their own dedicated antenna. 
In addition, there are lots of 2-way rigs, although I'd heard that the 
number of 2-way rigs there has fallen over the years. 

When I was actively involved in ham repeaters (about 25 years ago), the 
VHF trash was so bad that a VHF receiver probably saw a noise floor on 
the order of millivolts. Both the broadcast antennas and 2-way antennas 
use 1-2 bay antennas for low-band TV (Ch2-6) and FM, and 3-4 bays for 
high band (Ch7-13). This makes them relatively omni in the vertical 
plane, as compared to UHF antennas, which typically have 12-16 dB of 
vertical gain. That difference puts more signal in the downtown area 
under these VHF antennas. 

Power levels for TV broadcasting depend on where you live. In the 
eastern half of the country and in major metro areas in the west, 
licensed transmitter powers are typically 3-10 dB lower than in the wide 
open areas of the west. There are also some factors related to antenna 
height. The biggest high band VHF antennas in Chicago typically are 
getting 100-316 kW ERP (that ERP includes feedline and combiner losses 
and antenna gain). Low band Vs in Chicago are down around 50 kW. 

You can see details for transmitter powers and antennas anywhere in the 
US by visiting this FCC site  


and asking for a TV or FM Query, plugging in your Lat/Lon coordinates 
and a radius that you'd like to consider. 

Up to now, the vast majority of broadcasters have run their digital rigs 
at significantly lower power levels than their licenses permit. Two 
reasons. First, the electic bill. Second, many digital licenses are on 
channels adjacent to the analog channel, 99% of viewers are watching 
analog, and the ratings surveys weren't counting digital viewers. 
Running the digital channel QRP minimizes interference to the analog 
channel, which is where the revenue is. 

As to that 20 dB increase in Portland -- I'd first ask how they measured 
it?  Were there a couple of good cavities and/or band pass filter in 
front of the measurement device so that it's front end wasn't getting 
blown away by the fundamentals of these transmitters?  

I'd also ask about the linearity of the transmitters (and how they're 
being driven/tuned). A house I rented in Chicago for 20 years was 3 
miles from Hancock, and I had a nice FM/TV yagi on the roof that put 
lots of signal into my Technics ST9030 FM RX (very bulletproof analog 
tuned front end). In all the time I lived there, I heard an FM spur only 
once, and it disappeared within an hour or two. 

BTW -- 20 dB is 100 times the power. 


Jim Brown K9YC

On Sat, 19 Apr 2008 17:35:53 -0400, kd4e wrote:

>Since they just ran a Bright House cable next door in our
>*very* rural neighbohood (10 acre lots) to a pole that is
>about 120 feet from the house should we anticipate a rise
>in RFI?

>If I we to allow them to run the cable to our house would
>that increase RFI?

>> First a comment.. This subject DOES impact ham radio, for in a larger
>> city with multiple Digital TV stations, which are normally "megawatt"
>> stations, the noise floor for VHF repeater inputs rises accordingly,
>> for digital pulses are full of spectrum noise!
>> In Portland, Oregon with channels 2, 6, 8, 10, and 12, and not all 
>> converted yet, it rose 20db! This translates out to a factor of some 
>> 120 times the origional noise!!
>> Dick, CET (I'm licensed TV tech!!)


>Thanks! & 73, doc, KD4E
>Free OS : http://www.ChurchPup.com
>Based on: http://www.PuppyLinux.com
>Personal: http://bibleseven.com/kd4e.html
>             |_|___|_|
>             | | & | |
>                {|
>        /\      {|
>       /  \     {|
>      /    \    {|
>     /   @  \   {|
>     |   |~_|~~~~|
>     |   -| |    |
>============\ # KD4E
>RFI mailing list
>RFI at contesting.com

More information about the RFI mailing list