[Top] [All Lists]

Re: [RFI] Low pass filter opinions

To: RFI RFI <rfi@contesting.com>
Subject: Re: [RFI] Low pass filter opinions
From: dalej <dj2001x@comcast.net>
Date: Wed, 12 Jan 2011 16:49:30 -0600
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
How would I cure a pin one problem in a particular piece of equipment.  Would I 
look at the input XLR or TRS and put that pin one directly to ground or the 
chassis somewhere close to the receptacle?  What about the PCB?  Some of those 
doggone connectors are mounted to PCB and not that easy to find the pin one 
connection.  I like the theory, but implementation is another matter, in some 

Dale, K9VUJ

On 12, Jan 2011, at 13:27, Jim Brown wrote:

> On 1/12/2011 9:22 AM, Scott Holisky wrote:
>> Lets not throw out those LPF yet. I live a few miles from the local
>> broadcast towers here in the Twin Cities. The ERP from the three towers is
>> ~30 Megawatts, give or take.
> I was similarly located in Chicago for nearly 20 years. For a research 
> project on RFI to audio systems, I did both calculations and 
> measurements (HP Spectrum Analyzer). At close in distances, field 
> strength is considerably less than a simple ERP calculation suggests, 
> because all those TX antennas have a LOT of gain in the vertical plane, 
> with VERY narrow beamwidth. Indeed, the greatest field strengths were 
> from low band VHF and FM stations, all of which were using one bay or 
> two-bay antennas and a lot less ERP.
>>  In the days of analog TV you could tune across
>> the bands and hear the occasional sync buzz of a TV station.
> I'd bet that most, if not all, of what you heard was low band VHF (Ch 
> 2-6). BTW -- video buzz is a better word than sync buzz -- the 
> repetition rate of NTSC video is 59.97 Hz, video is 75% of that 
> waveform, sync is only 25%, and you can (could) clearly hear the 
> character of the buzz vary as the video content changes.
>> I have used a LPF (the Drake version) for years. It eliminated the mixing or
>> overload in my radio's front end.
> While that's certainly a good solution, but the fact that it's needed 
> indicates it's a poor radio. If I had that sort of problem with a radio 
> today, I'd sell it and buy a better one. :)
> I strongly agree with others who say that an outboard LPF is rarely 
> needed in today's world.  Virtually all GOOD rigs and amps have very 
> good bandpass filters built in, and for all the reasons noted, VERY 
> little TVI is caused by harmonics of the TX unless something is badly 
> broken or badly mistuned. Antenna tuners also tend to minimize VHF/UHF 
> harmonics, and HF antennas don't radiate VHF/UHF very well. 174 MHz is 
> the lowest harmonic we need to worry about, because there are VERY few 
> US TV stations below Ch 7 (174-180 MHz). Most that were on Ch 2-6 (54-88 
> MHz) have moved to Ch 7 or above. Study 
> http://www.fcc.gov/dtv/markets/   to see who's on what RF channel in 
> your part of the country, or use 
> http://www.fcc.gov/fcc-bin/audio/tvq.html to search channel by channel. 
> (To avoid confusing non-techie viewers, stations that moved from one 
> channel to another continue to be identified by their old analog 
> designations.  For example, a station that was on Ch 9 in analog days is 
> still identified as Ch 9, even though they might actually be 
> transmitting on Ch 22.)
> Rather, the primary causes are poor shield connections in the RF system 
> (rooftop antenna, cable TV, and interconnections of those systems) and 
> pin 1 problems in equipment connected to the TV (like stereo rigs and 
> video recorders of various sorts), and an LPF does NOTHING to solve 
> these problems. Money would be far better spent on a good choke to keep 
> RF off the feedline, getting antennas a bit higher (thus further from 
> the victim equipment, and on chokes to kill RF current on wires 
> connected to the victim equipment.
> There's an RFI tutorial on my website.  
> http://audiosystemsgroup.com/RFI-Ham.pdf
> 73, Jim Brown K9YC
> _______________________________________________
> RFI mailing list
> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi

RFI mailing list

<Prev in Thread] Current Thread [Next in Thread>