[RFI] Strong RFI on 2m

[Lloyd - N9LB]

We had a full-power 50 KW Channel 3 VHF TV station here in Madison WI since the 1950s.
The transmitter was an all-tube RCA TT-50, worked well, reliable, and spectrally very clean.
Then about 30 years ago, an All Solid State Harris Transmitter was installed.  
That solid state transmitter totally destroyed 6m use for miles around.
( The old RCA was used on Tuesdays only, the Solid State was run the other 6 days each week. )
The local ham community complained bitterly about it to the station and to the FCC.
An FCC Field Engineer from Chicago came to Madison and made measurements.  He found:
1.  Wideband noise was being generated by the Solid State transmitter, not present with the tube rig.
2.  The noise was "within spectral limits" so the TV station was operating legally.  Too bad for the hams.

Years later, with the transition to digital, a change from channel 3 to channel 50 was made and no more RFI.

73,

Lloyd - N9LB  
( Former Transmitter Supervisor for that channel 3 transmitter site back when it was still all vacuum tube. )

-----Original Message-----
From: RFI [mailto:rfi-bounces+lloydberg=tds.net at contesting.com] On Behalf Of David Eckhardt
Sent: Wednesday, December 22, 2021 12:48 PM
To: nlsa at nlsa.com
Cc: RFI <rfi at contesting.com>
Subject: Re: [RFI] Strong RFI on 2m

I suspect what you are experiencing is broadband noise from solid state transmitter PAs.  During the days of vacuum tube amplifiers for commercial broadcast this was much less of a problem.  However, the newer solid state PAs produce *much* more broadband noise than their older vacuum tube counterparts.  There likely isn't much you can do about it other than not
point your antenna(s) at the source(s).   The noise is there and all the
filtering in the world is not going to eliminate it.  Not what you want to read for an EME setup.

With that many sources, both FM and TV, I doubt you would be able to convince all of them to install aggressive BPFs on their outputs.  Best case might be only one or two of the broadcasters are responsible, but I doubt you should be so lucky.

Dave - WØLEV

On Wed, Dec 22, 2021 at 3:18 PM <nlsa at nlsa.com> wrote:

> Dear friends,
>
> I have recently commissioned my 4-yagi 2m EME system.  To my unhappy 
> surprise, I find strong RFI in the direction of a large commercial 
> broadcast tower 3.5 miles away.  The noise is uniform across my 96-kHz 
> passband, as if the noise floor has been raised 20-30 dB.  I don't 
> detect any modulation or other structure to the noise; it is wideband 
> and constant.  It affects my RX in other directions via antenna 
> sidelobes.
>
> I have discounted fundamental overload to my system by placing a 
> 2-cavity bandpass filter in the RX line.  It should attenuate 
> everything outside the 2m band by at least 40 dB and yet there is no 
> effect on the RFI noise.  I conclude that the noise is "real," in 
> other words not an artifact of my RX chain.
>
> Questions:
>
> *       Should I expect modulation of some sort on spurious emissions from
> TV or FM broadcast stations? (I have five TV and five FM transmitters 
> within a 5-mile radius).
> *       Other than the usual direction-finding, what else might I try to
> investigate in order to identify the source?
>
> Thanks for any suggestions -
>
> Mike, W9IP
>
>
>
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>


--
*Dave - WØLEV*
*Just Let Darwin Work*
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