Message-ID: <002b01c1ee86$f0a29620$2e5301d5@dave>
From: "Dave Sergeant" <dsergeant@connectfree.co.uk>
To: rsgb_lf_group@blacksheep.org
References: <002001c1ee43$25ea0dc0$743f01d5@default>
Subject: LF: Re: Mains Harmonics ??
Date: Sun, 28 Apr 2002 08:21:23 +0100
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

>From Dave G3YMC

Thanks for the detailed analysis of TV etc interference to 136.

>I see drifty lines with 50Hz and 100Hz spacing, but a quick check at lower
>frequencies with teh Rx or SA shows that these are mains modulated signals
>with a funamental at 30 to 40kHz and emanate mainly from switch-mode power
>supplies, PCs and TVs being the worst offenders.

In fact most TV power supplies operate at a switching frequency of around 70kHz, and
it is the second harmonic content which appears on 136.  You usually also find a
strong carrier component at the ninth harmonic of the line frequency (140.625kHz) and
as the power supply is often locked to the line timebase this will be modulated with
power supply components and is often found to have sidebands spreading over the band
(as is the case with my neighbour's wide screen TV).  There is not likely to be much
50Hz component with this radiation unless the power supply smoothing cap is faulty.

>Another gotcha is that loops dont always help, they will couple magetically
>to TV and monitor scan coils at quite large distances (10m+)

I had thought my transmit loop was fairly good at not picking up these noise sources,
and it is the case in many cases.  However my neighbour's tv and its coax feed is
quite close to the loop and clearly there is more pick up in that case - the long
wire and vertical which are considerably further away pick up much less from that
source.

Incidentally I still have a page on my website
(http://www.dsergeant.btinternet.co.uk/tvqrm.htm) to collate tv qrm problems and
indicate the ones to avoid.  I have had very little input to this page - if you want
a set listed (good or bad) let me know.

73s Dave G3YMC
dsergeant@iee.org
dsergeant@btinternet.com
http://www.dsergeant.btinternet.co.uk