[Premium-Rx] Fw: Preselectors and strong signals

[Michael O'Beirne]

----- Original Message ----- 
From: "Michael O'Beirne" <michaelob666 at ntlworld.com>
To: "Premium-Rx" <premium-rx at mailman.qth.net>
Sent: Sunday, December 02, 2007 3:37 PM
Subject: Preselectors and strong signals


> Dear All
>
> I am intrigued as to where these 50V signals from our antennas are coming
> from.  To check on the typical input levels here at noon today I attached 
> my old Marconi TF2370 spectrum analyser connected directly to the
> output of a Wellbrook loop situated indoors with the analyser
> tuned to where the strongest signals are located here - in the Long Wave 
> and
> Medium Wave.
> >
> The second strongest station was BBC Radio 4 on 198kHz at a level
> of -26dBm (11.5mV PD), transmitting at 500kW from about 130 miles distant. 
>  >
> The strongest signal was BBC Radio 5 Live on
> 909 kHz at -23dBm (16mV PD).  That is transmitted at 150 kW from Brookmans
> Park about 25 miles away.  These levels compare reasonably well with the 
> cruder S meter display on the RA3701.
>
> Of course at night the LW and MW will be full of strong Continentals
> but at levels not much higher than shown here.
>
> So where are these mega 50V signals?  If it's a local commercial
> transmitter, I would assuredly have moved house long ago.
>
> If it's from a very long wire, frankly, ditch it and move to a decent 
> loop.
> You will get larger signals from a long wire but even more noise 
> (including
> all your neighbourly computer and SM PSU crud), and the impedance will
> almost certainly be wrong.  Reviews of the Wellbrook loop show 
> convincingly
> that the S+n/n it delivers is better than with a long wire.  You can also 
> achieve up to 30dB nulls by rotating it.
>
> If you have a giant Granger log periodic in your back field, then apply 
> 10dB
> attenuation and your IM products should disappear.
>
> HF receivers with wideband front ends are designed to cope linearly with 
> reasonable input
> levels - usually in the tens of mV - before IM products appear at
> unacceptable levels.  Most professional receivers have protective relays 
> to
> disconnect or earth the antenna input when the input level reaches about 
> 5V.
> Most receivers would otherwise fry if exposed to 50 V continuously.
>
> 73s
> Michael
> G8MOB
>
>