Re: Ground noise on 23cm?


Colin Ranson
 

Hi all,

 

I have a filter designed by G3XDY plumbed into my system after the amplifier.     https://www.qsl.net/g3xdy/QRO%2023cm%20Harmonic%20Filter%20and%20Reflectometer.htm

 

On RX I have an (almost) mast head pre-amp (located in loft, with the amplifier, under the antenna outside, a Tonna 35ele that seems to be intermittent at times so something else is needed). The pre-amp has an onboard hairpin bandpass filter and this also feeds an interdigital filter which connects via 30ft of coax to the SG Labs transverter.   Other than radar pulses I have never noticed any unusual noise from any direction – even with a well loaded water tower only 400 yards away. Mind you not saying this won’t change when I get a decent antenna up there.

 

Regards to all,

 

Colin de G8LBS.

 

 

 

 

 

Sent from Mail for Windows 10

 

From: Richard GD8EXI
Sent: 16 September 2020 22:27
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Ground noise on 23cm?

 

I doubt it can be ground noise if it is S5. On 23cms you are unlikely to see more than 3dB change in noise, between beaming into a hillside and beaming down hill. The antenna noise temp going from say 100 to 200K, so even with a RX noise temperature of 50K, only expect a 2.2dB difference.
 
As Andy stated more likely the presence of one or more very strong signals, mixing in your RX at the /P location.
 
Another common culprit is a DTV transmitter centred on about 650MHz. The intermodulation products look like white noise in a 3kHz bandwidth on 1296MHz. Also do not assume your beam will tell you where the source is, as the polar diagram will look very different on 650MHz.
 
Best test is a good 1296MHz bandpass filter in front of any amplification. The trouble is the path loss of about 1dB. I use a 650MHz notch filter in front of my masthead preamp and an interdigital one after it. The noise floor drops by 10dB on some beam headings, on adding this combination, even after correcting for insertion loss.

73s
Richard
GD8EXI




On 16/09/2020, 20:20, "Andy G4JNT" <andy.g4jnt@...> wrote:

The product is 2160MHz  [which is on the edge of a 3G channel]  -   2 x 1152 [LO]  =  144MHz

Andy
www.g4jnt.com <http://www.g4jnt.com>



On Wed, 16 Sep 2020 at 20:18, Andy Talbot <andy.g4jnt@...> wrote:

If you're using aa 144MHz IF, there's a chance you're seeing IM products between your mixer LO and one of the 3G base station channels.   I'd have to look up the channel that cases the problems, but it was bad here until I added an additional filter in the Rx chain

Andy
www.g4jnt.com <http://www.g4jnt.com>



On Wed, 16 Sep 2020 at 20:13, G0FVI via groups.io <http://groups.io>  <Andrew.gilfillan@...> wrote:

I'm still learning being relatively new to uwaves so apologies if this question seems a bit 'dumb'. I'm noticing fairly high noise levels on 23cm UKAC from my /P location (S5) but it's not constant in all directions and tails off to zero in several directions particularly due east to the continent (ground slope away to east and there are no obvious nearby man made sources of noise although there are HT pylons about 100 yards away). I'm not getting this at home with the same setup mounted on mast at about 35 feet (/P mast is only 20 feet), so I'm guessing it is perhaps ground noise? 

Andy G0FVI

 

 

Join UKMicrowaves@groups.io to automatically receive all group messages.