As of 26.09.12, this page becomes the home page for the 3cm Mow Cop  .....................................receiver running via WebSDR

The Bawdsey remote SDR installation was removed off site in 2009, when permission to operate there was revoked. No internet connection was ever realised from that site, but it has now found a very good new home at Mow Cop and is about to be made accessible via WebSDR thanks to the efforts of Martin, G7CKX.

Initially, a single 96 kHz window centred on 10 368.855 MHz is being used. The following UK beacons are therefore covered:

10 368.810  GB3XGH   Rochdale
10 368.830  GB3MHX  Martlesham
10 368.850  GB3SEE   Reigate
10 368.870  GB3KBQ  Taunton
10 368.900  GB3AZA  Scarborough (currently off-air)

There are also at least 11 mainland Europe beacons using this frequency window.

A Connor-Winfield ASOV5S3E 10 MHz OCXO should ensure that the frequency stability of the receiver remains better than +- 200Hz.
IF section
 The same antenna and LNB combination that was used for the Bawdsey receiver is used here, but a different IF is employed, based on an '80m Finningley SDR' board. In a bit of a Heath Robinson arrangement, the on-board 15 MHz crystal oscillator is replaced with an oscillator/multiplier board using a 12.570 MHz crystal multiplied by a factor of 6 to 75.420 MHz (in fact, it's another of the same multiplier boards used in the front-end 'Berniebox').  This results in an IF coverage of 18.855 MHz +- 48 kHz.

Output from the SDR feeds an SoundBlaster AudigyZS sound card fitted to the server.




Notes on the original Bawdsey 3cm receiver

This receiver uses the LNB based converter described elsewhere on the index page.

To obtain a nominally even azimuth gain, a slotted waveguide antenna with side wings has been used. This has been produced by David Wrigley G6GXK, and is shown below. Multiple slots provide elevation gain.
There are 12 pairs of slots resulting in a gain
of 13.5dBi. Extensions, or wings, added to the
narrow waveguide face reduce the variation
in azimuthal gain to about +-2.3dB.


(view radiation pattern picture in a seperate)
window



(antenna pics/plot courtesy of David Wrigley)

 slot antenna
 slot antenna with radome
 radiation pattern

Waveguide input LNB

Although both WR-20 and C120 waveguide input LNBs are available, they are not very common (particularly with 9.75GHz LO and extended lower band edge of 10.75GHz). It was discovered that on a couple of brands of mini-dish LNB that the first choke ring provided a tight fit for a 22mm straight copper coupler, providing a suitable interface for 22mm copper pipe to be used as waveguide.
LNB 22mm coupler LNB front-on
 This particular LNB is manufactured by WNG, and
the 22mm coupler is a good tight intereference fit,
which means that the mating copper tube wave-
guide can be rotated without having to worry about
the coupler itself rotating or coming loose, which is
actually quite a useful feature.

Probe skew (relative to the eliptical feed horn)
ensures that it is not obvious what the correct
alignment is for the copper tube waveguide, so
being able to rotate this for best signal-to-noise
ratio is very useful.


The next requirement is a transition from 22mm circular waveguide to WR-16.

transition lnbfeed

Amazing what can be done with a rubber mallet...

Tuning screws were added to correct the inevitable mismatch, though this turned out not to be too bad, the un-tweaked return loss being about 10dB. Originally, a seperate coupler was envisaged to insert the reference (LO) signal whilst providing some directivity, but has not been entirely satisfactory, so a simple coupling probe has been added to the transition - this explains the short sma flying lead.


Some noise figure measurements

An HP 346A noise source was used to feed an sma-to-WG16 adaptor, which was bolted to the end of the transition section shown above, and the tuning screws adjusted for best sensitivity (measured via the Bernie box at 18MHz). Repeatedly switching the noise source on and off gave the following result:
sensitivity set up

Housing

The LNB  housing is a 1m length of Wickes soil pipe. This provides a moderately good fit to the 4" mast. Note that the reference feed tail on the transition has been replaced by a bulkhead sma connector. Not only does this give a mechanically beter arrangement, but it also allos the probe length to be reliably set (and adjusted).
housing1
 housing2

housing 3


 pedestal

LNB, with David's antenna fitted:

dawdsey ant

A final tweak of the matching screws on the transition piece was then made - it did make half a dB difference, which is a slight supprise.
The following set-up was used for the tweak.

final tweak

A 30 dB amplifier was added to the HP noise source, which fed the antenna fastened to the ladder on the lhs of the picture (probe fitted within an old Grundig LNB).  The receive antenna/LNB combination (rhs of picture atop three wooden pallets) then fed the converter via 8m of twin coax. Output from the converter was split between the noise figure meter input and a spectrum analyser, used as a monitor to check all was well. Using the noise figure meter for this adjustment is good, because the meter can be set for heavy averaging.