Notes on the 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.

First light....

Dave, Alan and Martin hoist up the antenna, but Murphy's law ensures that the plastic pipe is neither too big nor too small to fit concentrically to the mast. On the Spectravue display, GB3MHX can seen some 40 dB out of the noise:

 first antenna first reception


Results
Since no automatic monitoring software is available yet, Dave G4HUP has had to switch on the receiver and record relatively short files from the SDR-IQ iinterface whenever he has been on site. Nothing (other than GB3MHX) was heard until March 2010, when on one morning, a beacon above 'MHX was spotted. This turned out to be GB3SEE, and a screen dump of this is shown opposite top. In the afternoon, another beacon was spotted, this time on the low side of 'MHZ (opposite, bottom pic). This was DB0GHz.
 gb3see

db0ghz