|As of 26.09.12, this page becomes
the home page for the 3cm Mow Cop .....................................receiver
running via WebSDR
dx so far: HB9G at 952km
Latest news (25.05.15) - Head end failure.
The receiver had seemed a little deaf for the best part of a
year, and it finally failed at the beginning of May.
Inevitably, the cause was water ingress. For a Full report,
please click here.
Also, a second front-end unit has been built so that we can
have something actually on the tower, with full 360 degree
coverage of the far horizon. Details here.
* The beacon receiver is currently running
1.14 kHz low (16.02.15)*
(use GB3XGH as your frequency reference - it is GPS locked)
The Bawdsey remote SDR installation was removed off site in
2009, when permission to operate there was effectively revoked.
No internet connection was ever realised from that site, though
Dave G4HUP did record some decent dx, and 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.
To date, the antenna has remained firmly attached to the
site cabin roof - at some point, either it or a duplicate
antenna should make it onto the tower, adding at least 10m
to the height. This additional height above ground would
ensure that the current shadowing to the North and
North-West will be eliminated.
A single192 kHz window
centred on 10 368.855 MHz is currently being used. The
following UK beacons are therefore covered:
10 368.810 GB3XGH Rochdale (heard)
10 368.830 GB3MHX Martlesham (heard - though only rarely)
10 368.850 GB3SEE
10 368.870 GB3KBQ Taunton (heard)
10 368.895 GB3NGI Antrim
368.900 GB3AZA Scarborough (currently
10 368.905 GB3SCX Bell
10 368.940 GB3CCX Cleeve
GB3PKT Clacton (heard)
Plus, just outside of +-96 kHz, but still available as alias*
10 368.752 GB3FNY Sandtoft (alias
appears at 10 368.944) (heard)
GB3CAM Wyton (alias appears at 10
10 368.955 GB3LEX Leicester (alias appears at 10
Plus well over a dozen
mainland Europe beacons.
Here are some screen shots: Flat band
GB3LEX out of
alias signal reception:
have been a few comments/queries via the receiver
text screen regarding the reception of GB3LEX and
GB3CAM. The former is 100 kHz higher in frequency
than the sound card sampling frequency whilst the
latter, by co-incidence, is 100 kHz below, so both
are at first thought, out of range of this
receiver. However, if you consider sound card
sampling as being a mixing process, you would
expect an image response. This ought to be removed
at source by the sound card anti-alias filter. In
practice though, it takes the filter some 10 kHz
beyond the 96 kHz sampling frequency to provide a
lot of attenuation. This is easy enough to show by
feeding a signal into the receiver, initially at
the centre frequency (ie, DC as far as the sound
card is concerned) and then moving away higher, as
the picture shows below
particular signal was 30dB above noise floor, 1
kHz above centre frequency. As its frequency was
increased, an eye was kept on this signal to noise
ratio. It remained within a dB of this right up to
88 kHz above centre frequency. At 96 kHz it was 3
dB down. Increasing beyond the 96 kHz sampling
frequency, the trace disappears off screen on the
rhs, only to reappear on the lhs. It is the
phasing nature of the receiver that causes this
effect - if you detect using only the I or the Q
audio signal, you see a more familiar sight:
ie, the image
appears both sides. At 100 kHz offset, the signal to
noise ratio has decreased by 10 dB, so when you see
either GB3LEX or GB3CAM, the signal is actually 10 dB
stronger than it appears.
A Connor-Winfield ASOV5S3E 10 MHz OCXO should
ensure that the frequency stability of the
receiver remains better than +- 200Hz.
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 +- 96 kHz.
Output from the SDR feeds
an Asus Xonar D1 sound
card fitted to the server.
Notes on the original
Bawdsey 3cm receiver
This receiver uses the LNB based
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
13.5dBi. Extensions, or wings, added to the
narrow waveguide face reduce the variation
azimuthal gain to about +-2.3dB.
radiation pattern picture in a seperate)
(antenna pics/plot courtesy of David Wrigley)
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.
|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
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.
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
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
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).
David's antenna fitted:
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.
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