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Surplus
LNB’s at 10GHZ. By
Brian V Davies GW4KAZ
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With the change from analogue to
digital taking place in satellite broadcasting, the old analogue systems are
being removed with the equipment-becoming surplus. A number of LNB's have been given to me for experimentation; these include
the good old blue cap Marconi LNB and many others. Before starting work on
any LNB is worth noting that internally they are static sensitive,
therefore suitable precautions have to be taken! The blue cap LNB, I have
converted into 10GHz transmitters with great success. I found information
in passed issues of CQ-TV and various web sites, in particular PA3GCO's
site at www.qsl.net/pa3gco. |
The
challenge then was to convert an LNB to receive on 10GHz, to work with a domestic
satellite TV receiver. LNB's in the main have either 10GHz or 9.750GHz
local oscillators. For the LNB to work on 10GHz receive, then the local
oscillator would preferably be working at 9GHz.The method I used to covert
the local oscillator to 9GHz was to glue two dielectric resonators
‘puck’ together, using clear nail varnish, clamping the two together
and leaving for about 24 hrs to dry. Then grinding the combined dro, until
the oscillator was at 9GHz, this was a slow process initially, but as
experience was gained, it became easier. It was more of a mechanical
problem than electronic, so a micrometer was used to measure the width of
the combined dielectric resonators, as I ground them down. I have
converted various types of LNB's to 9GHz, but mainly the Cambridge Juno
type, with great success.
Following on from this, I had a requirement to use this LNB
with a feed horn and WG16.After careful study of the Cambridge type LNB, a
plan was worked out to achieve this transition, with references taken from
the R.S.G.B Microwave Handbook, volume 3, page 18.12.
The plan was the use
of a hacksaw, and a lot of elbow grease!
Fig 1 shows
the original Cambridge LNB, also the LNB with the circular feed-horn
removed. Before starting to cut away at the LNB, there is one important
job to be done that is to remove the pcb from the LNB housing; it’s a
simple job. Access to the pcb is by removing the lid and internal
screening of the LNB. All that keeps the pcb it in place now is the
connection to the “F” connector, unsolder this connection, and with
care the pcb together with the rf coupling into the wave-guide can be
removed, and safely stored. With the pcb removed, cutting the surplus
parts off the LNB casing can start, removing the parts as in
Fig
1, one important feature here is the cut with the face of the LNB,
this needs to be as level as possible with the rest of the casing. This is
because the WG16 needs to have a good level fit with the casing. Having
completed this modification to the LNB casing, I needed to build up the
WG16 as per the reference above. I used the dimension of 23.1mm for the
coupling into the WG16 from the blanked off end of the wave-guide. The
WG16 was drilled at this location ready to take the coupling from the pcb
into the wave-guide, with sufficient clearance. Holes are already drilled
in the LNB casing where the couplings protrude into the original circular
wave-guide. Note that there are two couplings; this is because the
original system needed horizontal and vertical polarization. In my case I
only needed one, the active coupling with 12v on the LNB was in my case
the coupling nearest the edge of the LNB. I also fitted matching screws in
the WG16 as per Fig 18.21 on page 18.13 of the Microwave Handbook, the
pitch for the 6BA screws is 5mm, these were drilled and tapped into the
WG16.
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I soldered 4 lugs onto my piece
of WG16; the length of WG16 can be any length required, I kept mine short
to suit my purpose. Match the hole in the WG16, with that in the LNB
casing, and mark the location of the lugs onto the LNB casing, drill and
tap these as required to enable the WG16 to fit tightly to the casing. Now
the transition metal work is complete, care is taken with the length of
thread of the screws attaching the WG16 to the casing, if they protrude
into the inner side of the casing, they will damage the pcb when its re
fitted into the case. |
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As mentioned previously only one
coupling is required into the WG16, there are two attached to the pcb, so
removal of the appropriate coupling is required, together with adjustment
in the length of the remaining coupling. As in the reference above, the
coupling length into the WG16 should be 6mm. It is therefore a good idea
to unsolder both couplings and file down one to the appropriate size,
taking into account the thickness of the LNB casing and WG16 wall
thickness, which the coupling has to pass through, the appropriate
coupling can now be re-soldered to the pcb. A flange could be fitted onto
the WG16 for future use, if required. The transition is now complete, with
the pcb re-installed into the casing with lid and screening.
Fig 2
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I needed to fit an antenna onto
the WG16. The requirement was a very broad beam-width; again I turned to
the Microwave Handbook. Page 18.83 gave me the answer. A sectoral horn,
this should give 180 degrees beam width, just what the doctor ordered.
Again some planning took place, and decided to use CAD software to design
the shapes of each side of the horn, printing these out, then sticking the
cut out paper template onto copper sheet. This gave me an accurate cutting
guide. The copper sheet was cut carefully and the pieces solder together
to construct the horn. I did not have any flanges, so decided to solder
the appropriate face of the horn to the WG16 directly, making sure all the
soldering joints had a minimum amount of solder on the inside faces of the
joints. I understand that solder absorbs microwave radiation well, which
reduces the efficiency of the system. The finished unit is shown in
Fig 3 and
4, and it works well, except it did not
give me the 180 degrees beam width, but 150 degrees at the -3dB points.
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I hope that this article helps others to look
at 10GHz projects for experimentation.
My thanks go to Max MW1KDP for the donation of LNB’s,
which allowed me to experiment at 10GHz.
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