The GB3IV IOW Repeater Working Group of SCART, plan to create a second repeater input and output on the 10GHz band. The motivation for this enhancement is two fold: Firstly to offer input and output frequencies free from Radar interference, secondly to provide for the expansion of the repeater range by offering "linking" facilities to nearby sister repeater groups such as the Bournemouth Amateur Tv Group (BATG) in Dorset.
Indirectly the 10GHz band will allow wider channel widths for analogue signals with two audio channels.
Co-incidentally with the 10GHz enhancements it is intended that digital receive and transmit facilities will be made available. Any 10GHz expansion will also allow for digital reception and transmission, if BATG would prefer a digital link then the default setting may be arranged to be digital from the outset. It is recognised that simpler FM ATV transmitters will be the norm for most individual stations so it is planned to be able to have an analogue FM receiver and a digital receiver on each of the three antennas.
Antennas and coverage.
Path attenuation values will be higher at 10GHz than at 23cm, however for stations with reasonable LOS paths, only small dishes may be required.
Three antennas are planned for receive: For Portsmouth and Southampton we plan to use broad beam width LNBs (Low noise converters) only. Beamwidths will be approximately 50 degrees to 3dB points.
The third antenna will be an LNB on a small sky satellite dish pointing at the BATG repeater group site at Pokesdown, beamwidth will be approximately 8 degrees to the 3dB points. A field test is planned with BATG before the dish antenna is confirmed.
The LNB for Portsmouth and Southampton are quite small and will be arranged on the mast to suit the beam heading, and a low loss feeder run will be brought into the equipment room.
The Dorset dish antenna may be mounted on a rotator at a later stage and be rotatable in azimuth only.
Three low cost satellite receivers will provide LNB power through the LNB down lead, each receiver has an L band loop-through connector to deliver the L band 1st IF to analogue receivers. The analogue receivers may be packaged Comtech units or older satellite analogue receivers.
10Ghz LNB with 9Ghz LO.
The transmission on 10GHz will be an up-converted copy of the GB3IV transmission on 23cm. A DTMF code selection will allow the up-converted signal to be changed to a digital MPEG2 source on 1316MHz using 4MS/s, with two mono audio channels (initially connected to the 5.5 and 6.0 MHz analogue channels).
At the outset the video feed to the digital encoder will be the same as that used for the 23m Tx. Other options may become available at a later stage.
Home station equipment.
Receivers will be relatively simple to acquire and install. The LNB designed for the amateur band covers the whole range of 10.0 to 10.45GHz using a DRO local oscillator of 9GHz. Therefore the IF will be 1 to 1.45GHz and well within the normal tuning range of older analogue satellite receivers or the 23cm Comtech modules. Low cost digital receivers are commonplace and will cover the same IF range.
Small dish antennas will give the best margins and if only used for the repeater can be on a fixed bearing. For some stations with a good path it is anticipated that an LNB only will work quite well, and will be very unobtrusive.
Providing a home station Tx is a more difficult task, it is expected that analogue FM 10GHz Tx will provide the link to the repeater for some time.
A key part of the SCART 10GHz project is to make available the x4 multiplier to work with the cost effective Comtech 13cm Tx to provide a 10GHz output.
The aim is to make the Tx unit as small as possible to allow the Tx to be mounted on a mast or building.
The Comtech module is already quite small and the multiplier module is a similar length. With just modules the user can expect an accurate synthesised generator with a filtered driver amplifier delivering 25mW to the dish.
The multiplier is designed by John G8ACE at Winchester and the internal filter allows operation over 10 to 10.45GHz. A small 25mW PA is included.
The unit operates from 12volt dc.
Dave G0GMK has produced an alternative PIC for use in the Comtech 13cm Tx to cover the part of the band required to multiply into 10GHz. Amateurs do not have the entire band from 10.0 to 10.5GHz – only the 10.0 to 10.125 and 10.225 to 10.5GHz segments. It may be that the PIC processor defining the Comtech TX range could be modified to only output the segments of the band we have in our licence schedule. We will find a way to copy the new PIC as part for the project.
The majority of ATV repeaters have an output in the low segment 10.065 and input high around 10.350GHz, positive modulation is the norm for FM signals.[ie for a increasing video transition the Tx frequency increases].
John G8ACE is investigating the design of an add-on PA with a power output of 150mW.
SCART project support.
Several club evenings are planned to assist stations who wish to build the 10GHz ATV systems.
This will include workshop evenings to test and tune-up the modules.
Arrangements will be made to copy the new PICs which are plug in DIL packages.
Multiplier/Amp modules will be made as finished items as the very small surface mount components do need specialist tools and test gear. The units will be made in small batches of 10 or 15, requests should be given to G3PYB. The price of the multiplier modules should be available in the next 4 weeks.
Advice on packaging the units will be given at the SCART workshops, together with information on interconnecting the modules and avoiding the losses that can accrue if care is not taken. The multiplier module and any additional PA will almost certainly use SMA connectors.
Antennas for external mounting will depend on the path the station has to the repeater and advice is available. The best method is to build the receive system first and become familiar with the 10GHz band. If you have both an analogue L band receiver and a digital receiver (such as Comag’s Porty) you will soon gain experience of the two modulation techniques.
Even with low power at ground level you will be able to confirm the operation of the complete Tx /Rx pair.