Tech

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GB3HV

IO91OF - Farnham Surrey

GB3HV - 144 and 432 MHz antennas

 

 

A Diamond X30 colinear is used for 144 MHz monitoring and 432.750 MHz DTMF control channel reception.

 

For 432 MHz horizontal DATV reception, GB3HV uses a 6 element directional beam - see user guide for details on how to control direction - with bandpass filter and pre-amp at the masthead.

 

 

 

 

 

 

 

 

 

 

GB3HV - 70cms DATV recieve system

 

The 70cms aerial is connected to a narrow bandpass 70cms cavity filter followed by an SPF 5053 pre-amplifier at masthead and then fed to the shack using feeder 4. DC is fed up the feeder to the pre-amp.

 

The signal is fed in to a modified Key Radio 450 base station front end providing a 3 stage helical filter followed by a gain stage and a second 3 stage helical filter. The output of the front end has a 25 dB notch filter on 433.375 to remove GB3FN (we do not see any desense) and then is fed into the input of a modified Zinwell SUP 2400 up converter.

 

The 1963 MHz signal is fed in to a Comaq SL30 receiver which is set to receive 2 Msymbols DATV with auto FEC sensing. The receiver is modified to include a lock sense relay which switches the AV to the phono connectors on the rear of the receiver.

 

 

 

 

 

GB3HV - 23cms antennas

 

 

On transmit, GB3HV will use 4 plate antennas designed and made by Roy G8CKN, phased together with a quarter wave line transformer/combiner in to 1 common feeder.

 

 

On recieive, a similar set of plates are used with a separate pre-amp for each plate. The pre-amps were made by Mike G8LES and have a sub 1 dB Noise Figure and 18 dB of gain at 1248 MHz .

 

The output of the pre-amps is phased together and fed into a common feeder.

 

Currently all 4 pre-amps are fed by a common DC feed up the rx coax, but we have the facility to feed each pre-amp separately to enable a system of switched pre-amp depending on received signal strength and quality..

 

The pictures shows Tony, G1HBD, alongside the array - note we have had to currently reduce the spacing between the arrays to improve mechanical stability on the wooden tower. We will review the arrangement once we have a transmit NoV.

 

 

 

GB3HV - 23cms recieve system

 

The 1248 MHz signal is fed via a 4 pole inter digital filter, followed by a 12 dB line amplifier in to a Comaq SL30 digital receiver which is set to receive 4 Msymbols DATV with auto FEC sensing. DATV receive system performance measured on the bench using an SPF 5753 pre-amp in place of the actual units showed a digital lock at -103 dBm.

 

The 1248 MHz signal is fed from the L Band output of the DATV receiver in to a Comtech FM receiver frontend tuned to 1248 MHz. The baseband signal output from the tuner modulle before de-emphasis or sound filtering ts then fed directly in to the baseband input of Standard communications MT620 professional satellite receiver to provide professional standard of video and audio processing.

 

The composite video and 6 MHz audio from the MT620 are then fed back in to the DATV receiver chassis. The DATV receiver is modified to include a lock sense relay and this is used to switch between analogue and digital 23cms reception. By default the analogue signal is fed out of the phono connectors on the rear of the receiver in to the automatic logic input 3 which will detect and relay the input when an analogue signal is present. When a DATV signal on 1248 MHz is received the digital lock relay in the SL30 will switch and present the main logic with the DATV signal which will then be relayed.

 

GB3HV - Logic and control system

 

The main logic is an Intuitive Circuits controller modified to give indication of valid signal lock and a K sequence when signals are cleared from the repeater input.

 

The logic actually has 5 main inputs, 3 of which are presented for external use, and the highest number input will take priority over other inputs.

 

The main logic input 1 is internally connected to the output of the 8 way DTMF selector described below. This means that when a valid video signal is selected on the 8 way switch, this input is selected, it will override the automatic selection of 70cms or 23cms ATV signals and so HV will appear to ignore a genuine ATV signal on an input. Users should always send 777 to close the manual selection down when finished.

 

An additional CW generator has been added on a sub board mounted in the socket of U3 (ULN 2003) . The sync detect lines from input 2, 3 and 4 are taken to spare gates on the chip and used to drive a PIC CW generator via diode matrix to generate a CW “1, 2 or 3” depending on which input has detected a valid signal. The sync detect line which was already present on pin2 of chip U3 is used to generate a “K” when signal is lost.

 

An Intuitive Circuits 8 way DTMF controlled video and audio switch is used to expand the number of inputs available and is user controlled by DTMF on 432.750 or via the Skype input..

 

The output of the switch is connected to input 1 on the main controller and therefore when a valid video signal is selected on the 8 way switch, this input is selected, it will override the automatic selection of 70cms or 23cms ATV signals and so HV will appear to ignore a genuine ATV signal on an input. This is to allow the manual selection of the quad when valid signals, which would otherwise overide the manual choice, are present on the main inputs.

 

The unit has an additional sub board to ensure that any input selected by DTMF will automatically time out after approximately 18 minutes and the repeater will go back to automatic mode

 

 

 

 

 

 

 

 

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