Circuit Notebook 86 - 3cms Input Indication and Switching for the BATC I²C Repeater Control System (By John Lawrence GW3JGA and David Ellis Jones GW8PBX)


GB3TM is our local 23cms ATV Repeater and is located on the North East corner of the Isle of Anglesey in North Wales. It has been operational since July 1994 and serves the North Wales Coast and under lift conditions EI, GI, GD, also G along the Lancashire coast.

Input is on 1249 MHz and the output is on 1316 MHz. It uses the BATC I²C logic control [1] and when in beacon mode it displays a sequence of captions including two news pages. When a valid input signal is received, the logic switches over to repeater mode and re-transmits the incoming video and audio signals. There are certain points in the sequence of captions where the response to a valid signal is immediate, otherwise there may be a delay of up to 30 seconds until the display of a caption has been completed. A block diagram of the 23cms repeater prior to 2004 is shown in Fig.1

BATC Logic

The BATC I²C logic control has two audio/video inputs, one for 23cms and one for 3cms. The logic detects which input carries a valid video signal and switches this input through to the output and to the 23cms transmitter. In the event of both inputs being valid, the first one which becomes active takes priority, until it drops out, at which time the other input is switched through to the transmitter.

3cms Receiver

In 2004 an experimental 3cms receiver was installed on site (and connected to the 3cms input of the I²C system) to determine the possible service area on this band. It consisted of a conventional satellite receiver and an open-ended LNB. Tests were carried out using simple 3cms Gunn diode transmitters with PW dishes [2] and by monitoring the return signal on 23cms. Because of the logic delays in switching over to repeater mode on 3cms it was difficult to beam the transmitting dishes on to the repeater. It was necessary to swing the dish very slowly or the correct direction was passed before the signal was re-transmitted. It was decided that an immediate indication of a valid 3cms signal was necessary. See 'Immediate indication of 3cms Signal' below.

Dual Receivers

Because of the location of GB3TM, with rising ground and the Irish Sea to the north, the most likely direction of a 3cms signal would be from the East, South or West. To cover this area, two LNBs were installed, each covering about 90 degrees and each having a satellite receiver, thus providing two audio/video receive channels, both on 10.340GHz.

The A/V signal from each receiver is taken to an A/V switching unit [3] to sequentially sample each receiver's output (East - South, South - West) at about 1 second intervals. The output from this switching unit is taken to a Valid Video Detector [4]. On receipt of a valid signal from one of the receivers, the switching sequence is halted and the valid signal is passed to the 3cms input of the repeater logic system. If the signal drops out then the switching sequence resumes. If a valid signal is received on both receivers then the receiver capturing the signal first, holds it.

Immediate indication of 3cms Signal

So far, so good. The valid 3cms signal is taken to the 3cms input of the repeater logic. To provide an instantaneous display of, for example '3cms Active', use is made of a second A/V switch and a Caption Generator [5]. This second A/V switch is controlled by the Valid Signal Detector to switch between the "straight" video from the logic unit and the same video but with caption inserted. When any 3cms activity is detected, the '3cms Active' caption is carried by the video line to the transmitter and so appears immediately over any pictures being transmitted, as shown in Fig.3.

In due course, the repeater logic senses the 3cms input signal and switches this through to the 23cms transmitter in the usual way, the '3cms Active' caption remains in the picture until the 3cms signal drops out.


The various parts of the circuit are built on separate circuit boards and the complete system is housed in a die-cast mega-box, as shown in Fig.4. The 5v regulator on the caption generator has been removed as a stable 5v supply is provided within the box.

Future modification - Emergency bypass

A relay is to be incorporated to switch the output of the logic unit directly to the transmitter input in the event of a power failure in the box. This will bypass the box completely, allowing the repeater call sign and standard 23cms operation to continue in the event of such a failure.


[1] An Intro to the I²C Book, Trevor Brown G8CJS, CQ-TV 152 p.18

[2] 10th Anniversary for GB3TM, CQ-TV 208 p.25

[3] A/V Switcher, Circuit Notebook 85, CQ-TV 208 p.35

[4] Video Detector, Circuit Notebook 83, CQ-TV 204 p.39

[5] A Low Cost Caption Generator, Steve Drury G6ALU CQ-TV 201. p.23


Fig.1. 23cms ATV Repeater

Fig.2. 23cms/3cms ATV Repeater

Fig.3. '3cms Active' Display

Fig.4. 3cms GW8PBX Control Box