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GB3YT - Technical description

RF Inputs:

Input 1 - 1276MHz Digital 4Ms/s FEC 1/2 (Horizontal polarisation on 23cm)

Input 2 - 1276MHz Analogue (FM PAL 625 - 6MHZ sound sub-carrier)

Input 3 - 437MHz Digital 2Ms/s FEC1/2 (Vertical polarisation on 70cm)

Input 4 - 146.5MHz Digital 333Ks/s FEC 7/8 (In planning/design early stages!)

RF Output:

Output - 1316MHz Digital 4Ms/s FEC 1/2

Digital Video PID - 256 (0100 hex)

Digital Audio PID - 257 (0101 hex) - See Audio channelTX useage below

Digital Tx Power - 25w ERP

Audio:

2m Talkback - 144.750MHz (CTCSS 118.8Hz) - Retransmitted on the digital right audio channel only.

Input digital audio - Retransmitted on the digital left audio channel only.

Input analogue audio 6MHz Sub-Carrier - Retransmitted on the digital left audio channel only.

Standby audio/testcards transmitted in stereo.

Aerials:

23cm TX/RX - Alford slot at 35ft AGL

2m/70cm RX - Diamond collinear vertical

Ground level - 550ft ASL

Location - IO93dp - Mirfield, West Yorkshire

 

General & Power

The GB3YT repeater is a combination of home brew and high quality commercial units built into two 19in compact 9u racks. The transmitter rack is powered by two large switch mode power supplies, one at 12v the other at 24v, and the receiver rack has a smaller 12v one also.  A general layout block diagram is shown under the pictures tab above. Power failures and system problems are notified directly to us by email and text messages.

 

Antenna system

The antenna system consists of an Alford slot mounted on a small mast which is mounted to the side of the building. This gives a total of around 35ft elevation above ground level. The mast also contains a high sensitivity colour camera which is used to generate a panoramic view from the hill top in Mirfield - this being broadcast when the repeater is not in use. This may be replaced by a testcard from time to time when we are performing testing, or if the camera view is poor or not available.

The feed from the antenna is directed into a set of small cavity duplexers which splits the receive and transmit signals, which are then in turn fed through two seperate five pole interdigital filters. This results in a very clean transmit signal, and provides a very clean receive signal into the low noise/high gain pre-amp.

The 23cm transmit signal is fed to the Alford slot which along with various feeder cable losses gives a calculated ERP of 25w - The cables losses equals the antenna gain!

For 2m/70cm receive, we have a Diamond colinear vertical. Due to our site owner requesting we use the same mast, this is mounted vertically downwards which should not cause any problem, however, it is sheilded by the building to the West therefore reducing sensitivity in that direction.

 

Receive

23cm: The antenna is first connected to a lightning arrester, then through a Bird Thruline SWR monitor, a low loss cavity duplexer and then fed into a five pole interdigital filter. This in turn is then fed into the 23cm VLNA which has 44db gain. The signal from the VLNA is fed into a very low loss signal splitter giving an output to a Tandberg TT1220 for digital receive, and a DX Antenna DSA646W for analogue receive.

70cm: The antenna is a 2/70 colinear which is fed into a duplexer/splitter. The 2m feed is used for the talkback on 144.750MHz, and the 70cm is fed into a series of bandpass filters and pre-amps. The resulting 70cm signal is then fed into an upconverter module which gives an output fed to another Tandberg TT1220 at 1.9GHz. The only problem will be that signals from the West will be partially sheilded by the building - although the coverage from the West is already limited by the pennines.

2m: The 2m signal will also be split and used for 2m RB digital receive when the development has been finished. The receive rack is ready to accept the fourth input when ready.

 

Talkback Receive

The talkback receiver is a crystal controlled PYE M290 receiver section from a F494 base station, which is tuned to 144.750MHz. The 2m audio is muted until a valid signal with a CTCSS of 118.8Hz is received. This talkback audio is re-transmitted on the right audio channel only, and ONLY when a valid ATV signal is being received. 118.8Hz CTCSS was chosen since the frequency is not normally used in our area, therefore reducing the chances of spurious audio being re-transmitted or interfering with the talkback input.

 

Audio Channel TX Usage

The receive audio, standby audio and takback audio are routed through to different transmit channels following a similar method adopted by other repeaters. The receive analogue and digital L/R audio is retransmitted on the left audio channel only, the CW ident during use is transmitted on both the LEFT & RIGHT audio channel, and the 2m talkback audio is retransmitted on the right audio channel only.

The 'K' and standby ident audio are transmitted on both LEFT & RIGHT audio channels. If you exprerience no/wrong sound, check you are transmitting your audio on a 6MHz sub-carrier if using analogue, and monitoring the left channel if listening for through audio. Remember the right channel is only used for talkback (2m) and stereo testcards.

 

Transmit

The transmitter system is based on the BATC DTX1 modulator. This provides all the necessary digital conversions to change from an analogue video/audio input, into a fully encoded digital RF signal - albeit at a very low level (-5dbm). The output from the DTX1 is fed into a MMIC pre-amp contained in the driver amplifier which boosts the low signal level to around 4watts. The output of this feeds a 150w final PA running at an output of around 40w. This signal is then filtered by the interdigital filter and then fed into the duplexer (a total of 2db loss through both). This gives a transmit output power of 25w into the antenna feeder.

The PA modules are mounted on two very large heatsinks which are fan cooled, with fan speed controlled dependant on heatsink temperature.

 

Logic/Switching

The signal path has various bits of logic to time and switch the audio/video path. This is all home made in both design and construction. The CW audio ident is home made based on a tone generator circuit and an EPROM to contain the callsign information. Switching between the video and audio feeds are performed by a Kramer A/V switcher which is controlled by the home made logic controller.

The logic has a quad video picture for multiple display of input signals - if more than one signal is being received. The logic will only display one full screen received signal, and then switch to quad display if more than one is being received. In other words, a 70cm AND a 23cm signal can be viewed at the same time - DUPLEX!

 

External Camera

The external camera is showing the view in a south easterly direction, and has the following specs;

1/3in. Colour Sony EXVIEW II CCD
High resolution
700TV Line (Colour)
Day and Night
Minimum Illumination : 0.005 Lux (F:1.2) (No IR illumination fitted)
Auto White Balance
Automatic Gain Control
Waterproof

Emley moor TV transmitter mast can be seen on the extreme right of the picture - navigation lights being easily visible at night time.