GB3SS Technical Information

Hardware • Logic • Specification • Downloads and Links

Hardware

Tait rack chassis with prototype logic under test (click for larger photo)

HOZ Logic module between RX (left) and TX. The PA is at far right (click for larger photo)

Two of the five masts at the site. The GB3SS antennas are at the top of the right mast (click for larger photo)

Originally based on the GB3US/G3RKL logic and a "surgically separated" Uniden transceiver in a homemade chassis, the repeater was overhauled in early 2005 and now consists of Tait T830 series RX, TX and PA in a Tait T800 rack chassis. This ex-PMR system offers better performance and is physically much sturdier. The new logic controller is based on the HOZ Logic design used on several UK repeaters.

The Tait T830 series system consists of a synthesised receiver, synthesised exciter and rugged power amplifier, housed in bays in a rack chassis. The receiver and exciter have configurable signal processing, including filtering and compression. The power amplifier is capable of 50W continuous into poor loads, with over-temperature and high-SWR shutdown and alarms.

The logic controller is housed in a spare bay in the Tait chassis, with the front panel designed to match the Tait modules. All modules slide out of the chassis from the front for easy service. The logic and PA have "umbilical" cables connecting to the chassis wiring so they can be pulled out and adjusted while the repeater is in operation. See below for details of the logic operation.

The transmit/receive duplexing is performed by a six-section cavity filter duplexer (3 RX, 3 TX). These filters were constructed by club members to a design by W1GAN in QST magazine and the ARRL handbook. An additional single cavity and a 3-stage helical filter in the RX line provide rejection of VHF paging transmitters (whose antennas are mounted adjacent to the GB3SS antennas).

The repeater unit is housed in a rack cabinet along with the cavity filters and a purpose-built power supply with capacity to spare. The cabinet shares one of several crowded rooms in a blockhouse with everything from microwave data links to broadcast radio.

The antenna system comprises two stacked (collinear) folded dipoles mounted at about 30m above ground on a shared (and very crowded!) pylon. The antennas are connected to the duplexer by about 45m of LDF4-50 heliax cable.

GB3SS has no battery backup, since the mains power supply to the site appears to be very reliable. The logic has provision for monitoring mains/battery power and backup could easily be added to the system should it be needed.

The repeater hardware is summarized in the block diagram below. Click the diagram for a full-size version.

GB3SS system block diagram (click for larger version)

Logic

View inside the logic module (click for larger photo)

The Tait RF deck installed in the cabinet. The main power supply is at top. The shelf below the Tait unit holds the log, manuals and old PA. (Click for larger photo)

The GB3SS logic is a slightly modified version of the "HOZ" logic used on GB3GD, GB3LD and other repeaters. The logic was designed by GD4HOZ (hence the name) for the Lake District group and has been in use in the UK since 1999. The design is freely available and not particularly expensive to build. The changes for GB3SS are mostly customisation of timings and callsign, with some changes to the remote control features and toneburst access. New features can be added by modifying the program code.

The logic is based on a Microchip PIC microcontroller programmed in assembler. Audio routing, processing and tone generation (CTCSS, beacon and "pip" tones) are performed by a single chip - the FX828 Selcall/CTCSS processor from CML. CTCSS decoding is performed by a FX365C also from CML, and the HT9170B/MT8870 provides DTMF detection (for remote control and configuration). The logic status is indicated by 3 LEDs and an alpha-numeric LCD panel.

The logic features full CTCSS encode and decode in addition to the standard toneburst access, includes alignment tests (test tone generation) to make setup easy, and generates different acknowledgement pips to indicate different conditions. Callsign beacons are transmitted every 5 minutes, with a longer beacon including locator every 25 minutes. There is also a special "Raynet" mode which turns the system into a simple carrier-operated repeater with no access requirements or timeout, which can be used for public event or emergency support.

For more information on the HOZ logic design, visit the Lake District Repeater Group web site or join the HozLogic Yahoo group where you can download the design documentation.

GB3SS Specification

Channel	RV48 (R0), RX 145.000MHz, TX 145.600MHz, 12.5kHz channel
Location	IO87MO (NJ 424 582), Tor Sliasg (south of Buckie), Moray, Scotland
Keeper	GM7LSI (MM5AHO, MM0HAR, MM0JMB secondary key holders)
Owner	Moray Firth Amateur Radio Society (MFARS)
CTCSS	67.0Hz (RSGB RMC tone “A”) decode and encode (250Hz deviation)
Access	CTCSS or 1750Hz toneburst (min 2 sec carrier to activate talkthrough)
Reaccess	Carrier (CTCSS optional) of at least 1 second
Timeout	4 minutes (no timeout when using CTCSS). Timer reset after end-of-over pip.
End of Over signal	Morse "E" pip if not using CTCSS. Morse "T" (for "tone") pip if using CTCSS. Morse "5" if near-timeout occurred. Morse "C" (for "clear") if timeout occurred.
ID Beacon	Callsign sent at 20WPM every 5 minutes. Long beacon (including locator and county) every 25 minutes. Normal frequency is 1500Hz at high deviation.
Logic	GD4HOZ Logic customized for GB3SS
RF System	Tait T830 series RX, TX and PA in rack chassis; 6-cavity duplexer; LDF4-50 feeder; 2 collinear folded dipoles 30m AGL (320m ASL).

Downloads and Links

Downloads

GB3SS User Guide (Acrobat PDF format, 142KB)