23 Cm Repeaters in the UK
By Andy G8VLL

In the UK the split for this band is 6 MHz, and the preferred polarisation is horizontal. Channels are RM0 = 1297.000 O/P, with 1291.000 I/P etc.

The TX’s run all the time with FSK id… as a beacon, when accessed FSK is inhibited and CTCSS enabled, id then becomes AFSK at less than 15 minute intervals.

Implementation of this on GB3NO.

The TX consists of a G4DDK  source modified to 1297 MHz, with fsk/afsk inputs added from memory this runs about 8 milliwatts out, it is amplified discreetly to about 1 watt and then to 5 watts, BLV93 transistor. A bandpass filter is fitted at the o/p and an isolator between the driver and PA. The resulting output is extremely clean with very low sideband noise. I do not recommend the transverter approach for TX.

The RX is a 7 pole interdigital converter, (as ARRL handbook but 2 extra fingers and LO multiplier removed and I seem to remember that the LO input would not resonate at 1146 so longer tuning screws were used and hobby brass tubing instead of solid fingers). LO is G4DDK board again. A 2-stage preamp using MAR6 etc as per KK7B transverter precedes the converter and a narrow 7 pole IDF is fitted before the preamp; sensitivity is about .2uV 12dB sinad. The 145 MHz output is passed via attenuators to a Phillips (PYE) R4002, a good quality commercial RX.

Crystal heaters are fitted to the driver and LO.

The control logic is a GB3US MK1 board modified to control the keying changes and CTCSS keying.

The antenna is a dual Alford slot constructed from a single piece of 1.5 inch OD 16 gauge tube with an ABS cover (drain pipe) There is 55dB separation between these 2 antenna’s.

Construction info. See RSGB Microwave books and ARRL Microwave projects manual

A single 450 MHz cavity used in ¾ wave mode is all that is required on the TX line to remove any desense.

I have also experimented with vertical polarisation using a modified cell antenna.

The repeater is located on a tower block in Norwich (JO02PP) antenna height approx 180 ft agl, site 100 ft asl, A UHF  repeater, GB3NR 433.000/434.600 and a 2320.890 MHz beacon  GB3ANT, are co located.

See attached pix of; experimental vertically polarised antenna, new 23cms pa, uhf cavity, antenna on site, repeater.
(NOT Available just yet)
 
 
 
 
 
 
 
 

Copyright April 22, 2001 G8VLL
HTML Copyright April 22, 2001 Kevin Custer  W3KKC
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