Rotorua linear repeater  built 1988

                           Up mixer Transmitter                                                          Receiver

This photo was taken about march 1988, not long after it was all put together and tested.There is a Receiver unit and a separate up mixer transmitter unit . They   connected  together via a piece of coax carrying the 10.7 MHz intermediate frequency. This unit pictured here is the mark 3 , the original mark 1 was built in a Hudson tinplate biscuit box by Bob Sutton ZL4DO in Dunedin, New Zealand, when we were both living in the same town. I forget the actual transmit receive split but we used CB crystals and it came out to 3.8 MHz or something like that. I remember it was fairly low powered too. I will try and dig out the original article in Break IN magazine NOV 1982  (now in website)

As usual. I didn't keep good notes on the construction of the device back in '88,   but I can remember most of it very well,and  sufficient of the pertinent   information and pitfalls for anyone who would like to build a similar unit.

Receiver.

The front end device was a  low noise device,a  J310 running in grounded gate configuration and biased to about 10 mA standing current. It had a single 2M tuned circuit , link coupled to the 50 ohm input from the antenna. The output was to a single stage 2 M tuned circuit that fed a 50 ohm RF port of a mini circuits SBL-1 dBm. The L.O. port was fed from the single crystal controlled buffered +10 dBm 137 MHz oscillator  via a 3 dB 50 Ohm pad. The IF output was fed through 10.7 MHz L/C  duplexer that is designed to present 50 ohms at all frequencies to the SBL-1. the output of the duplexer was connected to another low gain +9dB j310 in grounded gate (Norton configured ) before it feeds the 30 Khz crystal Filter via a 10.7 MHz if transformer .

The post mixer IF amplifier is to protect the SBL-1 dBm 50 ohm mixer from the high reflected reactance's / impedance's  the crystal filter produces to frequencies outside its  10.7 MHz pass-band. Possibly lowering its intercept point and strong out of band signal handling properties. Remember there is not too much front end preselection in the form of 2M tuned circuits (to keep the overall Rx noise figure low). The output of the crystal filter is capacitively top coupled to the if amp 10.7 matching tuned circuit. The IF chain was built using PLESSEY 1600 series IC's The first if amp was a low noise SL1610 . the second and third was using SL1615's. They were coupled together with 10.7 interstage transformers , The PLESSEY RF bible says they can be  capacitively coupled together to provide stage gain blocks BUT they produce an awful lot of  lot of broadband noise , too much for my liking ,I felt this would upset the AGC action and mean a lot of unwanted RF output on the wanted frequency after upconversion.The 10.7 interstage transformers help limit the broadband nature of the IF to a few 100's of Khz bandwidth. the last IF amp tuned circuit feeds the PLESSEY SL1623 am detector AGC controller IC . I did not put AGC on the Front end RF amp as this can increase the noise figure of the RX . It is important to remember that after the "wide band" 10.7 MHz IF amp the noise extends at quite high levels, 100's of kilohertz above and below the center frequency . If this output is not refiltered to a minimum required bandwidth i.e. that of the receive front end crystal filter , then this broadband noise will be upmixed to the new output frequency(144.350)and extend 100 of KHz's either side of the carrier.  The quality of the pre-upmixer IF amp can be mediocre with a poor shape factor ,but its bandwidth ideally should match that of the receiver.

The upmixer was a balanced transistor affair  , driven from a simple single transistor overtone crystal oscillator with a buffer stage. the configuration such that the balance is to minimize up mixer local oscillator level and reduce it as much as possible before feeding the up-mixed IF  into a top coupled triple tuned circuit to select for the wanted 144.350 mixer product.  From there it fed a J310 grounded gate RF oscillator that was coupled into a bipolar 2N4427 biased into a linear mode these two transistors provide a total gain of 20 dB the output of this stage (100mW) drives a typical Japanese linear 2 m power block  (100 mW in for around 10W out)

Note: to prevent the retransmitted signals, sideband inversion, the up mixer local oscillator must be on the lowerside of the output frequency minus the intermediate frequency. i.e. usb in gives usb out!!

Mute Operation

Our linear repeater site is remote and has only wind and solar power. So it was necessary to devise a suitable mute arrangement that would power down the system, such that only the bare minimum of receiver operated to conserve precious power.  However on receipt of a signal, the mute circuit will switch all the wanted upmixer low level amplifier and power block circuitry, back out of class C to linear class A . The type of mute was arrived at after a little experimentation, to use a typical FM noise mute   (CA3075) , it had quite a fast attack for ssb, could be easily arranged to hang for a number of seconds after loss of a signal. and also by providing an FM demodulated audio output so DTMF tones could be used for control purposes.

The system is very sensitive and a 0.2uV (-122 dBm) 144.950 MHz signal is readily heard from the transmitter at 144.350 MHz output. This means that you must have very good transmitter +40 dBm to receiver , that is a signal difference of 162db !!   a lot of the required isolation will be provided by the receiver being 600kHz away from the transmitter. However there will be broadband noise produced by the upmixer and linear amplifiers, this can amount to many dBms at the input frequency ,this could easily mask a moderately weak signal on the input frequency. You need to notch out, by many tens of dBs , the transmitted broadband noise at the receive frequency  i.e. the broadband noise caused by the transmitter at 144.950 MHz, and place a rejection notch in the receive   signal path to remove any signals at 144.350 MHz to stop the transmitter causing intermod and overloading in the receiver.  To operate a 10w output repeater effectively, you require about 110 dB of isolation between the transmitter and receiver.This isolation is achieved by utilising High Q ,low loss tuned circuits, usually in the form of multiple coaxial cavity filters (see section on coaxial cavity filters elsewhere in the website) a minimum of two in the transmit coax and two in the receive coax are required , and the rest of the isolation provided by utilising separate receive and transmit antennas. By using three cavity filters in each feed line and a circulator for added measure you can run the entire repeater off one lone antenna!!

A circulator is a passive frequency selective  ferro magnetic device with three ports(connectors) that has the ability to split and divide  the RF signals depending on the direction the signals are going, and direct  them to different ports or connectors. you can get up to 30 dB isolation between two ports on a circulator and only a few tenths of a dB loss in the process   they are not cheapto buy new  but sometimes surplus ones seen at junk sales as not many people know what they are.

The Rotorua Linear repeater uses two 10 element yagis as a DX repeater beaming up the North Island from Rotorua towards Hamilton / Auckland direction (regions of higher population density) the antennas are vertically polarised and stacked about eight feet apart one above the other ,this gave a measured isolation of about 40 dB .  Thus we can easily achieve our 110dB isolation without difficulty . Watch the quality of the feedline coax used preferably double screened but keep the TX and RX coax well separated to minimise unwanted coupling,  you can loose precious TX RX isolation this way.

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