So, what is a repeater?
A repeater receives the signals (messages) sent from someone, and sends them again. That increases the reach of the original sender.
So, why a duplexer?
Strictly speaking, you don't need a duplexer for repeating signals, but it's damn near impossible to work without one.
First of all, if your repeater's receiver part listens on the same frequencies that its transmitter is working on, you'll get into a lot of trouble – because they typically share an antenna (or closely adjacent antennas), the transmit power could easily fry a receiver. Even if it doesn't, you'd have to tell your own transmitted signals from the ones you want to receive apart. That's technically very expensive (you'd need a huge dynamic range or a circulator. I won't go into either.).
Also, the original sender might already be reaching stations with their own signal – and those stations would suddenly see the original transmission and your repeater's copy of that. That's a very strange multipath channel, in other words, highly undesirable.
So, you make sure that you take the signal, and transmit it on another frequency, not to interfere with
- the original transmission and
- your own receiver.
So, to keep your receiver protected from your very high power PA output, you add a diplexer, which really is just two filters, which separate the "incoming" from the "outgoing" channel.
The suppression of that diplexer filter usually is sufficient, so you don't need more filters. If you do: add them, SAW filters for the 70 cm band aren't expensive and can be bought in SMD shape.
What equipment / design procedure
- Antenna: now, 70cm isn't really huge. If you have a roof or something of the like, it's thoroughly possible you can get multiple antennas running; even having things like a directive RX (e.g. a Yagi-Uda or a biquad) antenna and an omnidirectional or only slightly directive TX might be an option. This depends on your motivation to have that repeater! Cheapest, and simplest, will be to have a single antenna that does both RX and TX. We'll focus on that.
- Diplexer: to split the RX from the TX, which are already separate in frequency domain, into different signal paths. This is the only hardware in your repeater that you have to buy/build exactly for the frequencies you'll need to work with!
Decide upon which frequencies you want to listen, and on which your repeater will transmit. Ideally, you want all the receiving frequencies to be in a continuous frequency range, and the transmitting ones in a separate one.
I think it's convention in the 70cm to have the matching frequency-transposed outgoing channel 5 MHz from the receive channel, but I'd be very happy if someone could verify/comment on that.
The frequencies around 440 MHz are relatively benign: They are low enough that you don't have to be afraid that even well-designed circuits simply act like capacitors and let through the signal, and they are high enough that frequency-selective components are small and cheap.
The "gold standard" for such frequencies are SAW filters (surface acoustic wave, basically, you excite a piezo crystal to mechanically oscillate, and by its geometrical properties, only certain wavelengths can travel). They are very steep, have low insertion losses, can be made very accurately and don't suffer very much from temperature/air moisture/moon phase & black cats-induced drifting. Also, they are produced in the billions for commercial devices, so they are cheap as hay.
I've taken the liberty too look up a few potential filters on Mouser.
You'll need one for the signal going into your transmit amplifier, and one for the signal going into your receiver, on different frequencies. The transmitter's one isn't as important - it's just to stop crosstalk from your receiver getting into your transmit amplifier.
- Splitter/Coupler: The antenna needs to be connected to both the output of your TX amplifier, and the input of the filter that goes into your receiver. A circulator would be perfect, but meh, they don't sell these for prices that I'd be willing to pay :) so, something like a 3dB splitter must do.
- Transmitter: TBD. Really, needs to output the power you want. The signal going into the transmitter comes from the receiver, and that's where the interesting action happens.
- Receiver: There's two ways of forwarding messages. These are amplify and forward (where your receive signal just gets... amplified and given to a mixer that puts it onto the transmit frequency; in that case, your transmitter+receiver is just the RX filter, an LNA, a 5 MHz oscillator, a mixer, the TX filter, and a power amplifier) and decode and forward.
The second means that your receiver takes the signal it gets and interprets (or tries to) it as a message. That improves SNR. For example, if you're expecting FM voice transmissions, it would frequency demodulate it, squelch away noise, clean up the resulting audio, and then re-modulate it as FM. With digital modes, this becomes even easier to motivate: Assume your repeater "sees" a packet that, due to noise at the receiver, has a single bit error. Now, it decodes that package, and that includes forward error correction, so that the packet has 0 bit errors after FEC, and re-modulates and transmits it again.
Personally, I'm a software defined radio guy. Which means there's no question for me – the perfect repeater would observe all the incoming channels at once by digitizing the incoming RF at a sufficient rate, detects active transmitters, automatically classifies the transmissions (e.g. APRS, FM voice, SSB AM voice, D-Star, low-bandwidth DVB-T), applies the correct decoder for the individual transmission, regenerates the original signal digitally, and combines the regenerated transmissions digitally, before giving it to the digital-to-analog converter. In this scenario, the receiving hardware needs a good filter, LNA, a relatively boring SDR receiver (and an RTL dongle might do, for a start), and a PC running appropriately designed software. The transmitter is that software + TX SDR hardware, e.g. the HackRF (add a good filter between HackRF and the PA, the HackRF's spectral cleanliness is... insufficient for repeater usage), or a USRP (which can receive while it transmits, so you could use it simultaneously as receive SDR and transmit SDR), or or or.