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I have a 50 Ω VHF airband antenna which I’m planning to install in my loft, for use with a RaspberryPi-based RTLSDR scanner/receiver.

I already have a UK-standard UHF/Freeview antenna using standard 75 Ω coaxial cabling running from the loft into the lounge and want to make use of this rather than running new cable.

I am not planning on transmitting on the VHF band, and the antenna will only be used for receiving/monitoring UK airband traffic.

Am I correct in thinking that:

  • Running two antennae for two diverse frequency bands across the same coax is OK, providing that both antennae are RX only

  • I will need a UHF/VHF diplexer on either end to suitably merge/split the signals from each antenna

  • The loss resulting from using a 50 Ω antenna and a 75 Ω antenna/cable is negligible enough to make the setup viable

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  • I will need a UHF/VHF diplexer on either end to suitably merge/split the signals from each antenna

Yes, this is correct.

A tangent: If you wanted to save some money by using mass-market parts, you could use 75 Ω power dividers (coax splitters) instead of diplexers. This has 3 dB loss because the signals are not directed exclusively to the intended receivers for each band, but evenly split between them.

  • Running two antennae for two diverse frequency bands across the same coax is OK, providing that both antennae are RX only

This type of configuration can work for transmitting as well, provided the diplexers are rated for the transmit power and the receivers can tolerate the leakage through the wrong port of the diplexer.

  • The loss resulting from using a 50 Ω antenna and a 75 Ω antenna/cable is negligible enough to make the setup viable

Yes, this is correct, unless you are seeking to receive extremely weak signals.

A 75-50 Ω mismatch has a reflection coefficient of 0.2. That is, each point of mismatch will reflect 20% of the signal, or $10\log_{10}(1 - 0.2) \approx 0.96\,\text{dB}$. So (discounting secondary reflections), you'll have a total loss of 2 dB. That will most likely be fine.

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  • $\begingroup$ When you say "power dividers", I think you mean "signal dividers" and not those things that allow you to put DC (i.e. 'power') on the coax as well as signal, separating the DC from the signal at the other end, usually used to power a masthead amplifier or similar ... $\endgroup$ – Scott Earle Jul 8 at 7:57
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    $\begingroup$ @ScottEarle "Power divider" is the correct term. The thing you are thinking of is a bias tee. $\endgroup$ – Kevin Reid AG6YO Jul 8 at 13:56
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To answer your questions in order:

  • As long as you’re not transmitting, you can run as many signals as you want into a piece of coax. In fact, your receiving antenna is passing a whole band of frequencies into it already
  • Ideally you would want a combiner at one end and a splitter at the other. These can be passive (a few coils and a capacitor or two), or active (with some electronics in, and power being supplied)
  • Certainly for receive, you won’t notice anything from using 75ohm instead of 50ohm cable
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For ultimate receive performance you would need a LNA at the antenna for UHF to avoid the losses of the coax and to improve the system noise figure. A LNA for VHF would not hurt (probably) and then you could use simple T-connectors to send the outputs through the same 70 ohm cable without loss of S/N. A receive only system is by far easier than a RX/TX system :-)

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