SDR antenna - 50 or 75Ohm cable? Answered
Once you get going with your new SDR dongle you soon realise that it is little fun with the supplied antenna - if it came with one...
Local stations, police (unencrypted) and so on is all fine with the short antennas.
But if you want to get weaker stations or even track some satellites you are as lost as trying to get anything in the 160m band on your screen.
Obvious way out is to get an external antenna.
Depending on your needs and layout at home you might be lucky to find a ready to go option at some cost.
In most cases though you will need more than the 3-5m that come with the usual antennas for car use.
And not all base station antennas come with enough cable - keep in mind that hieght above ground and no obstructions close by count...
With RG58 cable already being quite expensive a lot people consider the cheap 75Ohm TV cable as an alternative...
Can we use 75Ohm antenna cable for our SDR needs?
Of course we can!
We only need to make sure that at the antenna end AND the receiver end the cable acts and appears like a 50Ohm cable.
Especially if you need a really long cable in range of over 20m the TV cable shines with less loss that can't be ignored.
Let me try to explain it the simple way first:
You TV needs 75Ohm but the good old dipole antenna provides 300Ohm.
The so called BALUN that connects the actual antenna with cable matches the antenna to the required 75Ohm impedance for the cable and TV.
Like the TV, we don't transmit anything, so we don't have to care about what happens if power would go to the antenna - it is not a radio in the common sense, we just listen ;)
There are many options available to match any impedance to any other but in our case a lenght of coax pairs is all that is needed.
1/12 of the wavelength for the 75 and 50Ohm cable and we have a match.
You are right this simple option does not really work if you want to listen to a lot of different frequencies.
What works fine for the TV also works fine on both ends!
I assume you use an antenna for the common 50Ohm system and a standard 50Ohm SDR module or receiver.
Two simple transformers will do the trick just fine and allow us to use cheap 75Ohm TV cable.
The one on the antenna end provides 50Ohm for the antenna and 75Ohm for the cable.
For the receiver the transformer is the same type but connected in reverse.
You can look up how to make one with a simple ferrite torioid core and wire or coax but also order them from your favourite Chinese seller in the usual online markets.
Add an antenna switch and you can use multiple antennas optimised for the frequency in question with dirt cheap TV cable.
Just make sure to put the balun in a metal enclosure that is connected to the braid of the coax on both ends.