Impedance mismatch

[Steve714]

Another basic newcomer question. It seems that the detector end of amateur radio telescope equipment eg LNA, LNB, filters etc are almost exclusively 75 ohm input impedance while receivers / analysers are almost exclusively 50 ohm impedance. How is this mismatch managed in the rf circuit avoiding relatively high insertion loss and more importantly reflections resulting in degradation of the source signal?

[John101]

Steve,

Welcome aboard.

I suggest google, 50 Ohm 75 Ohm mismatch. The return is outstandig in turms of why and how to take advantage of the situation.

72, 73

John

N3AAZ

[niallk]

One approach is to use an impedance matching circuit to transform between 50 and 75 Ohm domains.  Low loss transformation is easier to achieve over lower bandwidths.

A very useful reference for matching is 'RF Circuit Design Handbook' by Chris Bowick:

https://www.pdf-archive.com/2013/06/22/rf-circuit-design-second-edition/

 

[Steve714]

Thanks for your replies. I was hoping to find an off-the-shelf solution rather than getting involved in Smith charts and circuit building again. To be more specific about the circuit problem, I have a standard commercial LNB, an in-line amplifier, a power inserter and dc block which are all 75 ohm F-type and a Funcube pro which is 50 ohm SMA. The impedance mismatch at present is 1.5:1 which will result in 20% of the power being reflected at the junction between the two impedances. An impedance matching transformer would reduce this to around 10%. I guess the only other option would be to buy a 75 ohm SDR but so far I've been unable to source one.

[The Admiral]

I'm not being facetious, but does it really matter? Are you going to see the effects? As far as I'm aware, for receiving it's a lot less of a problem than with transmitting. I have a Funcube connected to my nominal 50 ohm impedance antenna using 50 ohm cables and connectors, but given that my antenna is in the loft I suspect that its impedance is quite complex. It doesn't worry me, but then, ignorance is bliss .

I'll get my coat!

Ian

[wxsatuser]

On 09/11/2016 at 12:44, Steve714 said:

Thanks for your replies. I was hoping to find an off-the-shelf solution rather than getting involved in Smith charts and circuit building again. To be more specific about the circuit problem, I have a standard commercial LNB, an in-line amplifier, a power inserter and dc block which are all 75 ohm F-type and a Funcube pro which is 50 ohm SMA. The impedance mismatch at present is 1.5:1 which will result in 20% of the power being reflected at the junction between the two impedances. An impedance matching transformer would reduce this to around 10%. I guess the only other option would be to buy a 75 ohm SDR but so far I've been unable to source one.

Only 4% of the power and 20% of the voltage will be reflected at 1.5:1

I would guess the in line amp is used to negate any losses.

When I had my NOAA HRPT setup, 1700Mhz, we always needed an inline amp even though we had a very low loss LNB
and very sensitive receiver. Unless your using very long lengths of feeder, of questionable loss, I would'nt worry about it.
Use the very best quality low loss feeder and keep the run as short as possible.