FlightAware Discussions

What is the input impedance of the receiver stick?

Hi

This is a question that seem very basic, but in fact it is not.
What is the input impedance of the FlightAware Pro Stick and Pro Stick Plus ?

SMA connectors are 50 ohms so it should be 50 ohms. However, the R820T2 chip has an input impedance of 75 ohms according to RTL-SDR. This is because it was meant for TV receivers which use 75 ohms antenna systems.

However it is possible there is an impedance matching in the stick.

And unfortunately the specifications do not list the input impedance so it is not possible to know… so I have to ask ! I think only FlightAware can answer as they are the only ones to know what hardware they did put (or not put) inside.

Why does it matter ? Because I am trying to impedance match my antenna and I need to know what impedance the receiver is expecting…

Thanks

@uski
Here is your answer :slight_smile:

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When receiving radio signals, do not worry about mixing 50 ohm with 75 ohm. Please see the explanation below.

For transmitting, the impedance matching between antenna and transmitter is important due to power involved, which is in few watts to hundreds of watts.

Our case is receiving. In this case the power received by antenna and transferred to radio is in milli watts or even micro watts, and any reflected power can very easily be compensated by the front-end amplifier.

The mismatch loss when 50 Ohm antenna+cable feeds a 75 ohm radio will be very minimal at around 0.177 dB.

How I arrived at the figure of 0.177 dB?
When 50 ohm mixed with 75 ohm,
Reflection Coefficient Γ = (75-50)/(75+50) = 0.2
SWR = (1+Γ)/(1-Γ) = (1+0.2)/(1-0.2) = 1.2 / 0.8 = 1.5

Mismatch Loss in dB = -10 log (1 - Γ²) = -10 log (1 - 0.2²) = -10 log 0.96 = 0.177 dB

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If you want to fine tune that way, also consider the cable.
From a practical perspective i’ve seen lots of mixed environments, especially with 50/75 Ohm cables. All were running without issues

I too run a mix of 50/75 Ohm cables and connectors except for the RTL-SDR V3 and Radarbox 24 dongles that have SMA connectors - they run 50 Ohm cables and connectors all the way.
My opinion is that the difference is minor, but when you say " running without issues", how have you quantified this?
As all of your receivers are indoor or have very short feeders, your systems may not be representative of a more general installations.

I am not talking about my feeders only

Um … ok. So what are you talking about?
(please don’t say you are just repeating what you have heard - the isn’t facebook!)

At least one of my colleagues is using a 75 Ohm Antenna cable with 15 Meters, connected through SMA adapters. For the other two i don’t know their cable length

Sure, lots of us are doing that.
I have never tried to quantify what performance penalty (issues) I pay.
You stated there were no issues, so I hoped you’d done some testing at least.

I am sorry if my comment left a wrong expectation. No i did not perform tests by myself.
I could ask my colleague. He was struggling a bit with the cabling at the beginning. He’s using the device outdoor, connected via LoraWAN and PoE, Antenna is somewhere else.

He does have a rough environment…

  • By actual use:
    I have checked by replacing following mix by “all 50Ω cales & adaptors”. No noticeable change in my stats/graphs.

    FA Antenna >> N-male to F-female connector >> F-male connector on both ends of 75Ω RG6 >> F to SMA pigtail at dongle end.

  • By theory:
    A loss of 0.177 dB per 50Ω/75Ω transition. With two transitions (one at antenna, other at dongle) the total loss will be 2x0.177 dB = 0.354 dB, which is very easily compensated by th 19 dB LNA chip of Pro Stick.

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Would be interesting to see what a sweep with the VNA looks like with the mix vs without the mix.

The very reason I replaced mix system by all 50Ω was that the sma termination of “F to SMA pigtail” became faulty (intermittent connection at sma termination). Since the total length of RG6 cable was about 2 meters, instead of purchasing the 30 cm pigtail, I purchased 2 meters long RG174 cable with SMA-male to SMA-male and an adaptor N-male to SMA-female for antenna end.

As I have thrown the intermittent F-female to SMA-male pigtail in trash, I cannot test the 50Ω/75Ω mix cabling. I can however do a VNA sweep on “all 50Ω cabling”, if you are interested.

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No need. I do always do an end-to-end system sweep before I put anything where it’s a pain to get at later (like the roof). I was just curious about the 50Ω/75Ω mix if you had it handy. I actually have a spool of RG-6 somewhere but I’m not that motivated to actually look for it or do a test. :slight_smile:

I am using a “spool” of RG6 myself - 150’. After a LNA filter with some 27dB gain. There are plenty of signals decoded.

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In the end, that’s all that matters.

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Out of interest I put my ProStick Plus on the VNA to see what it measured.
Sweep is between 840MHz and 1340MHz. Drive level -15dBm
ProStick Plus was powered and running.
Looks to be about 20 ohms in reality :astonished:
See attached:

@LawrenceHill

Nice plot.
Which VNA model you have?
Can you plesse give link to the shop from where you purchased it? Thank you.

It’s a nanovna v2.
I actually got it from RTL-SDR but I don’t think they have them in stock at the moment.
https://www.rtl-sdr.com/tag/nanovna-v2/

Thank you LawrenceHill

 

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