You can just ignore problem of mixing 50 ohm with 75 ohm
What’s much more important for ADS-B reception than exact impedance matching is the coax’s attenuation loss at 1090MHz. 75 ohm RG6’s loss is a lot less (3.3dB for 10 meters/30 feet of coax) than 50 ohm RG58’s (5.4dB: never use RG58 at high frequencies for any coax run longer than a couple of feet or so. It’s too lossy at high frequencies.)
BTW not all RG6 is created equal. Cheap hardware store or dollar store RG6 designed for cable television is just that: cheap, and should be avoided. Better-quality and better-shielded RG6 meant for satellite TV antennas and dishes is fine for ADS-B.
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@jaymot
Fully agree 
My receiver is capable to show both, but the difference in graphs is zero.
Not sure where Mode-AC is still in use, but here over Germany i haven’t seen any Mode-A/C so far.
I would assume the receiver is not counting it twice, even if Mode-A/C is available
Yes, this is true - it’s the transition that causes loss.
eg antenna 75, cable 75, input 50 will have the same loss as antenna 75, cable 50, input 50
more mismatches will increase the losses as will poorly assembled connectors, poor quality connectors / adapters / cable.
A single mismatch is unlikely to be noticeable.
@abcd567 , Nice math!
Means that in my case, with 2 mismatches, I will have around 0.35dB, right?
Well @jaymot , I saw this yesterday, when I try to create 2 “exactly” designed antennas and the cable just mess everything up! I will have to find some new length of RG6 just to make sure which one is the culprit: connectors, RG6, crimp, etc. Long story short: what should be a perfect match mmana antenna becomes a dummy load!
@geckoVN , reception is almost impossible, no matter what I try, unless the plane is facing the antenna and within 30 miles. Otherwise, just noise
@foxhunter , I asked because here in Brazil, in São Paulo, the biggest city of the country, my peak are around 200m/s, without mode AC, just S. I would love to see 2250 messages per second one day!
For 2250 per day you need perfect conditions and lots of traffic and a specific device
Last year i had around 160 aircraft peak which ended up in 1800 m/sec
I doubt that you really ave that improvement with Mode-AC enabled.
look at these two setups. it’s from @keithma and only one delivers > 2000
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Factor in the third one (at the MTG site) and two out of three isn’t bad 
http://essexradar.co.uk:747/graphs1090/
/edit - All using 50R aerials and 50R coax… Just sayin’ 
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This is my Jetvision-Device getting the graphs via readsb on the Raspberry.
Also not getting > 2000 due to lower traffic. Mode-S only
Changing it to the combined ModeS/AC network port doesn’t make any difference
Thanks, @foxhunter and @keithma
Mine is not that impressive, and my measures are per minute, so, 10k messages translates to ~165m/s
Not sure but i think the graphs1090 of @wiedehopf is also measuring per minute or at least build an average
Meanwhile I’m only getting ~500 messages/second at around 100 aircraft. Have an airspy mini coming in a few days, excited to see if I can get any more with that.
Just discovered a new fact: cable resonance…looks like something is very wrong and I will had to cut the cable on multiples of wavelength*speed factor.
The antennas were perfect, but the cable, by the length,was just pushing the swr to 7 and above, killing the signal.
Now I added some extensions to test, and even with a bad position and a lot of walls, it gave me a good response and a good distance (216km)
You are right. Please see these posts, calculations & diagrams:
https://discussions.flightaware.com/t/new-antenna-10x-increase-not-even-outside-yet/17108/70
https://discussions.flightaware.com/t/built-my-first-antenna-and-doubled-my-coverage/15770/723
Thanks @abcd567. I will have to review all my cables. Looks like I will have to have a nanoVNA to do it right. At least, fast.
/edit. I guess I can see it with a nanoVNA, right?
https://discussions.flightaware.com/t/measuring-antenna-performance/19651/10
AN EXPERIMENT TO DEMONSTRATE HOW THE MEASURED VALUES ARE AFFECTED BY LENGTH OF COAX BETWEEN ANTENNA ANALYZER AND DEVICE UNDER TEST.
MEASUREMENT OF 75 Ω DUMMY LOAD THROUGH DIFFERENT LENGTHS OF RG6 COAX
TEST SETUP
COAX PIECE #1
Physical length =130 mm (including connectors), Electrical length = 130/0.84 = 155 mm = 0.564 λ
Measured R= 44.92 ohm
COAX PIECE #2
Physical length =150 mm (including connectors), Electrical length = 150/0.84 = 179 mm = 0.651 λ
Measured R= 34.01 ohm
COAX PIECE #3
Physical length =230 mm (including connectors), Electrical length = 230/0.84 = 274 mm = 1.0 λ
Measured R= 77.26 ohm
COAX PIECE #4
Physical length =260 mm (including connectors), Electrical length = 260/0.84 = 391 mm = 1.42 λ
Measured R= 32.46 ohm
Thanks @abcd567
Now I just have to figure out the rg6 I have to see if it conforms with the specs, as of 83% velocity factor and do some calculations to get it right with the antenna.
In fact, because of the small wavelength, 1 single milimeter matters.
No need to calculate 1/2 wavelength using VF.
The coax will always have n numbers of 1/2 wavelength sections + one section less than 1/2 wavelength (n is an integer). Please see sketch below.
The trick is to trim coax from receiver end in small steps till SWR becomes low. This happenes when the last section of coax of unknown length has been cut to zero or near zero length. You now have only n number of 1/2 wavelength sections.
Measure SWR, first at “as is” length. If it is high, cut say 10mm. Redo F connector of rg6, and measure SWR. Repeat cutting in 10mm steps till you get low SWR.
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