The generic(standard) DVB-T’s frond end has 75 ohm impedance.
In RTL SDR blog V3 they have modified front end matching circuit of DVB-T, and it seems that in addition to improving it’s quality, they have also changed its impedance from 75 ohms to 50 ohm.
Improved front end circuit The standard matching circuit on the RTL SDR was designed for DVB-T use, and tends to attenuate signals above ~1 GHz. The new matching circuit has less attenuation above 1 GHz and similar performance below. We have used high quality, high SRF, high Q inductors in this circuit.
When ADS-B started, the two Wireless systems (50 ohm Voice and 75 ohms Video) were already in place and both well established. ADS-B is none of the two, and could have adopted any one system, as both are equally good for ADS-B.
It so happened that the pioneers who made first ADS-B Receivers originated from Voice Communication branch of Wireless, and they simply used what expertise they had for ADS-B receivers. Others followed the suite. As a result all ADS-B receivers and equipment today are 50 Ohms.
Had these pioneers originated from Video branch of Wireless communications, today we would be having all ADS-B equipment 75 ohms using stuff used by TV, Satellite and Dish.
The history of 50 ohm VS 75 ohm impedance (not resistance!) is more complicated, and they appeared basically in the same time, following experiments done in 1929.
Radio SW (voice) was using 75 ohm frequently, so it’s not just a “TV” thing…
For reception, therefore low power levels, and 1GHz, 75 ohm is as good as 50 ohm. At higher frequencies, best-loss impedance for common core insulation materials is at value between 52–64 Ω. Example: the lowest loss cables at 2.4GHz are made in 50 ohm variant, but they are expensive (LMR series).
With some 75 ohm connectors and cables made primarily to be cheaper, quality issues might make them a catastrophe. So, don’t go cheap. RG 59 and RG 6 are the 75 ohm standards, with the difference being that RG59 has a thinner core wire (20 AWG versus 18 AWG for RG 6) and less shielding (no foil), more losses, at 1GHz frequencies. Therefore RG 6 should used exclusively for this frequency.
For power transmission (emitters), the 50 ohm is a better choice (the only one actually for over 100W) due to better power handling (higher voltage). RG58 is the standard for 50 ohms. RG174 is very thin 50 ohm cable (good for GPS antennas).
Regarding the type N male connector needed to connect to the FA antenna, what impedance (center pin diameter) do I need?
I don’t know what the center pin inner diameter is so buy one and solder/crimp it on. Should be fine either way. The bigger problem may be in the diameter of the coax wire for the strain relief. Try to find an SMA connector if you are using the FA dongle otherwise use “F” if using other MFGr Dongle.
Knowing the ideal installation should have minimal connections, what should I do on the ProStick end? With semi-rigid nature of the RG11, I’m thinking about using a type N on it and then a 6”-9” N-to-SMA whip to the dongle. Thoughts?
No whips on either end. Try to use a good USB cable for additional flexibility at the Pi end.
What about a SMA connector directly on the RG11? I couldn’t find any connectors on eBay that specifically mentioned RG11. Any ideas?
Order solder on SMA and just install them. Forget about “the book”. It’s a hobby!!
Some folks are advocating adding a LNA to the setup. All of the ones that I’ve seen use type F connectors. I would then have to adapt from N to F at the antenna, then add a bias-t and power injector, and then F-to-SMA. The LNA seems like a lot of work. Worth it?
You will not need any amplification until you put up your system and test it and before you add amplification you need filtering. FA has an amplifier built in on the Plus model. I believe the amplification belongs after the filtering not before. No sense amplifying noise. I can see filtering and amplification on the tower but in that order. At any rate you need to see the base results before adding “stuff” to the line.
I dont understand why there is so strong and unjustified opposition to anything other than 50 ohm / N / SMA
I have been using 75 ohm RG6 with Flightaware antenna and Flightaware Pro Stick for last 3 years without any problem or reduction in performance. All I needed was an N-male to F-female connector at Antenna end and a F-female to SMA-male pigtail at ProStick end.
You don’t have a baseline to see how much better the setup would be if everything was kept at the same impedance
Changes in impedance reflect back some of the signal…
Yes, I do have, by calculation, which shows that the effect of 50 to 75 ohms mismatch is negligible.
The impedance mismatch between 50 ohm & 75 ohm results in:
reflection coefficient 0.2
VSWR 1.5
mismatch loss 0.177 dB
power reflected 4%, power reaching the load 96%.
Please see calculations below
When 50 ohm mixed with 75 ohm: Reflection Coefficient Γ = (75-50)/(75+50) = 0.2 VSWR = (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
My practical experience showed that poor quality 50 Ohms coax was much more detrimental to my hamradio station’s performance than to my switching to 75 Ohms RG6 coax.
The calculations show that good quality 50 Ohms coax is still not worth the price differential, and termination hassle, at least at the max power level I’ll ever use, < 100W pep.
If you reduce Gain of Dongle by just 3 dB, you cut signal level to 50%, so what is the importance of just 4% for each 75 to 50 transition? Two transitions, one at Antenna to coax, 2nd at coax to dongle, total 8%.
I would not argue about cable impedance but might like to suggest we keep the insertion loss at each connection to a minimum. No matter what you do if you use “F” connectors you need to change to SMA. I really have no problem using 50 OHM connectors on RG6. What do the calcs say about having correct connections without the pigtail (what is the ohms for the conversion cable)?
Getting the most signal is about hundreds of small things like antenna placement, RF interference, shielding of the cable, common mode on outside of cable, good quality connectors installed properly, insertion loss, length of the cable, superior receivers, antenna design,etc., etc. Some of them you can control and some you can not, And some don’t make enough difference to worry about too much. In the end your only calculation is adding up all the things you can control and didn’t. So the compromises are everywhere not just on RG6. Just some you can’t control. Cable is one you can.
Rejecting a cable because it has substantially higher attenuation (dB/feet), or poor shielding, or poor in withstanding weather, is understandable, but rejecting it just because it is 75 ohms instead of 50 ohms, or uses F instead of N or SMA is unreasonable.
In theory, theory and practice are the same.
In practice, they aren't.
For one thing it assumes all connections are perfectly terminated.
Another is you are assuming all connectors/adapters are created equal. A $1 “gold colour” adapter from china will not perform the same as a $50 ‘equivalent’ from Greenpar or Huber & Suhner.
Indeed.
Good quality components win every time. Knowing how much of a penalty you’ll pay for cutting corners, that’s where experience kicks in.
If you buy a $3 TV dinner from your suppermarket, don’t expect it to have a Michelin star - same goes for coax.
Connecting a SMA connector made for 50 ohm cable, with a center pin 0.9 mm to a 75 ohm cable, that has a center pin 1.024 mm is always perfect… Or close enough?
I don’t recall anybody saying, or even suggesting, that. In fact, myself and others, always emphasize the dimension differences between the two types. If the center pin is physically smaller than the receptacle, proper contact will not be made, regardless of impedance.
We are talking strictly impedance, and the effects that a 50 to 75 Ohms mismatch causes. An impedance mismatch still requires proper contact.
Not necessarily. The existance of radio waves (electro-magnetic waves) was proved mathematically by Maxwell in 1862, about 25 years before their physical existance was proved by experiments by Hertz in 1887.
This applies not only to F-connectors, it applies to N and SMA connectors as well, even if the entire system is 50 ohms, and no transition from 50 to 75.
