FlightAware Discussions

Shouldn't shorter cables work better?

Been running with a typical 8-legged spider antenna and about 14ft of RG6 from the antenna mount to the FlightAware Pro Stick Plus. So far, so good, getting 200+NM at times so that’s great.

Decided to make an enclosure for the FlightAware and Raspberry Pi ZeroW and move the receiver to just at the antenna mount. Doing so reduced distance to less than 100NM. Leaving everything in place, and just re-inserting the 14ft of cable and I’m back out to 200+NM again.

Shouldn’t signal reception/quality/etc be better with a shorter cable? Or is there some sort of minimum length needed on a spider antenna? Or will taking out 14ft of cable require significantly different gain settings for the same antenna?


Interference is probably overloading the LNA in the blue stick.
A filter might help.

Also sometimes SMA connectors are a bit out of spec and some pairs won’t mate together well.

Also also antennas are affected by things in the near field (i.e. close to the antenna, within a few wavelengths); directly connecting the antenna is likely to mean that the near field looks different.

What people also underestimate from time to time is the usage of an USB extension. I have two ones here at home, one works fine (the longer one), while the other ones decreases performance of the stick by 30%

i am using a good shielded Coax from stick to the antenna of approx 15ft and did not have a noticable change compared to a 3ft cable. Maybe it can be measured with equipment

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The length of the cable does matter but so do other factors.

Cable loss at a given frequency is measured in feet or meters. So, clearly a longer cable would have more loss. However, most, if not all, receivers use either automatic gain control or require a manual setting to put the input signal at the receiver’s “sweet spot”. In the case of the dongle it is obviously automatic so unless a cable is super long and the signal is lower than the AGC can compensate for, then the length of the cable isn’t the issue. Conversely it is possible to overload the input if the signal is too strong and, again, the AGC can’t attenuate it enough. In that case an external attenuator is required.

However, connector quality and issues like SWR can play a role. Poor quality connectors cause internal reflections. Never use homemade cable unless you have high-quality compression type connectors and the proper installation tools. Fewer connectors are better. (and this should go without saying but make sure you use 50 ohm cable! I just helped a friend who was struggling to receive a fraction of what he should have been and it turns out he built his own cables using 75 ohm CATV cable!!)

I’m going to be dealing with a similar issue shortly - will try to update on my findings.

For an FM or CW transmission, what you say is true, but for a transmission as short as an ADS-B message, too much of the message is lost before the AGC can react.

Many people use RG-6 without sacrificing performance.

I didn’t realize that. Seems to me to be a bad idea because impedance mismatch is a sure-fire way to have problems.

Thinking about it, it’s worse than that.

The ADS-B Preamble only lasts 8uS

Without “all” of the preamble, you don’t know where the message starts.
So to decode “all” of the Pramble, the AGC needs to have settled before the Preamble starts.
This might work if you had a (very) slow AGC and only one aircraft to track.

For a transmitting feed - absolutly. For receiving - not so much.
My “goto” example is Trimble - a reputable manufacturer of GPS, navigation and precision frequency references. On their original Thunderbolt GPSDO, the GPS engine is 50Ω, the antenna is 50Ω, but the antenna connectors are Type-F
Now RG-59 wouldn’t be my first choice, but the cable they specify is foil and copper braid, so is probably better than many RG-6’s

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Thank you, that’s a treasure-trove of knowledge :slight_smile: You are right for sure - my experience was indeed with CW type systems. Ironically, most of our problems with bad cables, impedance mismatch, etc were on the receive side.

The impedance mismatch causes a really tiny amount of loss but the consensus, from those who work with it, is that it’s nothing at all to worry about in ADS-B receiver land.

Is RG8 worse/better?

So, adding additional cable reduces the interference, or having additional cable pulls down everything being received, including the interference, so the LNA has a chance to clean things up?

I’m also guessing that with the way shorter cable (I’ve created a 3D printed enclosure that also acts as a cable holder for the spider antenna, so the cable length is now down to about 21") I’ll probably want to change gain from -10 and start trying for a value that cleans this up.

No, cable doesn’t filter, the opposite probably as it attenuates lower frequencies less compared to higher frequencies.

The LNA just overloads if the signal is stronger than a certain absolute power level.
Changing the gain does not change this.

As explained above, you likely won’t find a gain setting that works.
You need a filter or some attenuation.

If the antenna location has changed, that could also be an issue.
Actually the RPi or its power supply can interfere with reception as well.
It’s best to have plenty of vertical space between the antenna and any electronics.
Could also be that the new location has another source of interference.

Haven’t moved antenna (yet). Step one was to create a enclosure setup that I could relocate and I was checking that the new configuration performed approximately the same as the previous (which its not).

Apparently, location of the extra cable matters. Having it horizontal greatly improves things, vertical doesn’t at all.

Maybe you have a loose connector?

Yeah, trying those options as well and swapping out different SMA parts just in case.

When adding a filter to a Pro Stick is that going to interfere with the built-in filter on the Pro Stick? I’m assuming we are talking something like https://www.amazon.com/ADS-B-Dual-1090-Band-Pass-Filter/dp/B010GBQXK8

Additional filtering is never an issue unless you filter the frequency you are interested in.
It also weakens the wanted frequency a bit, but that’s not avoidable.

The filter you linked is actually pretty wideband, i wouldn’t recommend it i have to say.

This is actually the filter i would currently use: https://www.amazon.com/AirNav-RadarBox-1090-ADS-B-Filter/dp/B082WR1SDF

The filtered LNA is probably still a better solution, but it’s more complicated and more expensive:

(it’s not susceptible to overload at all compared to the LNA in the prostick+)

There is a long thread about this here on FA. Nobody has changed sides. I use RG6 from 160 to 6m. Are there losses? I’m sure there are. Are they significant? I don’t think so, others disagree.

Incidentally, I note that more and more dedicated 160m operators are using RG6. Yes, the frequency is much lower than ADS-B but we are talking impedance mismatch here.

Not in this straightforward sense. A longer cable adds losses, this reduces all signals, good and bad, but sometimes the SNR goes up, and that is even more important than just a high signal level.