Internal/External aerial - Not as different as you'd expect.


#1

External
Raspberry Pi3
FlightAware Pro Stick Plus.
15m EcoFlex 15 Coax.
Moonraker Radar-110 aerial at 38ft AGL.

Internal
Raspberry Pi3
RTL-SDR dongle
Uputronics combined preamp and ceramic filter.
Homebrew spider aerial, not tuned, just built and stuck in the loft. No coax.

Here you can see the positions of the aerials - The red circle is roughly where the internal aerial is located in the loft. As you can see, it’s right by some internal brickwork and is therefore directly shielded slightly west of south.

I’m feeding them both into VRS and here’s a range plot from the two sources. Red is the internal aerial, grey is the external.

Despite the fact that the external aerial has (allegedly) much higher gain and with a clearer view all around, the maximum range of the internal aerial isn’t too different in some directions although the internal aerial is well down in directions it’s directly blocked. The Moonraker collinear is in the clear with nothing really in the way to the horizon.

I spot almost exactly the same number of aircraft/day between the two systems although the reports received on the external aerial is about 10% higher than that on the internal.

I’ve tried this with the Pro Stick Plus in the loft and the RTL-SDR/Uputronics pre-amp on the external aerial with no real noticeable difference.

Next step is to get a Pi and receiver mounted at the base of the collinear, I’m not sure which one to stick up there, whether to go with the Pro Stick Plus or the RTL-SDR/Uputronics combo. The former will probably be easier to do but my ultimate goal is simple - Get rid of the coax as much as possible.

However, this just shows that a perfectly good system can be set up with a simple internal aerial.


#2

For proper comparison, try both antennas WITHOUT amplifier (internal to dvb-t or external to dvb-t), and at same location with same length of coax for both, and then check the difference.


#3

I’m not sure what you’re trying to say there. I’m deliberately comparing two very different setups against each other to demonstrate that one doesn’t need to put a large pole up outside to get decent results.


#4

Oh, I misunderstood that you are comparing only antennas.

Since you are comparing the systems as a whole, your approach and conclusions are right.


#5

They look pretty different to me. Antenna height is likely they main difference in your situation.
In many directions you get 2 rings extra range. to the southeast(Paris) it looks similar.
An amp can makeup for lack of antenna gain, especially in electrically quiet locations.

You can also try tuning the gain. I had to turn down the pro-sitck on one setup to 36db.

Don’t go by aircraft counts, try position reports.
I have tourist helos near me that take up a lot of bandwidth. I get 1 million position reports per day(Most days).
I can sometimes track an aircraft for over 300NM yet it only counts as one aircraft. The same goes for the helos.

I have a few Moonraker directional antennas. I am too close to sea level for them to help my range. I may put up the other two and use them for UAT978. I had intended to use 3 for inputs into a mode S Beast but only a few were designed to use 4 inputs and the code was somewhat limited.


#6

I’m deliberately comparing two very different setups against each other

But given the surprising internal results, what would that be like using the external antenna?

Just wondering what is contributing to those internal data so we can learn from them. Is it antenna, amplifier, filter, dongle, lack of coax? You’ve hit on something so now what is the key component?

Edit: because I want the same :laughing:


#7

I’ve done gain optimization but I think that’s a bit hit and miss really. The external one is running at 48 and the internal one at 44.5 and I adjusted them for position counts rather than messages or aircraft.

I think the big thing that makes the internal one work so well is the fact that there’s no coax involved. The amplifier and receiver are quite literally at the aerial. If I can do something similar outside then that should give really good results - There will be a short coax tail from the base of the collinear but that will only be 3ft or so of LDF-250 so the loss is negligible. I can be fairly sure I’ll need to reduce the gain a tad when that happens.


#8

I remember a guy from early last year did something similar.
ads-b-flight-tracking-f21/pre-amps-vs-low-loss-t36637.html
He powered his RPI via POE and put the setup in a plastic box on his tower.
He gets pretty good range too flightaware.com/adsb/stats/user/BellCountyTX


#9

In 2013, I have done some comparison of systems with & without Amplifier, and with 50 ft coax and with 12 ft coax. You can see these experiments and their results here:

https://forum.planefinder.net/threads/ads-b-diy-antenna.23/

.


#10

abcd

You generate so many posts, it’s hard to keep up with you :wink: . Particularly if I wasn’t bothered about amplifiers, etc, at the time you posted. I guess we need an index :laughing:

I’ll take a look at that post - thanks.


#11

I agree that there should be a bigger difference.

Something in your outside setup is not optimal.

Is the Moonraker considered a known quaility brand? Have you done a SWR check on the antenna? It claims 6.5dBd, thats 8.6 dBi. It’s a VERY complicated process to make that kind of gain and get it right.

Or, the antenna is too good, pulling in interference for example from GSM900. A good cavity filter inline would be interesting to see…


#12

We need hashtags!

#ADS-B_DIY_Antenna :mrgreen:


#13
 I did a similar thing last week and it was a big 50% reduction fail.  I put everything back the way it was and now I'm testing components.  I only had two new components, an Amazon 9.5 ft USB extension and a adapter.  I think my issue was the SMA to N adapter between the Flightstick and FA antenna but I so far cant see any issues.  It looks like it has a 50 ohm pin but I think I might measure it.  I'm currently using the USB cable between RPi and flightstick with no issues.

But to get back on topic, moving a non amplified cantenna from inside to outside at 4ft AGL made a big difference for me. Amplified flightstick made a big difference with no other changes. Raising the antenna to 35ft AGL made a smaller difference.


#14

RPA ?


#15

SMA. SMA male to N male.

Not sure what I was thinking when I typed that. I went back and corrected it.


#16

I would try one change at a time. It could be that the USB cable is the problem.
I used POE not a long USB cable. A lot of people have had issues with USB cables.
You also may need to turn down the gain to get better results.


#17

OK, and you have checked that you didn’t get an RP-SMA instead?

/M


#18

The USB cable is working fine, I’m using now just to test and I’m having one of my best days ever. It’s not a really long amplified cable, it’s a 9.5 foot cable.

And I am positive it is not a RP-SMA. :mrgreen:

The adapter appears to be a 50ohm Type N. It came from Cables to Go on Amazon.