Antenna building


#1

I have a setup that is now working well. The receiver dongle and small antenna are on the roof, the antenna on a 10 foot mast and the receiver just below in a waterproof box. There is a 10 meter powered USB cable that connects the roof setup to the raspberry pi.
Before the original pi succumbed to lightning I was getting some pretty good results, even a few hits at 250+ miles. That was the exception, most were within about 100 miles. I am greedy, however, and want better reception. I built a coaxial collinear antenna as described by several folks, and when I put it up (same setup with a very short cable run to the receiver) my number of aircraft tracked dropped nearly in half. The little tiny included antenna was much better!
I did check the antenna as I built it, there were no shorts.

I’m trying to figure out where things went awry. I used 75 ohm impedance coax, perhaps I needed 50 ohm? I bought some LMR-400 to use and then discovered it has a center conductor much too large for building this kind of antenna.

Any advice from someone who is seeing airplanes at great distance? I’m willing to go as tall as I can before the neighbors call the zoning board, and while not a radio person I am pretty good at building stuff. This has been a fun project.


#2

I am going through a similar process…
When starting i used a quarter wave groundplane antenna, outside the window but below the rooftop shielding the eastern part of the hemisphere. Eager to get more planes i built a 8 section coax collinear which gave a 3 times better result at the very same spot as the groundplane when testing it taped ad the side of a PVC tube.
However, when mounted within this PVC pipe and put on top of the roof performance dropped dramatically.
I suspected the grey PVC tube to contain carbon so i tried white PCV tube yesterday.
Again mounted it into the white tube real nice. Sticked it out of the window below the rooftop and saw fair results (roughly the same as the groundplane).
BUT… when i put it on the rooftop the result was… nil Absolutely nothing. After 10 minutes i saw just a single plane :imp:

Now recovering from this disappointment i might have found an explanation for this:
Below the rooftop my antenna is shielded from a 900MHz GSM tower at only 200m to the east. Together with the 20dB MMIC wideband preamp i guess this saturates the RTL receiver. Leaving out the preamp doesn’t solve this overload.
@SZCZ: So a high gain antenna might not always be beneficial especially with a cheap receiver.

I’m now looking for a bandpass filter design… (or a better receiver alltogether :wink: as this might be no job for a cheap RTL)
I will further investigate on monday by using the RTL on my PC with SDR# to see what’s going on.

/paul


#3

Now that is interesting. I have a cell tower about the same distance to my west. That’s great when I need to make a call, but I hate to think it’d be screwing up my airplane tracking.
When I had the antenna lower I saw more airplanes, not fewer. Hmmm…


#4

sounds like you need a resonant cavity filter … see f5ann.pagesperso-orange.fr/Cavit … index.html (it’s in French so run it through google translate)

don’t miss the link to the diagram at the bottom of the article.

The filter is made from 12-15mm / 1/2" copper pipe (plumbing supplies)

(I may make one of these - but make it tunable by soldering a 6mm machine screw to the end of the central rod and taping a thread though the end cap so it can be screwed up and down a little)


#5

Made some new discoveries yesterday.
I have been using an EzCAP RTL dongle which I had already available. These sticks are equiped with a Fitipower FC0013 tuner chip. I suspected that 1090 Mhz could be off limit for this stick but it looked to be doing well until…

Yesterday the FA proposed stick with RT820T arrived from banggood which immediately showed 10 times more planes. Sticking the collinear ouside the window multiplied this number by two, increasing height to look over the rooftop multiplied it again by two… No problems with strong neighbouring signals anymore which must have been caused by the poor RTL stick. I’m now tracking 100-120 planes constantly.
Adding the wideband pre-amp does not bring any big improvements (if any).

Next thing to try might be shielding the RTL as described here.

Meanwhile the bandpass filter is ready built but needs to be tuned. Experimenting will continue 8)
/paul


#6

To continue…

The new stick doesn’t like the preamp. Even with the collinear indoor it will get overloaded. Thus the bandpass filter was finished and tuned.
This combination does a great job and provided a noticeable improvement - even with the collinear antenna indoor. Will try to mount it outside over the weekend.


Find more details on the filter and other stuff at my blog: keptenkurk.wordpress.com/2014/1 … -b-filter/
/paul


#7

and to finalize…

I did put the antenna on the roof last weekend. This really does the job! The filter prevents the dongle from being overloaded as it did before (with the FC0013 dongle). This is what statistics look like now. For more pictures see this blog

/paul