Antenna Anomoly

[drawding]

I received a RTL-SDR RTL2832U 1PPM TCXO dongle for Christmas this year and have been having a blast with it!  After trying a number of iterations of software I settled on ModeSDeco2 and Virtual Radar.  Not content with the reception on the first floor of the house I scrounged up a machine with 8 GB RAM, 3 GHz Core2 Duo processor and wireless, put Ubuntu 16.4 LTS on it and placed it in the attic.  I took the mag mount with the large whip, extended it fully (approx 1.5m) and placed it on top of the air conditioner unit. 

This setup has been picking up aircraft up to about 100 miles away and working great.  Toady I decided to play around with the other whip that came with the dongle.  According to the product description, collapsed the second whip is 6 cm and should work well for ADS-B.  However when I swapped the antenna the performance was about 1/2 of the longer antenna.  The farthest I was picking up was only about 50 miles away.  Disappointed I decided to swap out the whip for a Diamond discone that I had out in the garage.  This discone had performed well for me in the past so I thought I'd see some improvement.  I connected the dongle directly to the discone and used a usb extension cable so that there would be as little loss as possible.  In theory this should have been the best performing setup.  Unfortunately it ended up only being marginally better than the short whip.  To make sure I was not crazy I hooked the long whip back up and right away started receiving back out to 80-100 miles. 

This makes absolutely no sense so I figured I'd reach out to the experts and see what you all think.  The only thing I can come up with is that I may have to play with the gain.  Maybe with a better performing antenna it's overloading the front end?  What is a good gain number to start with?

Any help is appreciated!

[Bethsalem]

Could be that the front-end of the Dongle is getting over-saturated by the strength of the signal reaching it.

You could try a 1090MHz filter to see if this improves your range. It certainly improved mine, as I have both FM and 3G/4G towers fairly close by.

[drawding]

But I would think that the longer antenna would make overload like that worse.  As it's fully extended it's supposed to be resonant on 50 MHz.  The shorter antenna is supposed to be resonant around 1100 MHz and should have been less prone to FM broadcast.

I ran kalibrate I couldn't find a cell signal on any band, but I do have a 50,000W FM radio station only 2-3 miles away.  After I made my original post I ended up ordering a FlightAware Pro Stick Plus and a second RTL-SDR RTL2832U!  When they come in next week I plan to do some realtime side by side comparisons.  The results should be interesting.

[Truetrack]

I cannot comment on adjusting sensivity on any RTL-SDR Dongles
As for antennas using a Discone is not that helpful. This type is intended for
mult freqs in a wide freq range of up to 1:10 e.g. 50 MHz to 500 Mhz, thus so widely used for scanners.
Makes no sense in having to again filter out the unwanted signals by a Bandpass Filter.
As for the ~ 1.5m whip glad you experienced better results. In principle one can get a long whip
in resonance at such UHF as long as it is pruned to total a certain number of 1/4 wavelength.
However again many possible unwanted signals come with it and is not best.
One can overcome this using a collinear-type, a number of phased sections cut for 1090 .
There are many DYI instructions in the net or commercial products available.

[drawding]

So I played around with the gain freq correction and agc settings and got about a 20% increase in reception distance.  Then I pulled out my old ARRL antenna handbook and calculated the length of a full wave vertical on 1090 MHz.  It came out to 0.262M which is significantly longer than the product description had mentioned.  This made another big difference netting me another 25% or so.  Now this makes sense!  I should have known better from the start and done my own calculations for antenna length, etc.

I'm now hearing out to over 100 nmi consistently.  Should be interesting when I get the FlightAware Pro Stick Plus next week and have some filtering in place.  Next will be to get the antennas outside!

[drawding]

Since my last post I've done some more experimentation.  I was able to tune the whip a little more and have been picking up planes around 125 miles away.  My FA Pro Stick + came in and not surprising it made a big difference again!  Picked up another 50 miles or so of reception.  Hearing planes around 195 miles away on the same antenna. 

I did a test with a splitter and fed both the PS+ and RTL-SDR.  It was no suprise that the FA picked up far more planes and at further distance.  But the RTL-SDR picked up some closer ones that the PS+ did not.  I wonder if it has to do with the 1 PPM TCXO in the RTL-SDR?  Next thing to try is to make some calculations as to how much drift there is in the PS+ and make some adjustments. 

This has been a lot of fun!