Poor coax or ground plane needed?

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arjscott
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Joined: Sat Feb 06, 2016 12:08 pm

Poor coax or ground plane needed?

Post by arjscott » Sat Feb 06, 2016 12:40 pm

I have the SCO-1090-MCX antenna from http://www.jetvision.de with eztune dongle for receiving ADS-B. 55cm antenna with 3m RG-179 to MCX. Works well enough - around 140Nm reception.

On trying SDR-Radio for the first time, around 1090MHz it did look like a lot of interference was present. On touching the coax with my hand, the waterfall graph cleared up a lot.

I read a comment stating a remedy for this could be adding a ground plane to the antenna. However, some vendors selling this type of ADS-B antenna state no ground plane is needed.

I would like to clear up the apparent interference. Any suggestions how?

Also, am I right thinking this 55cm antenna is designed as 2* wavelength? Fractional wavelength (quarter wave) antenna information is easy to find, multiples of wavelength - not so easy.

Thanks

rtlsdrblog
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Re: Poor coax or ground plane needed?

Post by rtlsdrblog » Sun Feb 07, 2016 1:08 am

I'm not sure what that antenna is like inside, but i'm guessing it's a collinear type antenna. If it's collinear no ground plane should be needed.

Usually if you touch the cable and the noise floor changes, then it means there is some sort of impedance mismatch going on though. Maybe try creating an ugly balun - make a few turns of coax near the antenna side.

Where have you mounted the antenna? If not mounted outside the impedance could change and cause the noise you're seeing.

arjscott
Posts: 4
Joined: Sat Feb 06, 2016 12:08 pm

Re: Poor coax or ground plane needed?

Post by arjscott » Sun Feb 07, 2016 9:40 am

Thanks for the tip!

With two turns of coax looped through a ferrite core the noise is gone. Not sure if this would be an ugly balun, but removing it from the coax confirms its making a difference.

The antenna is about 25ft up outside - luckily I can get the dongle inside to a windowsill with that short cable.

I can now see some weaker signals, probably not decoded by dump1090. I have an LNA + filter due from HAB supplies. That might increase the received data a little.

Probably right it is coliniear. Can't receive much else except for FM broadcast and a little aircraft band.

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