Dongle unusable using long wire antenna and 9:1 ballun.

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idioma
Posts: 4
Joined: Thu Feb 20, 2020 7:07 am

Dongle unusable using long wire antenna and 9:1 ballun.

Post by idioma » Thu Feb 20, 2020 7:38 am

Hi - I have a dongle purchased from rtl-sdr.com. I'm using a long wire antenna with a 9:1 ballun from NooElec. I have changed the sample mode to Direct sampling (Q Branch) and a sample rate of 2.048 MSPS.

With these settings, virtually *every* frequency on *every* band is a cacophony of local AM stations stomping on each other. It's not even possible to listen to an AM station clearly. No signal comes in with any other sampling mode. The long wire/ballun combination renders the dongle useless.

I do have a nanovna, which shows a very sharp saw tooth pattern across the entire spectrum. The antenna side of the ballun is about 30 feet of wire up about 12 feet in the air, and the ground side is grounded. It's not clear to me what I'm doing wrong or whether I need a trap or filter or something to get this to work. I don't really know what I'm doing...just enough to be dangerous. Any advice would be appreciated!

Every band looks like the image below...
sdr.png
sdr.png (265.41 KiB) Viewed 2985 times
Thanks

VE3EAR
Posts: 25
Joined: Sat Oct 05, 2019 10:13 pm

Re: Dongle unusable using long wire antenna and 9:1 ballun.

Post by VE3EAR » Thu Feb 20, 2020 2:44 pm

How long is this long wire? You may be overwhelming the front end of the dongle. Another option to try is to get a filter that attenuates the AM broadcast band, while passing any signals above.

73, Bruce, VE3EAR

hotpaw2
Posts: 126
Joined: Sat Jan 14, 2017 11:07 pm
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Re: Dongle unusable using long wire antenna and 9:1 ballun.

Post by hotpaw2 » Thu Feb 20, 2020 4:34 pm

Looks like intermodulation from multiple very strong stations nearby. You can try an AM bandstop filter. But your location (due to building wiring, power lines, other nearby electrical equipment, etc.) might be generating too much EMI/RF noise for any HF antenna there to be usable.

idioma
Posts: 4
Joined: Thu Feb 20, 2020 7:07 am

Re: Dongle unusable using long wire antenna and 9:1 ballun.

Post by idioma » Thu Feb 20, 2020 6:32 pm

Hey thanks for the help!!! Reading the replies, I suspect both responses may be correct.

1) I built a crystal radio way back. It would only pick up one station at first on all frequencies. I ended up with 3 wave traps and 2 antenna tuners before I could pick up anything else - but then it would pick up Chicago at night! The thing looks like something out of a 50s ScI Fi movie now with all the knobs :) but it works very well here at the same location.

2) I forgot to mention that the receiver is inside a metal RV and the antenna is hanging off the top of the RV on one side - so maybe that huge chunk of metal is interfering? I'm on 5 acres - the closest power line is further than it would be in a normal neighborhood and the closest radio station is at least several miles away - so hopefully that is the problem.

There are so many filters and what-not that I wasn't sure what to try next, but I will move the antenna and get an AM filter...and I will update the post when I have further results in case it helps someone else.

Thanks again!!!!! - Jon

rtlsdrblog
Site Admin
Posts: 2559
Joined: Mon Nov 19, 2012 11:54 pm

Re: Dongle unusable using long wire antenna and 9:1 ballun.

Post by rtlsdrblog » Fri Feb 21, 2020 1:13 pm

Definitely overload from broadcast AM. You'll probably want a good high pass filter to get rid of that.

idioma
Posts: 4
Joined: Thu Feb 20, 2020 7:07 am

Re: Dongle unusable using long wire antenna and 9:1 ballun.

Post by idioma » Tue Feb 25, 2020 5:16 am

Will do - thanks!

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