Balanced antenna for direct sampling

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jevgienij
Posts: 8
Joined: Sat Dec 09, 2017 9:13 pm

Balanced antenna for direct sampling

Post by jevgienij » Sun Jul 22, 2018 1:15 am

So I've been doing a lot of research on HF antennas and I've come to this conclusion: Why would I use an impedance transformer at RTL2832U pins, then the 50ohm feeder line and then again some balun when I can build a multiband dipole fed through a ladder line, right? This way I have a balanced antenna connected directly to a receiver with balanced input. Like this:
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Theoretically, it couldn't be simpler. What do you guys think? If this is a good idea, what impedance of the ladder line should I choose? 300, 450, 600 ohms or another?

rtlsdrblog
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Joined: Mon Nov 19, 2012 11:54 pm

Re: Balanced antenna for direct sampling

Post by rtlsdrblog » Sun Jul 22, 2018 5:42 am

Would probably work okay. Remember that ladder line is much more difficult to work with though. If you get it near any metal, gets wet, etc it can really mess it up.

Also the input impedance of the RTL2832U is close to about 3,300 Ohms: https://www.rtl-sdr.com/measuring-the-i ... nput-pins/.

Finally, be careful with static discharge when doing this. The advantage of a transformer is that it helps protect the ESD sensitive ADC inputs from static.

jevgienij
Posts: 8
Joined: Sat Dec 09, 2017 9:13 pm

Re: Balanced antenna for direct sampling

Post by jevgienij » Sun Jul 22, 2018 12:04 pm

Thanks. What about using standard TV antenna baluns (75 <-> 300 ohm) for this purpose? I understand they are not usually used by HAMs due to low TX power they could take but can I use them for RTL SDR direct sampling when I'm using RX only? Any cons?

snn47
Posts: 253
Joined: Tue Dec 27, 2016 11:00 pm

Re: Balanced antenna for direct sampling

Post by snn47 » Sun Jul 22, 2018 7:38 pm

WIth a 1/2 wave dipole you only get resonance at multiples of the dipole. e.g. if it's 40m in diameter (1/2 wave for 80) m then its more or less in resonance 40m 20 m 10 m wavelength, but the antenna pattern will vary with frequency and height above ground level.

Since you said you don't want to transmitt, why worry about missmatch of the antenna or getting it in resonance. A more or less long thin cooper wire, e.g. unwound from an old discarded transformer is easyer to install. I used for years a thin cooper wire that I stuck through a potato and then threw into trees or on roofs. With such a potato wire antenna you cannot damage anything, except glass. With a homebuilt transverter and my 2m transceiver I was able to receive signals from 77.5 kHz (DCF-77) upwards to 10 m. With an antenna matcher it also worked as transmitt antenna. I would rather look for a preselector to reduce the bandwidth you sample.

jevgienij
Posts: 8
Joined: Sat Dec 09, 2017 9:13 pm

Re: Balanced antenna for direct sampling

Post by jevgienij » Sun Jul 29, 2018 9:42 am

I just thought why would I even need a ladder line when I can mount the dongle directly at the antenna? And I made this:
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Then I attached a dipole antenna to both ends which works as its expected. It may not be perfect, weatherproof and it requires a long active USB cable but I think this is as much as I can get from it, because every balun, every coax cable would only make some signal drop, wouldn't it?

Kenn
Posts: 54
Joined: Fri Apr 08, 2016 8:43 pm

Re: Balanced antenna for direct sampling

Post by Kenn » Mon Jul 30, 2018 7:07 am

Your build looks good, thumbs up! My only concerns would be heat and static. I'd go for a V3 to solve that.

I've used TV baluns and RG-6 for many a project, they work fine. The feed-point impedance of a 1/2w dipole is ~73 ohms, super close to RG-6's 75 ohms so you wouldn't even need a balun for that. Just a good electrical contact, and your solder skills be lookin' good. :)

If it's HF you're after, consider the losses per foot <30MHz for RG-6 are very low. It's when we're working at UHF and higher that mounting the receiver at the feed-point becomes advantageous.


- Kenn

https://ranous.wordpress.com

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