Folded dipoles and quad loops with SDRs

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Plaska Z.
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Joined: Tue Aug 06, 2019 5:54 pm

Folded dipoles and quad loops with SDRs

Post by Plaska Z. » Tue Aug 06, 2019 6:01 pm

Are folded dipoles and quad loop antennas OK to use with SDR dongles? I seem to remember reading somewhere that they are not recommended since these antennas form an electrically conductive loop and would essentially appear as a short circuit to the SDR. But looking for more detail about this today I can't find any such warnings. Any thoughts or experiences?

snn47
Posts: 153
Joined: Tue Dec 27, 2016 11:00 pm

Re: Folded dipoles and quad loops with SDRs

Post by snn47 » Wed Aug 07, 2019 4:48 pm

Every antenna has a more or less limited bandwidth within which they function well.
Discone and Log Periodic about one octve, other antennas bandwidth depend on the operating frequency. You can consult HAM Radio literature for easy information.

Kenn
Posts: 51
Joined: Fri Apr 08, 2016 8:43 pm

Re: Folded dipoles and quad loops with SDRs

Post by Kenn » Wed Aug 07, 2019 5:25 pm

Yes, folded dipoles and loops are OK to use with SDR dongles. And from the DC resistance perspective, you are correct that it looks like a short. So you want to make sure the dongle's bias-T is off so you don't let the smoke out. :) The bias tee allows you to power antenna side devices like low noise amplifiers by putting DC voltage on the coaxial cable. Do a quick search from the rtl home page on bias for the details ....

-Ken

Plaska Z.
Posts: 2
Joined: Tue Aug 06, 2019 5:54 pm

Re: Folded dipoles and quad loops with SDRs

Post by Plaska Z. » Fri Aug 09, 2019 1:14 pm

Kenn wrote:
Wed Aug 07, 2019 5:25 pm
Yes, folded dipoles and loops are OK to use with SDR dongles. And from the DC resistance perspective, you are correct that it looks like a short. So you want to make sure the dongle's bias-T is off so you don't let the smoke out. :)

-Ken
Thanks Kenn, The warning against quad antennas that I vaguely remembered hearing was probably in connection with using a bias T powered preamp.

snn47
Posts: 153
Joined: Tue Dec 27, 2016 11:00 pm

Re: Folded dipoles and quad loops with SDRs

Post by snn47 » Sat Aug 10, 2019 7:23 pm

Those antenna types do not necessarily create a short for the DC path, but It depends on how the antenna is matched to the transmission. But mostly they do, but you can also add a a DC isolation via e.g. a HP-filter.

Kenn
Posts: 51
Joined: Fri Apr 08, 2016 8:43 pm

Re: Folded dipoles and quad loops with SDRs

Post by Kenn » Tue Aug 13, 2019 12:38 am

That's right, best way to check is with an ohm-meter. If there's infinite resistance across the antenna inputs you're safe. If it's zero or very low ohms, just be sure you're bias tee is off and you're good.

I made a 60" long foded dipole with 300 ohm TV twinlead. It has a 4:1 balun in the center for matching to 75 ohms coax. Mounted horizontally it works very well for broadcast FM.

-Ken

rtlsdrblog
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Re: Folded dipoles and quad loops with SDRs

Post by rtlsdrblog » Tue Aug 13, 2019 10:33 am

Yes DC shorted antennas are safe to use with the RTL-SDR V3, just don't turn on the bias tee without an LNA or DC block in between. RTL-SDRs without any bias tee will also be fine.

You only need to be very careful if you have an RTL-SDR with an always on bias tee as you could damage that with a DC short antenna.

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