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Folded dipoles and quad loops with SDRs

Posted: Tue Aug 06, 2019 6:01 pm
by Plaska Z.
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?

Re: Folded dipoles and quad loops with SDRs

Posted: Wed Aug 07, 2019 4:48 pm
by snn47
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.

Re: Folded dipoles and quad loops with SDRs

Posted: Wed Aug 07, 2019 5:25 pm
by Kenn
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

Re: Folded dipoles and quad loops with SDRs

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

Re: Folded dipoles and quad loops with SDRs

Posted: Sat Aug 10, 2019 7:23 pm
by snn47
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.

Re: Folded dipoles and quad loops with SDRs

Posted: Tue Aug 13, 2019 12:38 am
by Kenn
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

Re: Folded dipoles and quad loops with SDRs

Posted: Tue Aug 13, 2019 10:33 am
by rtlsdrblog
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.