Can I use the SDR very close to the RF source?

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Bastian Soto
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Joined: Tue Feb 25, 2020 8:10 pm

Can I use the SDR very close to the RF source?

Post by Bastian Soto » Tue Feb 25, 2020 8:19 pm

Hello, my query is if I can use my RTL SDR V3 near the RF source (less than one meter), next to an outdoor antenna, leaving on VHF 157.625 MHZ @ 40W (I would be listening to that particular frequency) ... This would destroy my SDR? Or shouldn't I worry?

And what would be the ideal distance in these cases?

Thanks friends. CD6BDS Radio Amateur from Chile. :geek:

rtlsdrblog
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Re: Can I use the SDR very close to the RF source?

Post by rtlsdrblog » Wed Feb 26, 2020 5:05 am

As with any RX SDR radio, it's very important to never TX right next to the RX antenna as it will destroy the receiver. I would advise getting a switch to ground the RX while transmitting, or getting a ~60dB attenuator.

You could try and calculate the path loss.

Convert to dbM: https://www.rapidtables.com/convert/pow ... o_dBm.html
Path Loss: https://www.everythingrf.com/rf-calcula ... calculator

max input power of an R820T2 is +10dBm, and to be safe you'll want to stay closer to 0dBm if you're transmitting for longer periods. So you need roughly 46dB loss in between antennas, so roughly 60m apart should do it.

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

Re: Can I use the SDR very close to the RF source?

Post by snn47 » Wed Feb 26, 2020 5:26 am

Why do you want to place the SDR antenna so close to the transmitter antenna ? Even a 50 ohm termination on your SDR should be enough to receive the transmitted signal at this distance and make it safe for your SDR.

With antennas at VHF beeing about 1 m appart you cannot calculate/estimate the isolation loss between those antennas, because you are not fare enough appart to apply far field propagation calculation. If you have a RF-voltmeter you could measure power received and estimate this way the isolation between both antennas.
One problem though, you would need to know the power levels that a SDR can withstand at the antenna input without beeing damaged, and the value that a SDR can withstand an still operate to answer your question.

vy 73
_.. .._. .... .__. _.__

Username
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Re: Can I use the SDR very close to the RF source?

Post by Username » Wed Feb 26, 2020 7:29 pm

My SDR Antenna is next to VHF, UHF, Ghz Antenna and have no problem.

rtlsdrblog
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Re: Can I use the SDR very close to the RF source?

Post by rtlsdrblog » Thu Feb 27, 2020 5:41 am

Username wrote:
Wed Feb 26, 2020 7:29 pm
My SDR Antenna is next to VHF, UHF, Ghz Antenna and have no problem.
How close, and what power do you TX at?

Username
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Joined: Sun Oct 09, 2016 7:27 am

Re: Can I use the SDR very close to the RF source?

Post by Username » Thu Feb 27, 2020 6:58 am

2m and 10W Peak with my CB Radio.

snn47
Posts: 213
Joined: Tue Dec 27, 2016 11:00 pm

Re: Can I use the SDR very close to the RF source?

Post by snn47 » Sat Feb 29, 2020 4:54 am

@rtlsdr
max input power of an R820T2 is +10dBm


Does this apply to the RTL-820T2 input without or with the antiparalell protecting diodes that I remember having seen.

Do you know the max. PAPR (Peak To Average Ratio) for the +10 dBm value, which is also important to know, since the PAPR for OFDM can be >10 dB depending on how software will limit the PAPR?

rtlsdrblog
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Re: Can I use the SDR very close to the RF source?

Post by rtlsdrblog » Mon Mar 02, 2020 1:52 am

i don't know the PAPR sorry, I only know the +10dBm specified in the datasheet.

The BAV99 will begin clipping and protecting from ~7-12 dBm, but it's not going to help if the input is too strong.

snn47
Posts: 213
Joined: Tue Dec 27, 2016 11:00 pm

PAPR and RMS of modulation

Post by snn47 » Mon Mar 02, 2020 9:54 am

Thank you, that is what I assumed, the Diodes are clipping to protect.

PS:it can be two causes that destroy the receiver,
either a to high voltage generated by the modulation power that will destroy the diodes junktion this is why PAPR is important
and/or
a to high continious RF RMS power power of the transmitter and it's modulation that will heats the solid state device until it is burnt to crispy toast. For FM you have a constan power output, for AM the PAPR varies by about 3 dB at max. modulation index unless the transmitter is clipping and can exceed the 3 dB. For other modulations the PAPR can be much higher and short peaks of power can damage solid state devices in a very short time, e.g. Radar pulse.

Bastian Soto
Posts: 3
Joined: Tue Feb 25, 2020 8:10 pm

Re: Can I use the SDR very close to the RF source?

Post by Bastian Soto » Mon Mar 02, 2020 2:11 pm

Thank you friends, it is clear to me now the matter... did anyone ask why I want to put it so close to the transmitting antenna? Well, to take advantage of an antenna tower... the thing is that the communications center transmits there, but I also need to listen to the mobile radios that run through the city...

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