To near to the RF source?

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huskzi30
Posts: 1
Joined: Thu Oct 22, 2020 11:01 am

To near to the RF source?

Post by huskzi30 » Thu Oct 22, 2020 11:32 am

Hi,

This is my first post here.

First, I read this: viewtopic.php?f=1&t=5299

Then I realized, I came with the RF source a few times too near to the dongle (+touching the antenna :/).
Should I worry about? How can I test the dongle?

And what are the symptoms of a dying dongle?

I am grateful for every answer.

huskzi30

UncleVinny
Posts: 10
Joined: Fri Oct 23, 2020 5:30 pm

Re: To near to the RF source?

Post by UncleVinny » Mon Oct 26, 2020 1:37 pm

A dying dongle!!??

I KNOW there's a joke in there somewhere, but I won't touch it, ha, ha!

(Ya know, like there's always Viagra!)

Seriously, I wouldn't worry about it:
The voltage from an antenna is minute and shouldn't damage anything.
That is, unless it's unless it's a 200 watt ham transmitter or something like that.

rtlsdrblog
Site Admin
Posts: 2783
Joined: Mon Nov 19, 2012 11:54 pm

Re: To near to the RF source?

Post by rtlsdrblog » Tue Oct 27, 2020 3:44 am

What was the RF source, and what was it's power output? If it's just something like a HackRF, it should be fine.

snn47
Posts: 223
Joined: Tue Dec 27, 2016 11:00 pm

Varying voltage with antenna type and location on antenna

Post by snn47 » Tue Oct 27, 2020 3:01 pm

There is no simple answer to your questions, it depends on many factors and not how close you are to the antenna.

The voltage on an antenna changes with antenna type, it's voltage distributions and therefore the position. The voltage changes therefore with where you come close to or touch an antenna.
It's not just the power into an antenna but also the definition if it is an average/peak power input to the antenna feedpoint.

- The antenna impedance used for your RTL-SDR Rx.

- Position on antenna size will decrease in size/wavelength for an increase in frequency. For a half-wave dipole you can see the voltage distribution is highest at the two ends, see here https://en.wikipedia.org/wiki/Antenna_(radio). On shortwave, e.g. 80 m both ends areless than 40m separated, at 100 MHz less than 1,5 m and at 1000 MHz less than 15 cm

- In case of a capacitive tuned magnetic antenna, a <<1/4 Lamba length of tubing bent into a square or circle and tuned with a capacitor between the two ends, even a few Watt of power into the antenna can increase voltage across the plates of an air capacitor, that you will get spark discharges between the plates. The reason is that the Q which multiplies the voltage and decrases bandwidth in good designs can exceed Q of >100 will increase the voltage across the capacitor by the same factor.

- Is the power of the signal/waveform referenced to peak power or averaged power?
The modulation and waveform used of extreme waveforms can have a high PAPR (Peak To Average Power) ratio. For OFDM type waveforms this means the peak power is up to ~17 dB or factor 50 higher compared to average power output of the transmitter.

If you plan to come close to antennas which may damage your Rx, you can add a 1 dB to 3 dB resistor attenuator in front of the input. The resistors will limit voltage builtup and protect the Rx, at the cost of a minor 1 to 3 dB loss of sensitivity. While some Rx have ESD Diodes intended to discahrge excessive voltages and protect the receiver frontend, there is a limit to how much protection the can provde to short discharges.

In my oppinion a small attenuation is preferable to the possibility of damaging a receiver.
And what are the symptoms of a dying dongle?
If you remember some weak VHF radio station or VHF HAM beacon that you received before, if they are still in operation and you still receive it, then your Rx is ok. Similarily you can detect if your Rx is losing sensitivity, if you had the same sensitivity settings and antenna you used before.

qrp
Posts: 53
Joined: Wed May 22, 2019 11:16 pm

Re: To near to the RF source?

Post by qrp » Fri Nov 06, 2020 1:21 am

huskzi30 wrote:
Thu Oct 22, 2020 11:32 am
How can I test the dongle?
you can test it with HDSDR and signal generator, for example FY6800 or PSG9800

1) Start HDSDR
2) Select Bandwidth = 2400000 Hz
3) Open Options => Calibration settings
4) Select S-meter Calibration tab and press Reset button to reset S-meter calibration to default
5) close calibration widnow
6) Open ExtIO setting window
7) Uncheck Tuner AGC and RTL AGC checkboxes
8) Setup gain = 0 dB
9) close ExtIO window
10) Select AM modulation with 12 kHz bandwidth
11) click on S-meter to select RMS value (your'e needs to click on "Peak" label to change it)

Now the VHF band test:
1) Open ExtIO setting window
2) Select Direct Sampling = Disabled (or Auto Q if you're using my ExtIO plugin)
3) close ExtIO setting window
4) Setup generator amplitude to 0.020 Vpp (-42 dBm for 50 Ohm load)
5) Setup 30 MHz on the generator
6) Attach generator output to RTLSDRv3 input
7) Setup LO = 29.800000 MHz
8) Setup Tune = 30.000000 MHz
9) Check S-meter level, it should be about -24.2 dB

HF band test (direct sampling mode for RTLSDRv3)
1) Open ExtIO setting window
2) Select Direct Sampling = "Q branch" (or "Auto Q" if you're using my ExtIO plugin)
3) close ExtIO setting window
4) Setup generator amplitude to 0.020 Vpp (-42 dBm for 50 Ohm load)
5) Setup 7 MHz on the generator
6) Attach generator output to RTLSDRv3 input
7) Setup LO = 6.800000 MHz
8) Setup Tune = 7.000000 MHz
9) Check S-meter level, it should be about -8.2 dB

These levels I got on my RTLSDRv3 and PSG9800 generator.
It may be a little different, depends on your RTLSDRv3 version and generator. But not much.


if you got much smaller signal level, this is strong indicator that your RTLSDRv3 input is burned out.

PS: note, if you're using RF voltmeter to setup generator amplitude, you're needs to setup 7.07 mV RMS or 10 mV peak amplitude for open generator output. And generator output needs to be 50 Ohm impdedance. This is because RTLSDRv3 has a little different than 50 Ohm input impedance.

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