Measuring VSWR

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Victorbktop
Posts: 1
Joined: Mon Mar 09, 2020 8:59 am

Measuring VSWR

Post by Victorbktop » Mon Mar 09, 2020 9:17 am

Hi, glad to meet you guys.

I bought some rtl-sdr dongle and antenna. and I have some questions.

Does rtl-sdr dongle match the antenna's impedance to 50ohm? or does it just choose the proper frequency by using filter??
I wonder because I think the antenna I bought can't cover all of the HF and VHF frequency.

And if the dongle tune the impedance to 50ohm, I wonder how to measure the VSWR of the antenna before tuning and after tuning.

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

VSWR is important for transmitter

Post by snn47 » Mon Mar 09, 2020 6:18 pm

Victorbktop wrote:
Mon Mar 09, 2020 9:17 am
I wonder because I think the antenna I bought can't cover all of the HF and VHF frequency.
It depends on your definition of cover. There is no antenna I know of that will support such a large range of frequency/wavelength and is matched for all frequencies. There are active antennaes that have small whips, small in relation to wavelength, which can cover this range, but if they perform well on all frequencies, for all polarisations is another question .
Victorbktop wrote:
Mon Mar 09, 2020 9:17 am
And if the dongle tune the impedance to 50ohm, I wonder how to measure the VSWR of the antenna before tuning and after tuning.
VSWR is the ratio of forward power provided by a transmitter and the power that is reflected back by the antenna or components in between due to missmatch of components ordefects (like defective cable or switches), to the transmitter minus the cable loss in order to avoid damage to the power amplifier. Since you don't transmitt VSWR is not easy to measure without test equipment.
If you are interested in the impedance you can built or loan a Antenna Noise Bridge and connect your antenna and Rx to it and you will get a direct readout of the impedance after you adjust the bridge.

radiolistener
Posts: 76
Joined: Thu Jan 25, 2018 10:48 pm

Re: Measuring VSWR

Post by radiolistener » Fri Mar 13, 2020 4:58 am

Victorbktop wrote:
Mon Mar 09, 2020 9:17 am
Does rtl-sdr dongle match the antenna's impedance to 50ohm? or does it just choose the proper frequency by using filter??
I wonder because I think the antenna I bought can't cover all of the HF and VHF frequency.
No. It has some curve which depends on selected mode.

For example, here is VSWR measurement for RTLSDRv3 input taken in usual mode, LO is set to 145.000 MHz:
RTLSDRv3-vswr-145.png
RTLSDRv3-vswr-145.png (65.1 KiB) Viewed 6227 times


And here is VSWR measurement for RTLSDRv3 input taken in Direct Sampling Mode, LO is set to 14.000 MHz:
RTLSDRv3-vswr-14.png
RTLSDRv3-vswr-14.png (93.54 KiB) Viewed 6227 times


As you can see, it has input impedance far away from 50 Ohm, about 65-130 Ohms, depends on the frequency. But it is pretty acceptable for receiver.

radiolistener
Posts: 76
Joined: Thu Jan 25, 2018 10:48 pm

Re: Measuring VSWR

Post by radiolistener » Fri Mar 13, 2020 5:04 am

Victorbktop wrote:
Mon Mar 09, 2020 9:17 am
I wonder how to measure the VSWR of the antenna before tuning and after tuning.
You can buy NanoVNA analyzer and check your antenna impedance at specified frequency.

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