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Post by WM4MM » Sat Dec 29, 2018 6:05 pm

I use a QRP 10W transceiver and I want to monitor my transmitted signal. If I tie the antenna input directly to the radio (preferred)it will blow the front end. Anyone know what size antenuater would be needed to keep the SDR safe?*

I also could feed the dongle with a separate antenna. If I do this what size attenuater would I need?

I might add that the dongle will receive the HF frequencies. R820T2 RTL2832U 1PPM TCXO HF

* after this post I realized that direct coupling is impracticable due to the fact that attenuater might interfere with Xmit power
Last edited by WM4MM on Sat Dec 29, 2018 7:15 pm, edited 1 time in total.

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required power rating and attenuation of attenuaters

Post by snn47 » Sat Dec 29, 2018 7:11 pm

The answer will depend on a lot of if/then, depending on your receiver and how you want to couple the receiver to the transmitter output, e.g. direct connection via attenuators, via a directional coupler or over the air?

The required attenuation depends on the max. allowable power that the receiver can operate with e.g. -20 dBm. If the dongle has two diodes connected in reverse for protection of the chip, then I would say it should be the diodes that define the max. receiver power. However only if they are undamaged by static or to high power, otherwise the receiver is without their protection. However I cannot recall having read the max. allowable input power for the R820 chips under which the receiver can operate.

Then there is a the max. input power that a receiver can withstand while not being functional, without that the receiver is damaged/destroyed, which is higher e.g. +20 dBm for commercial receiver designed to operate with only few feet separation between L-Band antennas on an aircraft.

Another factor is the necessary power that the attenuator has to dissipate. Depending on the modulation you use, it is important to that you know if the 10 W transmitter power output are rated peak, RMS or average power.

For protection I would advise to apply the peak power of any modulation or modulation index. So if your transmitter has 10 W (40 dBm) peak, and you directly connect the 10 W (or 40 dBm) transmitter with the receiver via the attenuators. The first attenuator is connected directly to the transmitter output and has to be rated for 10 W power dissipation. If it is a 10 dB attenuator, this would reduce the power at the output to 1 W (30 dBm), which is the power rating needed for the next attenuator . If you use different attenuators you have to adjust the power rating with the attenuation of the input.

The required attenuation for 10 W (40 dBm) minus the max. allowable receiver input power gives you the required attenuation e.g. 60 dB if the max. allowable receiver input power would be -20 dBm.

If you use a directional coupler or the pick up element in a Bird meter, the power requirement of the attenuator will be lower by the factor of the coupling loss, and vary with the coupling loss of the coupler. The coupling loss will be more or less frequency dependent.

If you use a separate antenna to pick up the signal the input power will vary with frequency, the antenna used and path loss to your transmit antenna.

If you intend to built the attenuators yourself you can consult HAM literature e.g. the ARRL or RSGB handbooks.

That should give you an idea what to do.

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