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Here's a quick guide to using your Tiny SA or Ultra to measure real Watts for low power levels. This page describes my actual personal set up which I use for setting up QRP rigs. The set up is valid only for 0.5 to 7 watts. For a Tiny SA Ultra, this will give you a 0-5GHz power meter. Switch off the LNA in the Tiny SA Ultra (essential).
1 . You need 70db of attenuation for a safe level to reach the spectrum analyser for levels up to about 7 Watts maximum.. Many folk purchase a 10w handling 40db attenuator followed by a 30db 2 Watt handling attenuator. Repeat warning - Switch off the LNA in the Tiny SA Ultra (essential).
Place the 40db attenuator at the transmitter end, and then follow it by the 30db attenuator. From there you can connect a small length of coax to the INPUT of the Tiny SA (Ultra). Make sure it's the RF input and not the CALIBRATION Port or you will destroy the CALIBRATION port !
2. Enter into the Tiny SA / Ultra how much attenuation you are using, which is 70db. On the left hand side of the screen you will see the value confirmed.
3. Enter the frequency span of interest, example measuring the 2m band, enter 140-150Mhz, or 28.000 - 30.00Mhz for the 10m ham band.
4. Now key the transmitter and observe the value on the display peak (10 seconds for a steady reading).
5. The following values should be displayed according to power output.
6.5 Watts = 38.13dbm
6.0 Watts = 37.78dbm
5.5 Watts = 37.40dbm
5.0 Watts = 36.99dbm
4.5 Watts = 36.53dbm
4.0 Watts = 36.03dbm
3.5 Watts = 35.44dbm
3.0 Watts = 34.77dbm
2.5 Watts = 33.98dbm
2.0 Watts = 33.01dbm
1.5 Watts = 31.76dbm
1.0 Watts = 30.00dbm
0.5 Watts = 27.00dbm
Below is an example of my own test set up. In this case it's a Quansheng HT feeding 5 Watts to the test set up. Pictures below show the set up as individual pieces for clarity, the assembled set up and finally a Tiny SA Ultra screeen grab while the 5w rig is keyed.
For lower power levels you may need to DECREASE the RBW (Resolution Bandwidth) of the Tiny SA / Ultra so that you can increase the dynamic range and see the signal come through the noise floor. Halving the RBW increases the range by 3db, but it's slower to scan. I like to use 30Khz.
WARNING. Always ensure that all connections between cables and attenuators are secure and tight. Failure to do so will probably result in the ground / earth not being properly connected and thus the incorrect attenuation being applied (not enough !) and you end up with too much power going in to the spectrum anaylser. Most RF amps are coupled to the outsIde world via a capacitor or isolated transformer, but if you are playing with some homebrew stuff maybe then always use a DC block. And it never hurts to have always have a DC Block in line anyway.
FURTHER NOTES. If you are calibrating things like Quansheng HT's which use Hexadecimal values to set the power output from the SDR chip to the PA FET then you may some small non linear jumps in the levels. That's because RF semiconductors don't actually work in Hexadecimal (smile).
USEFUL LINKS
https://www.everythingrf.com/rf-calculators/watt-to-dbm
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