RF Calibration using very low insertion loss 20+ dB isolation directional couplers fed with an active noise source of known noise power and spectral characteristics.

From Robert Meade, SARA mailing list 24/12/2025:

Rather than using Dicke switching of the RF path between the antenna and a resistor as a noise source with PIN diodes, physical relays, or solid state relays, an alternative is to consider very low insertion loss 20+ dB isolation directional couplers fed with an active noise source of known noise power and spectral characteristics. It seems that a noise diode input to one of these should be more of a 0.2 dB noise figure impact vice 1 to 2 dB for putting any of these types of switches in front of the first LNA stage. These would also likely integrate easily into the connectorized loop feed + SAWBird+H1.

If cost is the driver, used directional couplers can be found cheap on the used market, and a fancy lab noise diode source need not be used, but a cheap noise generator board should suffice. Even without lab test equipment, MIT SRT’s over the air noise diode calibration method suggests absorber foam can be used to characterize the noise source and then it can be used without any motorized absorber foam system to do reference switching.

20 dB directional coupler covering 1.4 GHz:

https://www.ebay.com/itm/234565272708

Noise Generator covering 1.4 GHz (Ignore the seller’s false “tracking generator” marketing presumably to appease the amazon compliance folks):

Robert