I have a HF transceiver transmitting at roughly 9MHz and 20dBm into a active monopole antenna. i would like to try and measure the radiated power from the antenna. Traditionally i would subsitute the DUT for a known source and measure the air gap losses. would this still work even though im measuring in the near field? how is ERP normally measured? i would also like to do the reverse and try and measure radiated receiver sensitivity.

 

This sounds straight forward but is really complicated in so many ways. ERP is measured on a range at some distance and includes the transmitter antenna gain- a low power transmitter with a lot of antenna gain could have an appreciable ERP. The antenna also has a specified pattern, high gain antennas usually have a lot of narrow lobes and discontinuities in the near field. ERP is generally not meaningful in the near field- the field isn't a plane wave in the near field. Real ERP measurements like these are generally done at far field- maybe 100 lambda- which would be something like 3 kilometers at 9 Mhz. You might consider something like the reverse beacon network that hams use to watch propagation. They receive signals and post them to the internet with signal strength. I probably need to know a bit more about your application. I worked on a tracking project in the HF spectrum to track birds worldwide with low power HF transmitters. Its a tough problem. I really don't understand your receiver sensitivity question but its a similar issue with antenna patterns getting convolved into the mix. Luckily most of this stuff is reciprocal and things work similarly whether you're transmitting or receiving at least from a Friis equation level- what I think you're calling "airgap".