This article documents a feasibility study of using the modified nanoVNA-H to measure the gain of a 4 element 144MHz Yagi, the DUT.
The intended configuration is the DUT will be connected to the tx port (Port 1 or CH0 in nanoVNA speak), and a known ‘sense’ antenna connected to its rx port (Port 2 or CH1 in nanoVNA speak).
nanoVNA |s21| noise floor
To make useful measurements of the received signal, the rx signal level must be a reasonable amount higher than the noise floor, 10dB should be sufficient.
Above is a plot of the |s21| noise floor around 146MHz.
We want the measured |s21| to exceed -55dB.
Sense antenna
It is proposed to use a small untuned loop with balun as the sense antenna.
Above is the small loop used for field strength measurement. It is 2mm hard drawn round copper wire formed into a circle 185mm in circumference, and a common mode choke is used to connect the loop to the rx. The common mode choke is 0.6m of RG58C/U with 0.5m of ferrite sleeves over it and its loss is accounted for in the “Other Loss” item.
The nanoVNA-H has a 10dB attenuator on Port1 to control the load impedance seen by the sense antenna.
Above is a calculation of loop Antenna Factor using Calculate small loop Antenna Factor.
The Antenna Factor, distance, and |s21|can be used to calculate transmit antenna gain.
Above is a calculation using FSC. We are not interested in the absolute value of rx and tx power, we are interested in the difference as that equates to |s21| in the measurement scenario. So, setting tx power to 0dBm, we see that rx power is -52.01dBm, we we expect |s21|=-52.01dB for a 10dBi tx antenna, and in a more general sense, tx antenna gain is |s21|-62.01dB.
We might expect then with the DUT expected to be around 1-dBi gain, there is just sufficient range in the nanoVNA-H to measure its gain using the sense antenna described above at 3m from the phase centre of the DUT.