Why am I receiving big Gain differences

Hi,

I have the following 3 antennas:
Tunable dipole: tuned to 375MHz for my measuements with known Gain
Broadband Dipole: 200-500MHz unknown Gain
UHF antenna: 240-380MHZ unknown Gain

I put the antennas 'Broadband Dipole' and 'UHF antenna' in an anechoic chamber 3 meters from each other. I measure -35dB in power on the network analyzer at 300-340MHz. Second, I replace the 'UHF antenna' with the 'Tunable Dipole'. I receive -37dB in Power. Let's say the known gain of the tunable dipole in 2dB between 300-340MHz.

Gain of UHF antenna = Gain of Tunable dipole + (Power to UHF Antenna - Power to Tunable dipole)
Gain of UHF antenna = 2dB + (-35dB - (-37dB)) = 4dB

The problem is when I move the Broadband Dipole to 5 feet. I understand that I should receive different power levels because the antennas are further away from each other, but my gain numbers are different too. After making the same calculations, i get 7dB of Gain.

Question: Why is there such a big difference of 3dB in Gain? Shouldn't the gain of the UHF antenna be the same regardless of position?

Thanks

One problem is that you did not mention if your anechoic chamber is specified for your frequeny of interest. Locating antennas so close as less than 10 wavelengths does introduce large errors due to coupling, ground proximity, etc.
The best solution is to use an open range for such low-frequency antennas. The distance between antennas and to the ground must also be considered.
Antenna gain is affected by close objects, close meaning 10 wavelengths. A higher antenna gain requires more separation or free space, refer to near field definition.

agree with above comments

you are not going to get any respectable results with the antennas are well within the near-field regions
you need to separate them significantly to get useable gain figures ... gain figures of an antenna always relate to far field measurements


Dave