I've been trying to optimise a quarter-wave antenna on a 433MHz transmitter using a T100 Vector Impedance Analyser.
Attaching just the antenna directly to the T100 shows that it is already a good impedance match to 50 ohms, so I thought I'd check it out whilst it is actually attached to the hand-held case into which the transmitter is fitted.
To do this I cut a length of RG174 co-ax cable (terminated in an SMA free plug) to be an exact half-wavelength long at 433MHz (remembering the velocity factor). I connected the cable to the T100's SMA connector and the other end of it to the SMA socket (other feed cable removed) mounted on the transmitter case. I then connected the antenna being tested to the SMA socket on the case. The transmitter case does offer a 'ground-plane' of sorts, although it is nothing like a quarter-wave in radius/length..
I had thought that the antenna impedance shown at an exact half-wavelength along a 50 ohm co-ax cable would have been fairly similar to that of the first test (but just at 433MHz), but the second measurement bears no sensible relationship to the first, and I'm puzzling to understand why..
No doubt it'll be immediately apparent to the experts in this NG that I'm not very familiar with the subtleties of antenna measurements. If someone who is knowledgeable about them reads this posting and would care to let me know what I'm doing wrong, I'd be most grateful to hear their comments and recommendations as to the correct way to make these measurements..
TIA - Dave