The input impedance can be measured reasonably well at ground level.

Align the antenna so that the reflector is 'down' and the last director
is 'up'. Ground effects are minimized due to the F/B of the antenna. The
antenna is radiating straight 'up'.

Next take a 1 wavelength, allowing for velocity factor, coax line and
connect it to the antenna feedpoint.

Finally, beg, borrow, requisition, pilfer, rustle, etc., an antenna
analyzer similar to the MFJ 259B. Connect it to the other end of the 1
wavelength coax.

Select the function to read impedance. Dial in your frequency and read
the impedance.

A one minute job once the antenna, coax and meter are at hand.

+ + +

Paul (Home) News wrote:

Hi,
I have done quite a bit of numerical analysis of VHF Yagi's with usually
very good agreement between theory and performance at least for gain and
pattern. One of the things that seems more difficult to correctly predict
however is the input impedance, and thus to design some sort of a match
without some sort of subsequent trial and error. In an effort to explore the
differences further I have been looking at ways of measuring the impedance
directly rather than indirectly by vswr with the matching in place. I have
made a number of different return loss bridges, and even tried the technique
described in an old Ham Radio Article where you take two VSWR readings with
and without an added series resistance. All of course making allowances for
coax length etc. The problem I have is all of them give, in some cases
wildly, different answers even when used on the same antenna. So my
questions a

What sort of accuracy can I expect from these sorts of methods?

Is there a better way (which doesn't involve large sums of money) to measure
antenna impedance at say 146Mhz?

Thanks

Paul VK3DIP