On 10 feb, 14:20, "Marcus Tait" wrote:
Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length of
wire). I connect a receive antenna to a spectum analyser and the antenna to
be tuned to the signal generators 50ohm output. I set the signal generator
to 400MHz and adjusted the length of the antenna to get a maximum peak on
the spectrum analyser. My question is when i have tuned the antenna for
maximum output at 400MHz is the antenna then tuned to 400MHz or is the
antenna's impeadance tuned to 50ohm or both?

Hello Marcus,

You method does not guarantee optimum match (hence maximum radiated
power). Matching involves reactive component = 0, resistive component
= 50 Ohm (for a 50 Ohms generator).

When your quarter wave wire (connected to RF generator) has a real
impedance (Im(Z) = 0). The impedance can be anything between about 18
Ohm and 50 Ohms. It depends on the termination of the coax shield at
the quarter wave wire connection. So resonance (generally spoken, no
reactive components), does not guarantee maximum power transfer.

Only when you can prove by other means that your quarter wave antenna
is 50+j0 Ohms when in resonance, maximum reading on the analyzer
corresponds to optimum match and maximum radiated power (assuming no
ohmic losses in the antenna). As a first approximation, you can see
the quarter wave antenna as a series resonant circuit with a series
resistor that represents the "losses" due to electro-magnetic field
radiation.

Best regards,

Wim
PA3DJS
www.tetech.nl