In article , dansawyeror
wrote:

Good morning,

This question is: Taking measurements at the antenna and what they mean?

The antenna is a loaded vertical over a set of untuned radials. The feed
is 50
Ohm coax with a 1:1 isolation transformer at the antenna. I intend to run a
separate coax for measurement purposes to the feed point, stimulate the
antenna
with a 10 mw signal at the operating frequency, and measure the voltage
across
the antenna feed and the ground connection.

Will the voltages indicate the relative RF impedance of antenna (including
loading coil) versus the ground? If the ground were near perfect the
voltage at
the feedpoint should be close to zero. These voltages should indicate
the power
disapation difference between the antenna and the ground.

Thanks,
Dan

Hello, Dan. If we have a quarter wavelength monopole the voltage at the
feedpoint is at a minimum but definitely not zero even if we had a
perfectly electrically conducting (PEC) ground. If we had a PEC ground
and a monopole with no resistive (ohmic) losses then the impedance seen at
the feedpoint at resonance would be about 37 ohms resistive. The 37 ohms
is the radiation resistance of the antenna. This is where the RF power
gets "dissipated" when that power is radiated into free space.

If we use an electrically short antenna and put in say, a loading coil at
the feedpoint to resonate the monopole at the desired frequency, we are
looking at a radiation resistance of less than 37 ohms. It could be quite
small depending on how short the antenna is. If in addition we have
finite losses in the monopole structure, loading coil and in the
ground/radials, we are measuring a resistance that is the sum of all ohmic
losses and radiation resistance. And that's a classic problem - how to
separate out all these losses. Without making radiation pattern
measurements (to obtain total power radiated) about all you can do is
calculate the radiation resistance for a PEC monopole fed against a PEC
ground (or use an electromagnetics computation program like NEC to model
the structure.). The measured value less the calculated radiation
resistance would reflect the ohmic losses. And you still don't know how
much of this is ground loss without knowledge of the other ohmic losses.

There exits another direct measurement technique (Wheeler Cap) for
determing antenna efficiency that can be readily applied to antennas in
the GHz range but is not practical at HF because of the scaling factor
(compare the size of 2.4 GHz yagi or log-periodic with the same type at 14
MHz to put things in perspective). Sincerely,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337