Dave,
Thanks.
This had been strictly an academic exercise.
It is now something to file under "Why short HF whips are so inefficient"
I need to start playing with one of the antenna simulation programs someday.
The tools available now (software simulations, nanoVNA, tinySA, DSP receivers,
cheap surface mount parts and board fab, linux on a $100 chromebook)
make it all way easier to play with than it was 50 years ago.
Jerry, KE7ER
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On Tue, Aug 18, 2020 at 11:58 AM, David Eckhardt wrote:
We should clarify something here regarding very short (as a function of
free space wavelength) radiators.
A short radiator (a short monopole or small open ended 'wires' of a
transmission line) exhibits an extremely *low real part *of the feed
impedance or radiation resistance (neglecting ohmic losses, for the time
being). The impedance is predominantly capacitive. It forms a capacitive
probe to free space and is typically a very inefficient radiator due to
resulting small currents whose fields (generated by those moving or
oscillating charge carriers) open onto free space. A large series inductor
is required to 'resonate' with this capacitance which boosts the generated
fields due to increased currents at resonance. Remember, resonance is
defined by the absence of the complex term in the feed or radiation
impedance. Most of the current in such a structure exists below the
inductor (loaded mobile whip) and on the structure to which the loaded
"antenna' is attached. What's left is the radiation resistance and the
ohmic resistance of the coil support and the wire of the inductor, itself.
Typically, the sum of the last two greatly exceeds the radiation resistance
- that which produces the energy radiated into free space - by a good
amount.
Small radiating structures exhibit a low real part and a large complex
(capacitive) portion of the feed impedance.
Dave - W?LEV
