[Elecraft] Elecraft AX-1 Whip Antenna

[Jim Brown]

On 9/21/2022 7:29 PM, David Gilbert wrote:
> It is my opinion, however, that most of the radiating on an AX1 (or any 
> other short loaded whip) is being done by the counterpoise wire and/or 
> the shield of the coax.  The coil and whip are matching aids.

In FACT, not opinion, if you model ANY end-fed antenna, including those 
matching elements, feedline and whatever it is connected to as part of 
the model as wires, then set the modeling software to display the 
currents on the wires, you will find that all of those wires are 
carrying RF current that varies along the antenna with the open far end 
near zero, with peaks and nulls corresponding to wavelength.

Antennas work by RF current producing an EM field, and it is the parts 
of the antenna carrying the greatest current that does the most 
radiating. If we're in the shack, all of the wiring that connects to the 
shield of the coax, including house wiring, bonding connections, etc., 
carries that current, if we haven't provided an intentional 
counterpoise, like a radial system.

The earth is a big resistor, and makes a lousy return for antenna 
current -- it's like adding a resistor in series with the antenna. So if 
we're in the field and want the antenna to work well, we throw a length 
on the ground to act as a counterpoise, or use a long piece of coax to 
be the counterpoise. An every better trick is to elevate the 
counterpoise by throwing it over some brush or even into a tree, which 
reduces ground losses. Wayne's suggestion for pedestrian or bicycle 
mobile is to trail that counterpoise.

Another point. NEC does not correctly model current distribution in an 
antenna around and inductor, and inductors don't radiate -- they're only 
matching elements, but the do change current distribution. There was 
excellent work in QEX 8-10 years ago showing that the base of an antenna 
that resonates to a quarter-wave was the worst place to put a loading 
coil, because that's where current is the greatest, and it's radiating 
capability is wasted. It was a two-part piece, the first part a thorough 
description of the measurement system (which included field strength), 
the second part field strength measurements for a half dozen conditions 
of where the loading was placed.

This principle is why smart builders of verticals for 160 and 80M use 
top loading in the form of horizontal wire(s) at the top to form a Tee 
or inverted-L, with the horizontal wire at the top adding capacitance.

So yes, it CAN be a fact that the coax feedline to the AX1 could be 
doing more of the radiating than the whip, depending on length. :)

73, Jim K9YC