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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
