[Elecraft] High Current only on 20 Meters

[Adrian]

Jim, I understand what you have said, and it is what I am familiar with.
My feedline is only 12 feet from kx3 to the lower corner feedpoint on my 80m FW delta loop outside the window, which has a male bnc termination.
I can work on it with a step ladder, it is only 16ft of the ground. The antenna high apex is at 70 ft in a tree out the front.

I measured my band impedance at the antenna itself which is direct fed with coax and using  a good coax choke balun. 
It varied from 1.3:1 at 80m to 3:1 at 20m and 1.4:1 at 6m, with other bands within those points.

When I say radiation resistance I include the small copper resistance also, which is negligible on this heavy copper wire delta loop.
My observation is that the band with the lowest real resistance at the feedpoint is the band that draws most current on my kx3 ,
 which did have the atu engaged, again feedline loss is very low, since it is so short, and quality LMR240 bnc terminated both ends.

So I do understand your correct explanation Jim, and it was good revision, however I should have explained my setup better to
 remove any assumptions on your part. The antenna works very well, and I have been working USA, Europe and Asia this week with it on most bands with FT8.

Adrian... vk4tux

My observation 

-----Original Message-----
From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Brown
Sent: Thursday, 5 March 2020 1:54 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] High Current only on 20 Meters

On 3/4/2020 2:56 PM, Adrian wrote:
> I get the same on my KX3 on 12m , When using an antenna analyser the 
> 12m radiation resistance is lower (much less than 50 ohm), with more 
> reactance component than  in the other bands, and when the ATU 
> corrects the reactance component of the load, the current tends to be 
> higher (multi-band antenna use with ATU). than the other bands with 
> more radiation resistance values.

I suspect an understanding gap here. What you're describing implies a severe mismatch AT THE ANTENNA. A mismatched load (the antenna) is transformed by the transmission line to an entirely different impedance at every point along the line. The mismatch establishes standing waves on the line, and SWR is one measurement of that, BUT -- loss in the line causes SWR to gradually get smaller along the line, reaching its lowest value at the transmitter.

Computing programs like SimSmith and AC6LA's Excel spreadsheets can take measured data from analyzers that are capable of producing suitable files, and, if you know the cable's characteristics, can transform a measurement made in the shack to the impedance of the antenna itself.

Radiation resistance has meaning ONLY at the antenna feedpoint, and it's a characteristic of the antenna itself. It represents power that is radiated by the antenna. IF we could connect the analzyer at the feedpoint, we would measure its feedpoint impedance, Rs +jXs, but Rs is NOT necessarily the radiation resistance.

A study of texts like the ARRL Antenna Book (or equivalent in VK-land) are in order. There's a bit of a tutorial in slide show form on my website that might get you started on understanding on the transmission line part of it. http://k9yc.com/PacificonSmithChart.pdf  The Smith Chart is a graphical means of understanding (and computing) what happens on a transmission line.

73, Jim K9YC
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