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Re: [TowerTalk] Good vacation antenna

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Good vacation antenna
From: Wes <wes_n7ws@triconet.org>
Date: Fri, 4 Jul 2025 08:39:35 -0700
List-post: <mailto:towertalk@contesting.com>
Then the essence of this of this is what effect does a base loading inductor have on the current profile on the radiator.  My answer is none.

It's too bad we can't have attachments here so I'll try to describe it with words.  I "built" a model of a vertical in AutoEZ and used EZNEC Pro/2+ V 7.0 running the NEC 5 engine to do the computations.  I used 7.25 MHz with a height of 1/8 wavelength at that frequency (16.55 feet), diameter of 0.1 inch and 33 segments.  I used perfect ground and conductor loss of zero.  The current source was 1.0A.

The feedpoint Z without any loading is 6.15 -j444.6 (SWR = 651). Using the current plotting feature of AutoEZ, the current profile v. segment is a nearly linear, negative slope line from (almost) 1.0A down to (almost) zero.  (Almost because current is reported along the length of a segment, not at the ends)  I used the snapshot feature to capture this plot.

Next I added a series inductor at the feedpoint of 1.48 +j444.6 (Q=300).  The calculated feedpoint Z=7.64 j0 (SWR = 6.55).  The current profile is nearly identical as before.  The snapshot completely overlays the newly calculated plot.

My conclusion: A non-radiating, high Q inductor at the feedpoint of an electrically short monopole affects the feedpoint Z but has no effect on the current distribution on the radiator, which is determined entirely by its geometry.  A load placed anywhere on the radiator other than the feedpoint is a different story.

Wes  N7WS



On 7/3/2025 4:19 PM, David Gilbert wrote:

The loss is the same (probably), but the current profile at the antenna is different (definitely).

Dave AB7E


On 7/3/2025 4:10 PM, Wes wrote:
I'll confess some difficulty (probably my failing) in understanding your point(s).  To try and simplify the situation to something my pea brain may understand, let's use a toroid at either location. Now tell me what the difference is.

Wes  N7WS



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