[TowerTalk] resonance/swr/impedance plots

[i4jmy@iol.it]

> ---------- Initial message -----------
> 
> From    : owner-towertalk@contesting.com
> To      : <i4jmy@iol.it>, <towertalk@contesting.com>
> Cc      : 
> Date    : Wed, 29 Nov 2000 21:33:18 -0500
> Subject : Re: [TowerTalk] resonance/swr/impedance plots
> 
> What you say is correct for ONE of the THREE factors involved in 
ground
> losses:
> 
> The three involved are:
> 
> 1)  Intermediate to Far field reflectivity losses due to vertical
> polarization, which, as you say, may increase if the phase center of 
the
> vertical radiating cross-section is raised, because of moving the
> "bounce point" or maximal INDUCED ground current farther away, and
> possibly off or to a worse part of the ground screen.
> 
> 2)  E-plane current induction loss directly underneath the feedpoint,
> usually defeated by a good ground screen. This is forgotten by those 
who
> say a true vertical dipole, shortened or not, doesn't need any 
attention
> to the ground underneath.
> 
> 3)  Series-resistance ohmic loss when using an end-fed wire against a
> ground current sink. It is hard to create a good ground screen. 120 
1/4
> wave radials? THAT is low resistance.
> 
> Starting with a wire loaded against a ground rod, factor 3) is by far
> the worst, then 2), then 1). You don't get the chance to play with
> factor 1) until you've eliminated 3) and 2) with a really good ground
> screen.
> 
> But suppose it isn't, say it's ten or 15 ohms (I've seen over 100 ohms
> loss). Then a 15 ohm antenna fed against it will loose 3db in ground
> ohmic losses, PLUS probably some of the E-plane loss, PLUS some
> reflectivity losses.
> 
> All that was said, was that a 100 ohm feed for the wire, in series 
with
> 15 ohms ground resistance, looses a lot less to ground ohmic losses 
than
> a 15 ohm feed for the wire in series with 15 ohm ground resistance.  A
> fifteen ohm antenna fed against a 100 ohm lossy ground won't radiate
> past the neighbor's house. It's like N6BT's light bulb directional
> array.
> 
> --... ...--  . ...  --.  .-..
> 
> Regards, Guy
> Apex, NC

Feed point impedance and radiation resistance are totally different 
matters. An half wave antenna in free space can be end feed with 
thousands Ohm impedance, but its Rr continue to be much lower. 
A short vertical like a car whip can be feed from its base to the top 
or the reactance reversed (shunt feeding) allowing to match with an -J 
(a capacitor) but its Rr doesn't change and the losses are the same, or 
possibly higher because of other factors.

A series capacitor to match with 50 ohm line and canceling the reactive 
antenna component works if the antenna is already electrically longer 
than a quarter wavelenght (and less than halfwave). Whose fact leads 
the real part of the antenna impedance, R, to be in the around of 50 
Ohm.

Given an Rr, given a ground plane and a ground loss, anything that 
moves away from the antenna base the max. current density point 
negatively affects the antenna efficiency.

With a vertical half wave radiator the main current return path is also 
well beyond a quarter wave from antenna base and, unless beeing over 
salt water, using quarter wave long radials with an half wave radiator 
one raises the antenna impedance but worsen the efficiency if compared 
with a quarter wave tall (or even shorter) radiator.
   
73,
Mauri I4JMY





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