[TowerTalk] resonance/swr/impedance plots

[Guy Olinger, K2AV]

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


----- Original Message -----
From: <i4jmy@iol.it>
To: <towertalk@contesting.com>
Sent: Wednesday, November 29, 2000 11:06 AM
Subject: Re: [TowerTalk] resonance/swr/impedance plots


.
>
> Make it longer, and a simple fixed series capacitor will take out the
> reactance and give you a 50 ohm feed point with no reactance, properly
> tuned. Further, the ground losses have been reduced by a factor of  4
> ( I squared R loss) since the current at the feedpoint is cut in half.
> The antenna is NOT resonant, as you point out, but it's BETTER.

No, the ground losses are exactly the same they were.
Symply the highest current point (and losses) are moved away from
antenna base so making a need to increase radials density at farther
distance from base.
Stretching to the limit, an Half wave vertical against a ground plane
has nearly zero current at base, but along radials the current is
present beeing maximal one quarterwave off.

73,
Mauri I4JMY


>
> Regards, Guy
> Apex, NC
>
>
> ----- Original Message -----
> From: alsopb <alsopb@gloryroad.net>
> To: Jerry Devine <krr2ak@juno.com>; <towertalk@contesting.com>
> Sent: Wednesday, November 29, 2000 9:12 AM
> Subject: [TowerTalk] resonance/swr/impedance plots
>
>
> > Jerry,
> >
> > To further illustrate my point.  Min SWR is NOT ALWAYS the indicator
> > of true resonance.
> >
> > Here are two tables of data generated by EZNEC for a dipole cut to
> > resonance at 13.8 Mhz.  One assumes a reference impedance of 50 ohms
> > for SWR calculation the other assumes 200 ohms.
> >
> > Zsource               =50 ohms                                 =200
> > ohms
> > freq  13.5  z = 95.2  SWR =1.905  (complex =-33.7, real =67.7)
3.045
> >       13.6  z = 81.5  SWR =1




--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com