Topband: Inverted L Dimensions

[mrtman777]

Guy,

Sounds great if the ground resistance stays the same for a given ground system 
with different antenna lengths.  But that is an assumption.

By moving the current maxima up in the antenna, you will be moving the current 
maxima out in the radial system as well.  I believe that that might change the 
resistance of the ground system  (i.e., need longer radials for the same 
effectiveness).   However, if that pesky ground resistance stays the same, a 3/8 
wl inverted L would be better than a 1/4wl L due to lower ground loss as your 
modeling indicates.   Since radials systems are hard to model accurately, I will 
have to do some experimentation.  So far, I have a transmitter and MFJ SWR meter 
for measurments.  That isn't accurate enough.  This winter when I get some 
better instrumentation together, I will lengthen my inverted L to 3/8 wl and 
measure the feedpoint resistance, subtracting the modeled 52 ohm radiation 
resistance to get ground resistance.  I want to see if the ground resistance is 
still 15 ohms with the 3/8 wl antenna like with the 1/4 wl antenna currently.   
If it is, I am sold on the 3/8 wl antenna.  


Surely someone has already done this experiment.  Did your ground resistance 
stay the same with both antenna lengths?  What length radials do you use?


73,
Trey KJ4FDV




________________________________
From: Guy Olinger K2AV <olinger at bellsouth.net>
To: Rik van Riel <riel at surriel.com>
Cc: topband at contesting.com
Sent: Sat, September 25, 2010 4:18:48 PM
Subject: Re: Topband: Inverted L Dimensions

Various visual mental simplifications of what is going on in inverted L's do
not map well with what a model will show if you vary one length, record the
results, vary it again, record, etc to create a mapping of what happens to
feed R, current max height, low angle field strength, etc, as a single
detail is varied.

To all those who are speculating, the answer is to model the situations.
Much of what occurs is not the least bit intuitive, some counter-intuitive.

Item (1) below is FALSE as a GENERAL statement because many up and out
combinations of 3/8 wave ALREADY have the current max somewhere in the
vertical, some in the BEST place.  Further, the BEST spot for the current
max for say 15 degrees takeoff moves around, depending.  It takes the math
of the model program to put that together.

These cases are easily apparent if a variety of 3/8 wave L situations are
modeled.

73, Guy.

On Sat, Sep 25, 2010 at 4:19 PM, Rik van Riel <riel at surriel.com> wrote:

> On 09/24/2010 07:15 AM, Edward Swynar wrote:
>
> > So allow me to run this by the group for comments&  criiques, i.e., what
> if
> > I was to shorten each "L" from the existing 3/8-wave length, to something
> > approaching, say, "...one-quater-wave-plus-ten-feet"...? And what if I
> left
> > the tuning capacitor at the base of each "L"...?
> >
> > I believe that by doing so I'd accomplish two things:
> >
> > (1) The current node of each "L" would be more concentrated in the
> vertical
> > portion of the wire away from the horizontal, thus aiding&  abetting the
> > pursuit of DX, and,
> >
> > (2) I could still benefit from resonating each "L" with base-mounted
> > variable capacitors (albeit with much SMALLER "C").
>
> Your idea makes perfect sense.
>
> One thing to keep in mind is that having the point of maximum
> current lower on the antenna could result in higher ground losses.
>
> This makes it a tradeoff between maximizing low takeoff angle
> radiation from the vertical partof the radiator and minimizing
> ground losses.
>
> --
> All rights reversed.
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK