Hi Dave,
Question A) From your description, I need to know the height above ground.
Question B) Exactly what did you mean by tuned radials? Do you mean radials
carefully adjusted to 1/4 wave, or smaller lengths and some series device
to tune them to resonance. If the latter, what is the circuit?
Question C) Are you using a common mode current block at the feedpoint. If
not, you MUST literally model the coax shield, and placement lengths at
your site, including places where the coax lays on the ground. Again
specifics in counterpoise make a huge difference.
Essential issue D) Source placement issue. See below.
Whether a ground type, counterpoise configuration or ground description
matters depends on what you have decided to do for the vertical's
counterpoise, and the variation in modeling issues and results vary
enormously depending on just exactly what you are doing with counterpoise.
It is all too common for a vertical antenna modeling project to completely
ignore the counterpoise and variations, and presume that all issues proceed
from the vertical conductor. Practically, the starting answer is solve a
vertical's counterpoise efficiently for the target situation and only then
start monkeying with the vertical.
You had not mentioned anything about the counterpoise, and the counterpoise
is the number one issue for verticals 95% of the time in correspondence I
get. That is why I asked about the counterpoise and ground. For all I knew
it was ground-mounted, and a plethora of considerations apply.
Essential issue D) Segment placement rule: Never place a source in a given
segment if either end of the segment 1) connects to more than one wire, 2)
connects to a wire at an angle, 3) connects to a wire of a different
diameter, or 4) connects to a wire with a large difference in segment
length. Does not always cause a problem, but can, depending on whatever. Do
NOT depend on geometry checks to warn you off. DO IT YOURSELF, EVERY TIME.
Discipline.
One good way to deal with that in advance, BEFORE problems pop up, is to
use a larger count of smaller segments everywhere, and always use segment
#2 instead of #1 or segment n-1 instead of segment n. If a single one
segment wire has to contain a source, break the wire into three segments
and place the source in the center segment.
Breaking the segment placement rule will often give you gain AND/OR
impedance errors of some degree. IF you break that rule you need to test
for sensitivity to the rule in the specific model to see if changing to the
rule makes a difference. So you had to create the compliant model to see if
the non-compliant model caused a problem. I finally figured out doing the
non-compliant (on-purpose) was stupid me (slow learner), and consciously go
to compliant placement to start with.
A test you can do, IF you are using small segments, is to run Z and max
gain with source placed in segment one, then segment two, then segment
three. If the gain varies at all, you can't use segment one. If the Z diff
1 vs. 2 is different than Z diff 2 vs. 3. You do it this way because the
movement up the wire will vary the Z normally. The three segment test says
that the difference as you move should be the same or very close for small
segments. I find that source in segment one often erroneously changes the
gain figures. Not so cool if you are putting together comparisons of
differing antenna solutions.
If you are worried about fractions of a dB, or at least somewhat accurate
feed impedance estimates, go to small segments and stay there.
Hope this has helped. Remember questions A) B) and C) above.
73, Guy K2AV
On Fri, Jul 12, 2019 at 9:39 AM David Gould <dave@g3ueg.co.uk> wrote:
> Hi Guy,
>
> Thanks for quick reply,
>
> If I just start with the 20m vertical. It is 38mm tubing with the base
> 10m off the ground, the source is in the first (bottom) segment. Then
> there are two tuned 1/4 wave 2mm wire radials for 20m connected to the
> bottom of the vertical and drooping down at 45degrees.
>
> Is the ground type that important when it is so far off the ground? For
> reference it is real/MININEC medium - would an alternative be better?
>
> Does that give you what you need?
>
> 73,
>
> Dave G3UEG
>
>
>
> On 12/07/2019 14:16, Guy Olinger K2AV wrote:
> > Hi Dave,
> >
> > What are you using for the vertical's counterpoise? What are you using
> > for the the ground type? Where is your source placed?
> >
> > These are essential to answer your question.
> >
> > 73, Guy
> >
> > On Fri, Jul 12, 2019 at 8:02 AM David Gould <dave@g3ueg.co.uk
> > <mailto:dave@g3ueg.co.uk>> wrote:
> >
> > Most of my modelling has been with wire antennas but now I am
> > modelling
> > some verticals with elements having diameters of between 38mm and
> > 25mm
> > for 20m and 40m (and using drooping elevated radials made of wire)
> >
> > I usually use around 9 or 11 segments for a 1/4 wave element. I
> > noticed
> > that when I changed the number of segments the results for things
> > like
> > feed impedance changed quite dramatically.
> >
> > Are there some guidelines for the number of segments for a 1/4 wave
> > straight wire element?
> >
> > How is the choice of segment length affected by the diameter of the
> > element tubing? Is there a limit on the ratio of segment length to
> > segment diameter?
> >
> > 73,
> >
> > Dave G3UEG
> >
> > _______________________________________________
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> >
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