Thanks to all for your comments.
Tuned or Untuned Radials - that is the question
I think the point of my previous post has been missed. The subject is tuned
elevated topband radials. I'm wondering how this technique has worked for
others and what your experience has been? If you do not want to clutter up the
list with your comments please contact me directly.
Another question I have on tuned radials is the magnitude of the additional
ground loss created by operating the short radials at a higher potential then
if they were left untuned. I take the results of NEC-2 modeling with a grain of
salt concerning the ground loss. I want to learn more about this subject from
others having experience or thoughts.
Engineer or Cut-and-try
To engineer a suitable balun (or anything else) the modern method is to begin
with design specifications. We then have a target to design towards, and we
know when we are finished with the design.
In this case I set the specification that the feedline shield current will be
12 dB below the antenna current. The shield current specification is
developed base on several parameters. This is a subject for a future
discussion. The
example used is four short radials that present an impedance of -j200 ohms to
the coaxial cable shield. In the example posed, the shield shield current spec
can be achieved using a balun having a CM impedance of 600 ohms. Now let's say
we series tune the radials with an inductance. And let's say that we do not
tune them prefectly but that we achieve a radial input impedance of -j50 ohms.
Now the balun impedance needs to be 150 ohms to meet the 12 dB spec. Another
way to view this is that if the 600 ohm balun is used with -j50 ohm tuned
radials the shield current will be 22 dB below the antenna current. Several
simplifications are made in this calculation. The impedances are used as
resistances
and the shield current is assumed to be at a maximum at the antenna base - the
shield is assumed to present an impedance of 0 ohms.
This method allows one to rough in a design on paper and make a go or no go
decision before spending time and resources modeling or building. There are
many ways to solve an engineering problem. I tell new engineers they can find
100
ways to meet a design spec. This allows them to open up the design space. For
a one-off home antenna the goal for me is to meet my spec with low effort.
Throwing ferrite at a problem goes against my grain.
These simplifications are made to get a feel for the magnitude of the design.
This example is presented to get one thinking in terms of shunt and series
impedance for a counterpoise design. The shunt Z refers to the radials and the
series Z refers to the feedline shield at the antenna base. Of course this
cannot apply to every situation and this is a general model to begin the design
process.
Armchair Design
And no, I'm not an armchair modeler of antennas. I have build elevated tuned
radial systems and have compared simulations to measurements. I have put in
time in the field. In fact I've got some field work comming up next month to
prove an antenna for a magazine. Additionally, I have spent years doing EMC
measurements, design, and mitigation (NARTE certified EMC Engineer) and that
has
given me another perspective on antennas and evils of common-mode current. For
this I use simulation and too much time in the anechoic chamber.
Modeling - good or bad
Given a particular installation vs. a generalization a more true-to-life
model (paper and pencil or computer simulation) can be made. having said that,
I'm
a big believer in modeling when it is done right. Where I work ICs are
designed completely in simulation. They are built and in many cases the first
run
meets the design specs and the part goes into production. I have similar
success
designing test instrumentation.
Some calculations can aim one toward a successful solution. Cut and try in
the field can be a lot of work. Some engineering can save time and money.
Dave WX7G
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