Topband: tuned elevated radials

[Telegrapher9 at aol.com]

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