Are these points true?

The following is the simplest way of obtaining a practical yagi design using
an antenna modeling program:

1 If you use a non conducting boom then you can build a practical model
using the element lengths in the model.

2 For a VHF yagi in the clear you can assume freespace.

3 A simple straight wire in your model that is made source is DE and
is treated as a simple hertz dipole by the program. So the values given for
R and J are the values you would measure having a hertz dipole as DE.
This is true for the model and would be fairly accurate if the boom was
non conducting and reasonably in the clear in the practical design.

4 It is easy to scale for frequency in antenna programs.

5 It is fairly easy to alter element diameters in antenna programs and to
obtain a redesign that maintains the original performance characteristics.*

It seems to me that if one is prepared to use a non conducting boom, a
hertz dipole as DE, then the model is pretty much the practical design.

Not sure about impact of a balun or the feedline.

---------------------------------------------
*
http://www.cebik.com/32m.html


The second kind of answer involves adapting a given antenna design to a new
diameter material. Suppose that a designers specifies 0.1875" (3/16" or 4.76
mm) elements. The builder has a stock of either 0.125" (1/8" or 3.18 mm)
rods or 0.25" (1/4" or 6.35 mm) rods. Will he or she need to adjust the
element lengths of spacings?

The answer is "yes." In fact, without making such adjustments, the antenna
will not perform as originally designed. There are two major reasons for
this result. In general and first, both driver and parasitic elements
lengths require adjustment with every change of diameter. The general goal
is to arrive at elements whose self-resonant frequencies are the same as in
the original array. Second, the inter-element coupling changes for a given
spacing of two elements if we change the element diameter. Element spacing
does not change as rapidly as the element length for a given level of
coupling when we change element diameter. However, it changes enough so that
we cannot ignore the effects.

One of the simplest ways to accommodate a revised element diameter is to
resort to a Yagi optimizing program. We simply plug into the program the
existing design and specify the new element diameter. The program then
churns out the revised design.

More antenna builders have general antenna modeling programs than have
optimizing programs. There is a procedure that we can use to reoptimize a
design for a new element diameter, although it has a pitfall from which we
must guard ourselves. Here is how the procedure works.

1. Create a model of the original design and establish its operational
characteristics.

2. Revise the model to use the new element diameter.

3. Find the frequency at which the new model shows the same operating
characteristics as the original model did at its initial design frequency.
If we are moving to a larger-diameter element, the new frequency will be
lower than the old one. If we are moving to a smaller-diameter element, the
new frequency will be higher than the old one.

4. Frequency scale the revised antenna model from the new design center to
the original design center. Retain the new element diameter: the amount of
performance change occasioned by the small frequency movement will usually
not require a reiteration of this step. However, when enlarging or shrinking
elements by more than a factor of 2, it may pay to make the change in two
steps of scaling and checking.

At this stage, check the performance of the antenna across the passband used
by the original design. In many instances, the model will suggest that we
need not make any further changes. However, in some cases, we may need to
adjust some element lengths to center the gain, front-to-back, and SWR
curves as closely as possible to their original form (assuming that the
original curves are the most desirable ones for our application). The driver
length will have the greatest effect upon the SWR curve. Juggling the
reflector length and the most forward director lengths can smooth out the
performance across the passband, although rechecking the SWR curve may be
necessary. For a given band-edge adjustment, alter the element that moves
gain and/or front-to-back performance values in the desired direction with
least adverse affect on the SWR curve. Finally, when reducing element
diameters, you may need to increase the reflector spacing from the driver to
raise the general impedance level back to that of the larger elements with
which you began.

The pitfall in this procedure involves stopping at this point. Although the
initial detection of the revised design center frequency and scaling that
back to the original center produced element lengths that are very close to
optimum, the element spacing moved in the wrong direction. The thin-element
model increased element spacing, while the fat element model decreased the
spacing. However, as element diameter increases, element spacing must
increase to maintain the same level of coupling. Because we have adjusted
element lengths, the spacing adjustments may not be dramatic, but they will
be noticeable. Therefore, we need one more step.

5. If increasing element diameter, increase the spacing among elements by
about twice the amount that the initial scaling decreased them. If
decreasing the element diameter, do the opposite. Do not use a simple
additive method, but instead find a multiplier based on the scaling ratio
used in step 4. Take into account any revised positioning of the reflector
in step 4. As well, check the driver, reflector, and most forward director
lengths to re-establish the performance curves for the array.


(From Notes on 6-Element Wide-Band 2-Meter Yagis by L. B. Cebik, W4RNL)

"Richard" wrote in message
...
Are these points true?

The following is the simplest way of obtaining a practical yagi design
using
an antenna modeling program:

1 If you use a non conducting boom then you can build a practical model
using the element lengths in the model.

2 For a VHF yagi in the clear you can assume freespace.

3 A simple straight wire in your model that is made source is DE and
is treated as a simple hertz dipole by the program. So the values given
for
R and J are the values you would measure having a hertz dipole as DE.
This is true for the model and would be fairly accurate if the boom was
non conducting and reasonably in the clear in the practical design.

4 It is easy to scale for frequency in antenna programs.

5 It is fairly easy to alter element diameters in antenna programs and
to
obtain a redesign that maintains the original performance
characteristics.*

It seems to me that if one is prepared to use a non conducting boom, a
hertz dipole as DE, then the model is pretty much the practical design.

Not sure about impact of a balun or the feedline.

http://www.cebik.com/scales.html

"Although I have omitted details--since they may vary widely--the general
principles separating the three mounting systems should be clear. The first
version uses a non-conductive plate or other means of insulating and
separating the element from the boom. If the separation is sufficient, the
boom has virtually no effect upon the element. In such cases, one may use
modeled dimensions directly, since the model (whether NEC or MININEC)
presumes that the elements are isolated from unmodeled conductors. An
alternative to the insulating plate is, of course, to use a non-conductive
boom. This technique permits through-boom mounting for the element, but with
no change of the effective length of the element."

(Scaling and Adjusting VHF/UHF Yagis L. B. Cebik, W4RNL)
--------------------------------

So, I could use a metalic boom but make sure the elements are insulated,
then the model is pretty much the practical design.

This means then that designing a practical antenna would be greatly
simplified either by using a non conducting boom or insulating the elements.

I'm still unsure of point 3. But I assume that for a single wire that is
made source in an antenna program, DE is a simple hertz dipole, the
dimensions and the electrical parameters would be reproduced in a
practical design cut to the dimensions as per the model - if the
elements are insulated or the boom is non conducting.

Richard wrote:

Are these points true?

The following is the simplest way of obtaining a practical yagi design using
an antenna modeling program:

1 If you use a non conducting boom then you can build a practical model
using the element lengths in the model.

I don't think it matters much what boom you use. Being at right angles
to the elements, the boom should be nearly "invisible" to the antenna.
Even if it's metal, Even if the elements are directly connected to the
boom.
Sure, there will be minor differences, but for your task, I don't think
it's worth worrying about. I sure don't. I'm not *that* picky.

2 For a VHF yagi in the clear you can assume freespace.

Yes. For all practical purposes...

3 A simple straight wire in your model that is made source is DE and
is treated as a simple hertz dipole by the program. So the values given for
R and J are the values you would measure having a hertz dipole as DE.

Yes. Normally will end up a fairly low impedance, unless you space the
elements for a appx 50 ohm feed. NBS yagi's will have a fairly low
feedpoint impedance. Thats why I prefer just to use a boom grounded DE,
and use a matching device. I almost never use a split driven element
unless the yagi plans call for a 50 ohm feed due to the element spacing.
My cushcraft A4S is of this ilk...

This is true for the model and would be fairly accurate if the boom was
non conducting and reasonably in the clear in the practical design.

Again, even if the boom is conducting, I don't think it's worth worrying
about..It doesn't make that much difference. Or to me anyway...



4 It is easy to scale for frequency in antenna programs.

Yes.

5 It is fairly easy to alter element diameters in antenna programs and to
obtain a redesign that maintains the original performance characteristics.*

If you simply rescale, the results should be the same. All dimensions
are rescaled. Of course, you can doodle with the program and redesign..

It seems to me that if one is prepared to use a non conducting boom, a
hertz dipole as DE, then the model is pretty much the practical design.

I think it is good enough, no matter what the boom is...

Not sure about impact of a balun or the feedline.

If you use a split driven element with a 50 ohm feed, you need a 1:1
balun, or equal choke, or whatever...


--
http://web.wt.net/~nm5k

Richard wrote:


So, I could use a metalic boom but make sure the elements are insulated,
then the model is pretty much the practical design.

This means then that designing a practical antenna would be greatly
simplified either by using a non conducting boom or insulating the elements.

At 2m, yes, especially if it's a decent broadband design, at 432,
definately no, even if broadband. The shielding from the boom is
significant at 432.

tom
K0TAR

And if you meant "above" the boom, rather than through the boom as
"insulated", then my comment about shielding is moot.

tom
K0TAR