Strategy is that you don't rely on other peoples' software models. Because
if you do you might find yourself having to engage in quite extensive design
work, that is probably out of your skill range anyway. (Assumption is that
there's no way that you can simply transfer dimensions from a software model
to an actual antenna. This may be true to a greater or lesser extent.)

You decide on someone else's electrical and mechanical design. But the
design is for another frequency. So, you chose a design closest to your
frequency. This means you must scale the antenna.

Scaling simply invoves altering the dimensions proportionally. If the
already designed antenna is for 144 Mhz and your desired antenna is for 159
Mhz then you have to scale size downwards by a factor of 144/159 = 0.90566.

If you were to scale to perfection every part of the antenna would be
reduced in size, the boom, the element diameters etc. But this is not quite
necessary in practice if there is no great difference in design frequency of
the original antenna and your desired antenna frequency.

In the example given, concerning the element length, if the element lengths
are reduced by 0.90566 then that should suffice. In other words, just
altering the element lengths should suffice, no need to scale everying.

Yet, there may be times when you wish to alter the element and the boom
sizes.

If the designed antenna is for a different frequency than what you want,
changing the element lengths (again, sometimes you can get away by just
changing the element lengths) is a matter of scaling.

However, you can also wish to change dimentions, but not scale the antenna.
For instance, you want an antenna for 144Mhz, and you chose a 144Mhz antenna
design, but you want to alter either the element diameters, or the boom
size. Alteration of the element diameters will mean the length of the
elements will have to change, to maintain resonance. Alteration of boom size
will also have an effect on element lenghs. In this situation what you
need is a G3SEK's program ELE.EXE which will calculate the new element
length sizes when you alter either element diameter or boom size.

So, concerning the example: To get your 159 Mhz yagi antenna, you simpy
scale the element lengths of the 144Mhz antenna. That should suffice as
long as you keep to the original element diameters and boom.

However, if thereafter you wish to alter element diameters or boom size you
have to use ELE.EXE. In that case you pretend that the scaled antenna is an
original antenna. Remember, all you did was scale element lengths, nothing
else, and because the scaling was minimal it's safe to say the scaled
antenna will work pretty close to the original. So when using ELE.EXE you
take the scaled antenna as the original. But, you have to be carefull,
changing the design configuration too much will not produce a good result.
For instance, if you change the elements from going through the boom, to
over the top of the boom, that may be a change too far.

This is how you "design" your antenna using someone elses original design.
Well, it's how I think you do, not being an antenna buff.

Ian, G3SEK, can you say anything more about ELE.EXE. What can you alter, I
presume the construction must remain the same, just that you may alter
element diameters and boom size. What I want to do, is scale, and then
possibly alter either all parasitic element diameters, keeping DE diameter
the same, or just alter DE diameter, keeping parasitic element the same. Or
alter both. Can ELE.EXE cope with that?

I put "scaling antennas" into Google and got a measly 18 hits. Probably
because as a concept it's simple.

Perhaps there should be a word for scaling then altering element or
boom diameters. A name given to this "design" approach. :c)