"Richard" wrote in message
...
http://www.boulder.nist.gov/timefreq...al/pdf/451.pdf

Can someone please comment on the following statements and questions?
Thanks.

1 The reflector and director element lengths and spacings of Table 1
are
good for a yagis where d/lambda is 0.0085 and boom is non conducting.

2 You make a correction to the lengths when d/lambda of your reflector
and director element diameters are other than 0.0085.

3 When you use a metallic boom, you have to make corrections to the
element lengths as per Table 10.

4 I take it that whatever you put in place for DE, all spacing and
reflector and director elements continue to remain correct. So you can
place
whatever you want for DE, hertz dipole, folded dipole, gamma match, T
match, DE with 1/"4 dia tube or 1.2" dia tube -it does not matter,
parasitic element lengths once calculated need not be changed whatever you
DE you place in the design. Spacing always remain the same.

5 How do you go about designing for DE?

6 Do the element length corrections due to use of a metalic boom assume
a
particular element fixing method? Such as through the boom or on top of
the
boom?

7 The designs are maximised for gain.

8 What is the bandwidth of a NBS designed antenna?

9 Has anyone written a program specifically for an NBS antenna so you
can
calculate the element lengths without relying on tables and graphs?

10 Once designed the NBS design will probably work very close to
parameters. But strictly speaking the antenna will only work very close to
parameters if the antenna is 3 wavelenghts above ground. But for reception
purposes on VHF, you can probably ignore effect of ground and no further
design is probably necessary. (Lets face it with most bought antennas for
receivimng, you just put them up. You don't start altering element
spacings
etc.)


Further:

Reason why I looked at technical note 688 was because I thought I could
avoid using models in antenna computer programs. As far as I can see in
4nec2 and Eznec you *cannot* input boom data. So, I thought following note
688 I can more easily design an antenna because it includes a table so you
can take account of the boom. The test people obviously constructed the yagi
designed for maximum gain and used a boom to figure out the boom
corrections. This is dandy or looks good when somone like me does not want
to spend ages designing.

The way it reads is that if you follow note 688 you can easily design your
own antenna.

But, as always I get stuck with DE. Whether looking at note 688 or when I'm
messing with 4nec2.

I see now that in the original test yagi, DE was a 1/2 wavelength folded
dipole with the test element diameter of 1/4" matched to 50 Ohms with a
double stubb tuner. There is not enough information as to the construction
of the folded dipole to be able to replicate one to use as DE. Who knows if
468/f was used to get length for DE, who knows what the spacing between
the elements of the folded dipole were.

Yet, I think whoever wrote note 688 expects a "designer" to put almost
whatever he wants for DE, and is expected to keep the calculated dimensions
regarding the parasitic elements.Obviously that is how note 688 reads. The
problem is this though isn't it, it's okay if DE is in the clear of any
parasitic elements, because then there is no influence on DE and you can l
look in textbooks to calculate what the dimentions should be etc etc. DE
would be a simple dipole on it's own in free space or whatever. But of
course in a yagi DE is immersed in the fields created by the parasitic
elements which influence a lot of things to do with DE. So how on earth can
you design DE and establish Z etc without some sophisticated mathematics
that take into account the effect of the parasitic elements on DE. Could you
(whoever is reading this) know what to do with DE?. Could you calculate Z
and dimensions for DE just looking at the information given in note 688?

(I think it's possibly implied that whatever DE is, it ought to be a folded
dipole - maybe. Not a simple hertz dipole for instance.)

Anyway reasons why looking at note 688 is a ruse: I'm wanting a wideband
antenna and I'n not sure if a design for 159 Mhz will have a bandwidth of 6
Mhz. I wanted NOT to use a folded dipole so an NBS design is no good. And
can you actually make a design relying on the procedure in note 688 anyway.
Possibly though a basis of a model in an antenna program.

All this is part of my learning curve BTW.