Richard wrote:
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.

Good for those particular yagi designs, maybe, but not necessarily good
for all.

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

Yes, you must do that for any yagi design.

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

That's Figure 10. Those corrections will only apply if you use the same
method as they did for mounting an element on a round metal boom...
which they don't specify in detail, so Fig 10 is almost worthless.

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.

It does in the NBS designs; but not in most more modern designs.

5 How do you go about designing for DE?

Too big a subject - look it up teh basic principles and some practical
examples in various antenna handbooks, and then go and get your hands
dirty.

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?

No, they *demand* a particular fixing method! (See 3.)

7 The designs are maximised for gain.

As well as NBS could do it with the tools available to them at the time
(a hacksaw, mostly). Almost 40 years later - the work was actually done
in the mid-1960s - we have much better tools and wouldn't say those are
optimised designs any more.

8 What is the bandwidth of a NBS designed antenna?

With or without losses due to the bandwidth of the matching circuit?
You'd have to model it; but the NBS designs are not noted for being
wideband.

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

I'm sure someone did, back in the MBASIC days - go search for it.

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.)

In order: yes; yes; it depends how on the particular yagi, the
particular installation, and how much performance you're prepared to
sacrifice; certainly lots of people do; but that doesn't make it right.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek