That's what I expected from people who have practical experience and
understand the principles (much better than I current do).

I managed to track down thinner aluminum rod that is much closer to
scaled values of elements in the original design.

The values should be 3.78mm and I can get 3.175mm
For the 4.73mm , I can get 4.763mm.

This should now be close enough to only require small trim in length
to compensate for small change in diameter.

Tom Ring wrote:
Jim - NN7K wrote:

And, consider: even then, It might not work as it does on another
frequency. Have a friend (this years ago), who modeled the Komosko-
Johnson yagi for 2 meters (a 13 el, antenna , that was emperically

snip

in the VHF handbook, for 144)! As info, Jim NN7K


When properly done, I have never had a scaled design fail to perform as
expected. I have done several dozen. The designs were 144, 220, 222,
and 432 as source and destination.

I don't expect this one will surprise me as to how it performs,
especially as it is insulated above the boom.

tom
K0TAR

David wrote:

That's what I expected from people who have practical experience and
understand the principles (much better than I current do).

I managed to track down thinner aluminum rod that is much closer to
scaled values of elements in the original design.

The values should be 3.78mm and I can get 3.175mm
For the 4.73mm , I can get 4.763mm.

This should now be close enough to only require small trim in length
to compensate for small change in diameter.

The diameter ratio would be fine given the first diameter you listed.
The others would be fine also. Just specify what you want to use.

tom
K0TAR

Tom,

I have heaps of 6mm in stock. The smaller diameter has to be ordered
specially and is more expensive.

So:

Frequency 921 MHz. (20MHz BW or more would be good).

Gain = 7dBi

F/B ratio not critical.

4 Element Yagi - input impedance of around 29 Ohms (can easily match
with 2 par 75 Ohm 1/4 wave section).

Unit to be operated vertically orientated at height greater than 2m.

Element diameters 6mm

Boom is 12mm square aluminium. Elements mounted 3-5mm above boom on
nylon insulator block.

Just need accurate lengths and spacing now.

Tom Ring wrote:
David wrote:

That's what I expected from people who have practical experience and
understand the principles (much better than I current do).

I managed to track down thinner aluminum rod that is much closer to
scaled values of elements in the original design.

The values should be 3.78mm and I can get 3.175mm
For the 4.73mm , I can get 4.763mm.

This should now be close enough to only require small trim in length
to compensate for small change in diameter.


The diameter ratio would be fine given the first diameter you listed.
The others would be fine also. Just specify what you want to use.

tom
K0TAR