Scaling of Yagi design

[David]

Hi,

I have a design for a Yagi antenna at 436MHz and would like to scale it
to 921 MHz. The calculations I used from the understanding I have
obtained from reading various articles appears below.

Would someone be kind enough to advise me if this procedure and values
look correct ?

Thanks in advance.

The original dimensions(mm) were

Boom = 15 or 20mm square aluminum, 350mm long

Element Length Spacing Diameter
RE 336 0 8
DE 316 80 10
D1 300 190 8
D2 292 330 8

My scaled values to 921MHz
Boom = 12mm x 12mm aluminum 166mm long

Element Length Spacing Diameter
RE 158.9 0 3.78
DE 149.5 38 4.73
D1 141.9 90 3.78
D2 138.2 142 3.78

Now I scaled the tubing to 6mm diameter that I have in stock
Element Length Spacing Diameter
RE 158.5 0 6
DE 148.7 38 6
D1 139.4 90 6
D2 135.3 142 6

Regards

David Huisman

[Tom Ring]

David wrote:

Are these through the boom, and if so, insulated, or uninsulated?

If not through the boom, give specifics on the mounting.

Please do this for both the original antenna, and how you propose to
build the scaled one.

tom
K0TAR

[David]

The elements are mounted approximately 3mm off the boom by nylon
insulator blocks on the original design.

The scaled design will use same construction.
http://www.qsl.net/dk7zb/start1.htm

--

Kind Regards

David Huisman
General Manager
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[Tom Ring]

David wrote:

> The elements are mounted approximately 3mm off the boom by nylon
> insulator blocks on the original design.
>
> The scaled design will use same construction.
> http://www.qsl.net/dk7zb/start1.htm
>

Ok, I'll try to run a scaled version for you in the next one or two days.

tom
K0TAR

[David]

Thanks Tom.

The smallest aluminum I have is 12mm square for the boom and 6mm round
for elements. I would like to stick with the aluminum rod as I think
this will be easier to secure than welding rod etc.

[Ian White GM3SEK]

David wrote:
>Hi,
>
>I have a design for a Yagi antenna at 436MHz and would like to scale it
>to 921 MHz. The calculations I used from the understanding I have
>obtained from reading various articles appears below.
>
>Would someone be kind enough to advise me if this procedure and values
>look correct ?

[...]
>
>Regards
>
>David Huisman

The basic principle of scaling antennas according to wavelength is that
you must make an *exact* scale model. That means scaling every
dimension, and not adding any new or different 'features'.

Often that isn't practical for mechanical engineering reasons, so then
you need to apply corrections. However, you should still aim to keep
those corrections as small as possible. For example, simple scaling is
likely to produce odd, unavailable values for the element diameter, so
it's OK to use corrections to change to the nearest commercially
available diameter. But if you also decide to double the diameter, the
correction is more risky - you're entering the territory of a completely
new design.

Sorry that I don't have time to check your particular dimensions in
detail, David, but you can find resources for scaling and constructional
techniques on my 'VHF/UHF Long Yagi Workshop' pages. There is a
downloadable program which leads you through the relevant questions, and
also a link to an online Javascript version.

(From about 1200utc I will not have access to this newsgroup for the
next few days, so I hope the above will keep you going.)

--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

[David]

Ian,

Thanks for that.

I did previously look at your site and bookmarked it. I am going to make
up the element cutting jig and bending jig (for when I want to
experiment with folded dipoles).

Great site.

Regards

David

[Jim - NN7K]

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
designed), long before modeling programs, and then antenna range
measured. This was a staple of the old VHF handbooks, and the
ARRL handbooks for years, and seemed to deliver exceptional results.
He had pull at his local company (not to name them)! and had an
EXACT copy made for 1296, and then entered it into a antenna contest-
it failed misserably! (this not only was element length/Dia, spaceing,
,but BOOM diameter/length)!! It was (and is) a work of ART,
but it wont make his retirement check (or Contest Score)
any higher! For those that
wish to try it- look for those names, or for W2NLY-W6QKI in the
ARRL handbooks mentioned. But it must leave to the builder one
of the following conclusions: 1) It was great on 144, but a dog
on 1296, or: 2) It wasn't the greatest design , for even 144 (tho
it compairs favorable with the range patterns that were shown
in the VHF handbook, for 144)! As info, Jim NN7K

[David]

Jim,

Thanks for your input of caution.

I'm sure that along with all the horror stories there are a multitude of
success stories.

My belief is that antennas have been around for years and millions have
been constructed on almost every frequency imaginable.

In my case I am new to antenna design and green when it comes to
determining the parameters that would best suit my applications.

My requirements are not critical with regard to front/back ratio or the
last drop of gain available for a specific design.

I find there are always such a range of responses to a request.
Some of these are the theorists that love to explain how black holes in
the universe some 10 million light years away might effect free
electrons floating on the end of a copper wire.

Then we have the ones that are not particularly experienced but like to
have a say on every topic. They would normally ask why you want it in
the first place. "Oh, you want blue, why not have red".

Between these extremes are a bunch of very kind people who have "played"
with antennas and gained practical experience that they are prepared to
pass on to others.

Modeling programs have come a long way and my understanding is that as
long as you model the unit correctly, your actual results should not be
too far off. (I don't know how to model yet so hence the reason to ask
for help).

I imagine there are still some disasters when engineers build new cars
but I believe this would mainly occur when something is overlooked or
they are trying to achieve something new.

I am not trying to break any new ground here, just looking for around 7
dBi gain from a Yagi at 921MHz.

Regards

David Huisman

[Tom Ring]

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]

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]

Jim - NN7K wrote:

> wish to try it- look for those names, or for W2NLY-W6QKI in the
> ARRL handbooks mentioned. But it must leave to the builder one
> of the following conclusions: 1) It was great on 144, but a dog
> on 1296, or: 2) It wasn't the greatest design , for even 144 (tho
> it compairs favorable with the range patterns that were shown
> in the VHF handbook, for 144)! As info, Jim NN7K

And, except for the K1FO designs, the VHF handbook contains one of the
worst collections of yagis ever assembled. They are at least as bad as
the Cushcraft VHF/UHF yagis that had the dual reflectors.

[Tom Ring]

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

[David]

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.