formula for UHF element spacing.
 

Anyone know the formula (if there is one)for
working out the spacing of the elements
on a UHF TV antenna?

Thanks, Greg

Sorry, there isn't one.

Roy Lewallen, W7EL

gregB wrote:
Anyone know the formula (if there is one)for
working out the spacing of the elements
on a UHF TV antenna?

Thanks, Greg

Roy, thanks for the reply.
So how do they do it?
Greg.


Roy Lewallen wrote:
Sorry, there isn't one.

Roy Lewallen, W7EL

gregB wrote:

Anyone know the formula (if there is one)for
working out the spacing of the elements
on a UHF TV antenna?

Thanks, Greg

So how do they do it?
Greg.

==============================

Easy!
They just copy somebody else's workmanship.
Very non-critical.
If it looks like an antenna it will work.

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"gregB"
Anyone know the formula (if there is one)for
working out the spacing of the elements
on a UHF TV antenna?
__________________

If any kind of UHF TV antenna can be considered, suggest using a corner
reflector, which can have gains ranging from around 11 to 14 dBi across the
whole UHF TV band, with low SWR. They are also quite easy to build (but a
lot easier to buy).

"Antennas For All Applications" 3rd edition (Kraus & Marhefka) discusses
these in detail, including its dimensions in terms or wavelength.

RF

gregB wrote:
Roy, thanks for the reply.
So how do they do it?
Greg.

Making a decent antenna that works over the very wide bandwidth of the
TV UHF band isn't at all trivial. And there's a great big handful of
tradeoffs -- gain, front/back ratio, impedance match, physical size,
number of elements, sidelobe levels, and bandwidth among them. The UHF
TV antennas I've seen are mostly a combination of log periodic, Yagi,
and corner reflector types -- something like a few log periodic elements
with some parasitic (Yagi) elements and a corner reflector. There *are*
some formulas which can be used for pure log periodics and corner
reflectors -- you should be able to find them on the web, and a
serviceable if not optimum antenna could be made with either or both
techniques. You'll have to decide how to best make the tradeoffs for
your application. The main difficulty with a corner reflector is making
a very broadband driven element.

I imagine that beyond designing a basic log periodic or corner reflector
structure, most manufacturers do a bunch of cut and try. These days, I'm
sure they use modeling programs for it. I'm also sure that after some
years of designing the things, the engineers get a feel for the general
effect of certain techniques -- say, how to improve the front/back ratio
with the minimal number of added elements. Some of the bigger companies
might have developed some optimization software. If they have, it's
probably kept as a trade secret.

Roy Lewallen, W7EL

Greg wrote:
"Anyone know the formula (if there is one) for working out the spacing
of the elemrnts on a UHF TV antenna?"

It`s a choice, not a formula, because of many tradeoffs.

There are many types of TV antennas For a particular frequency, Kraus
shows a Yagi on page 66 of his 3rd edition of "Antennas" which spaces
the reflector 1/4-wave behind the driven element, and (4) directors are
used ahead of the driven element. Each director is spaced at a distance
of WL/pi from its neighbors. This gives 15% bandwidth, plenty for a
single TV channel, even for Channel 2 at 50 MHz.

The UHF TV channels extend from about 500 to 800 MHz. 633 MHz would
would be the geometric mean frequency. 95 Mhz would be the antenna
bandwidth at 633 MHz.

The geometric mean of two numbers is the square root of their product

There are many tradeoffs in TV antenna design. Look at some good book
concerning the subject such as "TV and Other Receiving Antennas" by
Arnold B. Bailey. Chapter 12 deals with "Practical Aspects of TV
Receiving Antennas.

Best regards, Richard Harrison, KB5WZI

Thanks all, appreciate the info.
Greg.


Richard Harrison wrote:
Greg wrote:
"Anyone know the formula (if there is one) for working out the spacing
of the elemrnts on a UHF TV antenna?"

It`s a choice, not a formula, because of many tradeoffs.

There are many types of TV antennas For a particular frequency, Kraus
shows a Yagi on page 66 of his 3rd edition of "Antennas" which spaces
the reflector 1/4-wave behind the driven element, and (4) directors are
used ahead of the driven element. Each director is spaced at a distance
of WL/pi from its neighbors. This gives 15% bandwidth, plenty for a
single TV channel, even for Channel 2 at 50 MHz.

The UHF TV channels extend from about 500 to 800 MHz. 633 MHz would
would be the geometric mean frequency. 95 Mhz would be the antenna
bandwidth at 633 MHz.

The geometric mean of two numbers is the square root of their product

There are many tradeoffs in TV antenna design. Look at some good book
concerning the subject such as "TV and Other Receiving Antennas" by
Arnold B. Bailey. Chapter 12 deals with "Practical Aspects of TV
Receiving Antennas.

Best regards, Richard Harrison, KB5WZI