Randy Yates wrote:
Hi Folks,

This is my first post to this group. I'm a EE, so I've
had all the theory - now I want to know how these things
work.

Basically, my question is this: how can a a TV antanna
cover, what, 60 MHz to 800 MHz? That's over three octaves,
and if the antenna elements are designed to be a fixed
portion of a wavelength, why does this work over such a
large range?

It's because all of it is the driven element.

What you are thinking about is a Yagi antenna. A Yagi has
only one driven element a refelctor at the rear, and
directors in front of it. The more directors you add,
the narrower the beam width (how wide a signal
it transmits or the direction it receives from).
As you narrow the beam width the gain increases,
for each halving of the beam width, the gain doubles.

Eventually you reach a point of diminishing returns
which seems to be around 12db.

A log periodic antenna, such as a TV antenna consists of
many driven elements of varying sizes. I don't know if
it is necessary, or just works better, but they are fed
out of phase, i.e. the connection to the next element
(really a dipole) is reversed.

The easiest way to tell a Yagi from a log periodic is
that a yagi antenna the elements except the driven one
is a single piece and they are not insulated from the boom.

A log periodic antenna is made up of a set of dipoles, so
each one is split in the middle and insulated. Often you
can see the feed wires criss-crossed.

Note that log periodic antennas are not used everywhere for
TV signals. In the U.S. tv stations tend to be clustered around
large cities and often the same area. A log periodic makes sense
because you point it in the general direction of the TV stations.

Here it is quite different, there are only 2 TV channels over the air,
while at one time there were four. They were all in different directions
and different bands, 1 VHF to the east, one to the far north, and 2
UHF stations near the coast. So a log periodic made no sense and you
see lots of yagi antennas.

I was talking the other night to a friend who lives in the U.S.
and he is not sure what he is going to do. Although all the
transmitters in his city are close to each other and about 8
miles from his house in the same direction, he can't receive
HD TV reliably with them.

When analog TV is stopped, he'll probably have to go to cable
or DBS.

Geoff.


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Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
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