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?
--
% Randy Yates % "So now it's getting late,
%% Fuquay-Varina, NC % and those who hesitate
%%% 919-577-9882 % got no one..."
%%%% % 'Waterfall', *Face The Music*, ELO
http://www.digitalsignallabs.com

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.


--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

"Randy Yates" wrote in message
...
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?
--
% Randy Yates % "So now it's getting late,
%% Fuquay-Varina, NC % and those who hesitate
%%% 919-577-9882 % got no one..."
%%%% % 'Waterfall', *Face The Music*, ELO
http://www.digitalsignallabs.com

The more elements it has, the better it works. So a cheap one doesn't work
very well due to few elements. But there are ways to spread the bandwidth of
elements by making a radiator out of V-shaped pieces of metal or tubes or
wires in a V configuration - - the thicker the elements, the wider the
bandwidth. Also configurations related to the log periodic array widen the
bandwidth. A UHF antenna in the form of a single circular loop evidently has
a wide bandwidth.

On Mon, 07 Jan 2008 12:03:07 -0500, Randy Yates wrote:

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 all done in marketing. :-)

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?

Its just "magic".

"Randy Yates" wrote in message
...
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?
--
% Randy Yates % "So now it's getting late,
%% Fuquay-Varina, NC % and those who hesitate
%%% 919-577-9882 % got no one..."
%%%% % 'Waterfall', *Face The Music*, ELO
http://www.digitalsignallabs.com

Hi Randy

You have probably thought about Log Periodic Antennas. They work by
forcing a null in the direction away from their feed point. The "dipoles"
that most nearly match the line connecting them will be the most active of
all. The active dipoles will be near 1/2 wave long. Each succeeding
dipole is fed 180 degrees from the dipoles on either side.
When the frequency is trippled, the dipole patern changes such that the
lobes add in a way favorable to the desired pattern if the dipoles are bent
to make the Log Periodic Vee Antenna.
There is a 3 to 1 relation inherant in the LPV that coincides with the
near 3 to 1 spread in assigned TV channels.

Jerry KD6JDJ

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.

I highly recommend that you get a couple of basic antenna texts. Very
few people understand how antennas work, but they're nonetheless free
with advice and explanations. Texts are generally reliable. The _ARRL
Antenna Book_ is about the only book I can recommend that covers the
topic at less than an engineering level. For engineering texts, Kraus'
_Antennas_ is the gold standard. You can find earlier editions at very
reasonable prices. There are, of course, a number of other very good
texts. Be very careful of information you get from the Internet,
including newsgroups.

As a person who develops, sells, and uses antenna modeling software, I
also highly recommend this as an educational tool. I guarantee you'll
learn a lot about antennas by doing some modeling. The free demo version
of EZNEC is available at http://eznec.com, but there are other free
programs available.

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?

One answer to your question is that "work" isn't a binary quantity but a
continuum. TV antennas typically work considerably better at some
frequencies than others. They just have to work well enough to provide
an adequate signal to the receiver, and hopefully provide some
directionality to reduce multipath interference. Most are a sort of
combination of Yagi and log-periodic designs, with a lot of tweaking and
trickery. Another respondent mentioned that elements are sometimes bent
into a V shape. This little trick provides an element which has a
bidirectional pattern at two frequencies related by a factor of three --
about the ratio of the high VHF to low VHF channels. Elements naturally
are resonant at multiple frequencies, including frequencies related by a
factor of three.

Good luck with your endeavors!

Roy Lewallen, W7EL