Elementary question:
 

First of all, I'm a novice where taransmitting antennas are concerned.

It seems to me that many AM "broadcast band" antennas are four-footed
towers. Do these towers merely serve to hold the actual antenna (a "wire")
off the ground vertically. The "wire" runs up the center of the tower?

FM and TV (and some AM) towers tend to be slim "guyed" towers with some sort
of antenna element(s) at the top. Is this correct? Is the actual tower ever
a radiating element?

Also... Some tower structures (AM, FM & TV) tend to be cited on hight
ground -- mountains or atop high buildings. Other antenna structures tend to
be intentionally located on "low" flat ground. Why the difference?

Thanks,

Dave Pitzer
=================

On Sat, 08 May 2004 17:34:06 GMT, "Dave Pitzer"
wrote:

First of all, I'm a novice where taransmitting antennas are concerned.

It seems to me that many AM "broadcast band" antennas are four-footed
towers. Do these towers merely serve to hold the actual antenna (a "wire")
off the ground vertically. The "wire" runs up the center of the tower?

Hi Dave,

No, the physical structure, in this case, is also the radiating
structure. Some are isolated from ground (they sit on glass mounts)
or are grounded and fed by other means.

FM and TV (and some AM) towers tend to be slim "guyed" towers with some sort
of antenna element(s) at the top. Is this correct? Is the actual tower ever
a radiating element?

Depends on the wavelength and the physical dimension and if the
structure is constructed with isolated elements (glass barriers along
its length to become a stacked, phased system). But by-and-large,
those that you see are holding up antennas for the height advantage.

This is not to say that BOTH could not be achieved. Here in Seattle,
we have phased AM towers holding up hundreds of VHF/UHF/SHF antennas.

Also... Some tower structures (AM, FM & TV) tend to be cited on hight
ground -- mountains or atop high buildings. Other antenna structures tend to
be intentionally located on "low" flat ground. Why the difference?

Those same phased AM towers are in fact atop two of the city's largest
hills, Queen Anne and what we call Pill Hill (lots of hospitals
located there). We also have lots of lake country and estuaries
filled with AM stations. Economics of Real Estate and coverage drives
such things far, far more than engineering.

73's
Richard Clark, KB7QHC

"Dave Pitzer" wrote in message
...
First of all, I'm a novice where taransmitting antennas are concerned.

It seems to me that many AM "broadcast band" antennas are four-footed
towers. Do these towers merely serve to hold the actual antenna (a "wire")
off the ground vertically. The "wire" runs up the center of the tower?

FM and TV (and some AM) towers tend to be slim "guyed" towers with some
sort
of antenna element(s) at the top. Is this correct? Is the actual tower
ever
a radiating element?

The tower _IS_ the wire. Some towers have insulators at the base, some
don't, but the metal in the tower is what radiates. The guy wires might be a
part of the radiation system, but usually they're just anchors.

Also... Some tower structures (AM, FM & TV) tend to be cited on hight
ground -- mountains or atop high buildings. Other antenna structures tend
to
be intentionally located on "low" flat ground. Why the difference?

FM and TV signals use frequencies that travel best in a straight "line of
sight" from the transmitting to the receiving antenna; ergo, they're high up
so that more receiving antenna can "see" them. AM stations benefit from
being near water, and since the frequencies used for AM don't need to be in
a line of sight from transmitter to receiver, they can be low down where the
water is. If a station is using its AM tower to hold an FM antenna, the
location is a compromise between height (for FM) and proximity to water (for
AM).

Thanks,

Dave Pitzer

You're welcome. Please tell your teacher that r.r.a.a. sends its regards.
;-J

Bill

Hi Dave, (and remember- There are never DUMB QUESTIONS!) The towers you see
(even at low elevations) can be used for more than AM broadcast (and ath the
frequencies for am, the WHOLE tower is used for the radiator- not the "wire"
inside it.
in fact, under OSHA regulations, the "WIRE" you see (and probably associated
with a ladder, tho not necessarily, may be "FALL PROTECTION" for those that
must climb it to do repairs- what you DONT see is a device that will allow
free travel one way (up), but unless a catch is depressed, will not travel
down (in case a maintainence worker slips). You may also see smaller
antennas (normally towards the top), for fm, 2 way, pageing, and even
microwave-- occasionly you will even see metal plates, pich at a 45 degree
angle- periscope reflectors for microwave- the dish feeding them sits on the
base of the antenna. As stated, for AM, the antenna is USUALLY insulated
from ground (the "counterpoise" can consist of wire netting, or individual
radials running on top, or just below the surface of the soil surrounding
the area.) . When was going to college, worked at a fm station in Portland,
Or- that station had something like 8 - F.M stations, and 5 - two way radio
stations on a 350 foot tower (Healey Heights) and the tower was probably
30-40 foot at the base! (coax for each of the broadcast antennas was about
8 inches in Diameter!!). Tho because of security problems in todays world,
some of the Engineers at these instalations are quite proud to give a Guided
Tour- might find it interesting to see just what makes them tick! Jim NN7K


" First of all, I'm a novice where taransmitting antennas are concerned.

It seems to me that many AM "broadcast band" antennas are four-footed
towers. Do these towers merely serve to hold the actual antenna (a
"wire")
off the ground vertically. The "wire" runs up the center of the tower?

FM and TV (and some AM) towers tend to be slim "guyed" towers with some
sort
of antenna element(s) at the top. Is this correct? Is the actual tower
ever
a radiating element?

The tower _IS_ the wire. Some towers have insulators at the base, some
don't, but the metal in the tower is what radiates. The guy wires might be
a
part of the radiation system, but usually they're just anchors.

Also... Some tower structures (AM, FM & TV) tend to be cited on hight
ground -- mountains or atop high buildings. Other antenna structures
tend
to
be intentionally located on "low" flat ground. Why the difference?

FM and TV signals use frequencies that travel best in a straight "line of
sight" from the transmitting to the receiving antenna; ergo, they're high
up
so that more receiving antenna can "see" them. AM stations benefit from
being near water, and since the frequencies used for AM don't need to be
in
a line of sight from transmitter to receiver, they can be low down where
the
water is. If a station is using its AM tower to hold an FM antenna, the
location is a compromise between height (for FM) and proximity to water
(for
AM).

Thanks,

Dave Pitzer

You're welcome. Please tell your teacher that r.r.a.a. sends its regards.
;-J

Bill

OOPS-- Tho because of security problems, many stations are off limits, but
Some of these engineers are quite proud to give you a guided tour to show
how they "tick"!! Jim

Just in that last week I happened to notice two broadcasting antennas in the
vacinity of where I live. In both cases it appears to be two towers about 100'+
tall which are about 100-200' apart. They are in a field with nothing else
around except a small building. I see no wire strung between the towers. They
are guyed and the guys are broken up with insulators. Are both towers driven or
is one a (parsisitic) relflector? Is this generally what AM broadcast antennas
look like?


Bill K6TAJ

"zeno" wrote in message ...
Just in that last week I happened to notice two broadcasting antennas in
the
vacinity of where I live. In both cases it appears to be two towers about
100'+
tall which are about 100-200' apart. They are in a field with nothing else
around except a small building. I see no wire strung between the towers.
They
are guyed and the guys are broken up with insulators. Are both towers
driven or
is one a (parsisitic) relflector? Is this generally what AM broadcast
antennas
look like?

The usual practice is to drive both towers. Parasitic reflectors don't give
sharp nulls, and broadcast stations are usually "protecting" another
licensee on the same frequency, so the array will be designed to put a notch
in the signal toward the other "priviledged" station, while still assuring
the maximum possible coverage in the local area. That means driving both
towers.

The guys are broken with insulators in order to prevent the guy wires from
becoming parasitic elements, which could weaken the null(s).

This is generally what AM antennas look like if the station in question
wasn't licensed a long time back: some "clear channel" stations have
non-directional antennas, but that's rare, and some "sundowner" (i.e.,
daytime-only) stations are non-directional because they have low power and
aren't on at night.

HTH.

Bill

Thanks everyone!!

DP



"Dave Pitzer" wrote in message
...
First of all, I'm a novice where taransmitting antennas are concerned.

It seems to me that many AM "broadcast band" antennas are four-footed
towers. Do these towers merely serve to hold the actual antenna (a "wire")
off the ground vertically. The "wire" runs up the center of the tower?

FM and TV (and some AM) towers tend to be slim "guyed" towers with some
sort
of antenna element(s) at the top. Is this correct? Is the actual tower
ever
a radiating element?

Also... Some tower structures (AM, FM & TV) tend to be cited on hight
ground -- mountains or atop high buildings. Other antenna structures tend
to
be intentionally located on "low" flat ground. Why the difference?

Thanks,

Dave Pitzer
=================

Another point is since the entire tower is RF hot not only must it stand on
an insulator at the bottom it also needs a way to get the tower lighting
voltage to it. This is done w/ "Austin Rings" basicly a pair of interlocked
toroids or rings. In addition, for lightning protection they have "johnny
balls" a pair of air spaced balls to form a spark gap.

Do a google search for these and you should find pictures.
http://hawkins.pair.com/atcitynj.shtml is one such site, just scroll to near
the bottom of the page.

"Jim" wrote in message
...
OOPS-- Tho because of security problems, many stations are off limits, but
Some of these engineers are quite proud to give you a guided tour to show
how they "tick"!! Jim

The Masked Marvel wrote:
Another point is since the entire tower is RF hot not only must it stand on
an insulator at the bottom ...

I don't know much about commercial AM transmitting systems.
Why not feed the towers with a gamma match?
--
73, Cecil http://www.qsl.net/w5dxp



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