"Telamon" wrote in message
...
In article ,
"David Eduardo" wrote:

The folded dipole is also appropriate, like a Franklin, when there is
a bad ground system, such as antennas in marshes and salt flats where
they corrode, or where there are structures on the property.

Yeah. That's why I thought it would be more popular in central and south
America where expensive or difficult to install, due to terrain,
grounding systems requirements could be reduced.

Why would the terrain considerations, grounding requirements and such be any
different than in, say, Idaho or Wisconsin or Arizona or Alabama?

And why would it be more expensive to do in Central and South America? I can
see no logic in any of this set of statements.


A unipois also useful with a shorter than 1/4 wave tower because the
tuning network needed to tune out the capacitive reactance often
narrowbands the antenna (not the tower itself).

That my point. The shunt type coupling is more broadband.

Only for very short towers where a high capacitive reactance is found.
Otherwise, the bandwidth is much more the effect of the Q of the ATU and the
trandsmitter itself.

So a unipole is mostly used to compensate for bad ground systems and
the need to multitask the tower, not to reduce the noxious effects of
a less than conforming tower, as the FCC requires a very special
showing to allow low antennas.

I don't know what you mean by "the FCC requires a very special showing
to allow low antennas" but I wasn't after the shorter tower aspect.

That is the only place where the unipole offers a distinct bandwidth
advantage. It also gets a better field strength at 1 km than a short series
fed tower. But the main reason anyone uses them is either due to a bad
ground or the need to put other antennas on the tower.

A broader cross section will broadband the tower a bit, but the
difference
in a 24" to 30" cross section and a folded dipole is minimal.

That's not my understanding.

Go back to Carl Smith's AM antenna and DA handbooks, Unless you find a nice
old Blaw Knox with a 24 foot center cross section, there is not much gain
except cost, maintenance, etc., in adding outriggers insofar as
broadbanding.

The bandwidth for AM is, by NRSC, 10 kHz in each sideband... actually, a
little less. This is to avoid 10 kHz harmonics with adjacent channels.

You usually make measurements beyond spec to show the trend over the
entire specification. Here the spec is VSWR.

A well tuned tower of 1/4 wave has less than 1.08 to 1 vizwar. And, except
for test situations, a tower measurement is usually done at -10, licenced
frequency and +10 kHz. This is what is often asked for by outside
fabricators of ATUs.

A well tuned ATU, whether high Q or broadbanded, does not create a
significant amount of reflected power. A tower that is mismatched at
carrier does.

Yes that is my point. The tower itself will have a narrow resonant
bandwidth so a method is needed for the tower to not present a high VSWR
+/- 15 KHz around the carrier frequency.

A mismatch is generally considered to be a mismatch of impedance output of
the ATU with the tower itself at the fundamental. Since the audio is
brick-walled right under 10 kHz, there should be little or no excursions
beyond +/- 10 kHz.

ATU's that couple the coax transmission line to the tower using a series
connection to an insulated tower have a stronger tendency to be
narrowband in and of themselves.

Not for the last 40 years or so. High Q was much more common pre-60's when
AMs mostly ran network showsthat came over 5 kHz lines from very far away.
When music took over AM, stations wanted better bandwidth.

An ATU designed to couple the coax
transmission line to a shunt fed tower tends to tune a little less
sharply and the useful resonance range is broader.

Since true shunts are no longer licensed, this point is moot. Most US AMs,
for economy, zoning, FAA, etc. use quarter wave series fed towers. Since a
huge percentage are directional, there are very, very few Unipole
directionals, so in that area series fed is the only way to go.

The FCC no longer authorizes shunt fed towers.

Why?

I don't know. This has been the case for many decades. I suspect part has to
do with the slight directionality the shunt itself introduces, and the fact
that shunts would be very difficult to do with directional stations for this
reason.

The unipole is the closest you get to this; one manufacturer,
Kintronics, who makes kits to order, compares them with shunt fed
systems. So, except for the unipoles, all US towers for AM are
insulated from ground.

I don't understand why the FCC would care how transmission lines are
coupled to the tower or tuned / matched by an ATU.

There is a lot about AM radio in the US you don't understand. The fact is,
shunt fed towers are very seldom used. I suspect that the fact that they are
not particulary easy to tune on short, 1/2 wave towers may be part of it,
but there must be more.

Transmission lines are never couple to the tower (with maybe one or two
exceptions... more later) because so few towers are a perfect impedance
match with the coax and devoid of +j or -j. An antenna coupling unit is
placed between the coax and the tower, using a network to match the tower
to
line impedance and to bring reactance to zero at the carrier. The ATU is
typically attached to the tower with a copper strap, copper tubing or
sometimes even braid. In any case, it is silver soldered to a connector,
which is usually pressure bolted to the output of the ATU and to a leg or
the base plate of the tower.

That's all interesting information but we are mixing and matching
terminology. The ATU doesn't just tune the tower to resonance it is also
a part of circuitry that couples the transmission line to the tower.

An ATU is not necessary if the tower is 52 ohms and not reactive, found
around about 100 to 110 degrees in electrical height.

The ATU is a matching circuit, to make the coax "see" 52 ohms (or some other
impedance) when, in fact, the tower is not of that impedance. In some
senses, a top hat or top loading does the same thing... it makes the tower
change the apparent electrical height. You are trying to complicate
something that is relatively simple... cancelling the reactance and matching
impedance.


Reactance is near zero at resonance and the antenna looks like a
resistive load to the transmission line. The resistive load value also
needs to match the line impedance so there is little or no reflective
power. This is one reason why transmitters are getting damaged when IBOC
is turned on. The reflective power goes up over the increased power
bandwidth IBOC requires.

That is not why the very few cases of transmitter damage have occured. Most
transmitters will simply shut down over reactive or mismatched loads. The
issues reported (and one that took out 80% of the power modules at KTNQ) had
to do with the control interface of the HD exciter and the analog
transmitter.... a design defect in non-rf and non-af circuitry.

Several things happen when the reflected power goes up. Mainly the power
does not go into the antenna to be radiated and instead heats the
transmission line and transmitter finals. Another bad effect is the
reflected power can make the transmitter unstable and generate spurious
energy.

Or, in today's transmitters, it does neither. It detects VSWR and shuts off.
..

This method was used by a few high power AMs in Latin America in decades
past, ones like XEB and XEW. The rest, if they have a tower (many use
inverted L's of wire) use series fed towers. Since many towers are
diplexed
and even triplexed, a rejection network is required and that requires an
ATU. Shunt fed towers are generally half wave or similar, and shunt
feeding
is not and has never been common with quarter wave or less towers.

I have visited every AM in Mexico City, and only 3 had shunt fed towers
in
1963... today, I believe only XEW has one. In Colombia, I have visited
about
20 50 kw or higher sites, and none was shunt fed. In Ecuador, today and
in
the past, no station was shunt fed. Of the several hundred stations I
visited in Central America, none was shunt fed. The most powerful AM in
Argentina, Radio 10 on 710 with 100 kw, with a nice half wave tower, is
series fed.

The only other Latin American shunt fed I know about was CB106 Radio
Mineria
in Santiago. That station, saying AM was no longer viable in Chile,
turned
in its license and turned off the 100 kw transmitter nearly a decade ago.

You sure get around.

Yes, I have worked in every nation in Latin America save Cuba and Nicaragua,
and visited stations in each one. There are pictures of dozens and dozens
stations I took, including some shunt fed ones, on my web page, from trips I
made visiting stations.

The fact is you have said that shunt feeding is common in Latin America
where it is highly uncommon and always has been. You made statements about
costs, land and towers in Latin America that make it sound like you think we
are talking about another planet, not the same Hemisphere. In fact, the
conditions and terrain in South Florida are more difficult and hostile than
in most places in Latin America.