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

If you really were "frequently called on to tune towers" then you must
have had a method. Maybe you could explain this tuning method. There are
always times when you go to tune some circuit or box or tower and it
does not tune up right. Maybe you have a notable example of when things
did not go right and you had to change methods or trouble shoot the
tower/coupling circuits before it would tune up?

The method for tuning a tower is to design based on wavelength a
theroretical ATU tuning circuit based on line impedance (typically 50 or 52
ohms for coax fed towers) and the calculated base impedence of the tower.
Then, using either the actual transmitter, or an RF signal generator, RF is
fed to the tower throug an OIB and the impedence and reactance are measured.

Frequently you have a close match, and only slight adjustments of the coil
in the ATU are needed (most ATUs use strappable coils, but fixed vacuum
caps) will bring it into match. One of the issues that makes tuning harder
today is the desire to have the tower as broadband as possible. Many older
ATUs had an easy to adjust high Q network, but today most stations want a
broader bandwidth ATU, which makes the best tuning point harder to find.
Many engineers will begin with an OIB read at points a quarter of the way
from each end of the coil's winding.... this gives you an idea of which way
to go to get a match. Once the tuning area is reduced, then "half way
between" steps are usually used.

Most ATUs are not built by station staff. One either provides a measurement
made with an RF generator and a bridge (Some bridges have an RF generator
incorporated) or the description of the tower in electrical degrees at the
frequency and the fabricator, like Kintronics, will ship an ATU built for
the tower.

On directionals, most are designed by consulting engneers, and the phasors
are built by one of two or three fabricators like Kintronics. While not used
in the US, many simple directionals elsewhere are done by means of a dual
coax feed, of equal electrical length, which goes to each of two towers,
with just the spacing in degrees determining directionality.

This does not adress towers tuned by methods other than series fed base fed
towers. There are shunt fed and unipole antennas, both of which are not
insulated from ground at the tower base. And there are direct fed antennas,
mostly foded L's and T's, that are fed without an ATU right off the final
tank circuit of the transmitter, with the vertical component of the L or T
being the radiator and the horizontal portion becomeing a top hat or "top
load" to simulate greater electrical height.

My tricks for tower tuning included, 1. do not do it when there is a single
cloud. 2. do not do it when there are any atmospherics. 3. wear boots at all
times. 4. have a positive indication, such as a light bulb on a long cord,
whether the transmitter is plates on or plates off 5. never do this work
alone. 6. when I was doing it, I always carried my CEI slide rule for
calculations 6. watch out for cattle, goats and, especially, geese. Geese
bite.


You gave a very weak "someone else's explanation." I'm not asking hard
questions of you. I just want to know whether you understand the
terminology you use to make a point. You make many posts to the news
group. You should know what you are talking about. I suggest you sit
down with an engineer and have him explain them to you.

Actually, after I posted, I went to our engineering office to get the NAB
Engineering Handbook, and brought up the simple question "what is mV/m"
since it is a daily use term; both of the engineers present gave the same
definition I gave you: it expresses the field strength of a signal at a
particular point or along a particular contour.

And these are engineers who install and maintain and modify the transmitters
for 5 stations (All with HD), two of them with backup sites, 16 studios,
network feeds to one 28 station and another 12 station network, dozens of
remotes, remote TV studios for out talent who are on TV daily, and all the
related routers, processing, redundant STL systems, earthquake and disaster
preparedness installations like alternate studios and genny sets as well as
a 50 kw 5 tower, 2 pattern directional that is diplexed with another high
power AM only 130 kHz higher on the band... all of which use a counterpoise
ground that has acres of copper webbing, silver-soldered and clamped, 12
meters above ground!