Richard,

Both you and Reg have given excellent answers and I thank you both.

By the way, you mention directional patterns and nulls. Is there any place I
can find polar graphs of commercial broadcast station's antenna patterns?

Thanks,

Dave P.
====================


"Richard Harrison" wrote in message
...
Dave P. wrote:
"Why is it that I can receive WCBS @ 880 kc fairly well at night but
WABC @ 770 kc suffers from phase distortion like you wouldn`t believe?"

I havn`t looked up the directional patterns of the two stations and
don`t know your location. If you should happen to be at the edge of a
null in the nighttime directional pattern of WABC, that would likely
cause distorted fading. WABC may be clear-channel non-directional day
and night for all I know. Not many of these remain in the U.S.A. now.

At 175 miles from both transmitters, you suffer interference between the
ground wave and sky wave from either transmitter at night, at least
occasionally. You probably have solid daytime reception from both
stations, but at night, the signal may be stronger, though variable. The
sky wave is susceptable to variations in the reflecting layers of the
ionosphere at night. These are a function of frequency, reflecting
carrier and sidebands differently at times. This can produce
overmodulation at times in the received signal. Another factor is likely
other stations on the same or adjacent channels which may fade in and
out and cause variation from your automatic volume control action even
when the interfering stations can not be readily identified. An Adcock,
loop, or other directional antenna may produce a big improvement in
reception of the desired signal.

Finally, WABC is owned by the Walt Disney company. Maybe you should
expect Mickey Mouse performance.

Best regards, Richard Harrison, KB5WZI

There are undoubtedly official sources from the FCC, etc.

The best hand-held guide I could recommend would be the National Radio
Club's "Night Pattern Book," a fantastic resource for MW DXers and
broadcast listeners. Basically it is a book of maps of North America
for each domestic broadcast frequency, with dots representing
transmitting locations, and the night time radiation pattern around each.

It's available from the following link:

http://www.nrcdxas.org/catalog/books/

The 5th edition is sold out, but the new 2005-06 edition is scheduled to
be out soon.

Brent Taylor
VE1JH


Dave Pitzer wrote:
Richard,

Both you and Reg have given excellent answers and I thank you both.

By the way, you mention directional patterns and nulls. Is there any place I
can find polar graphs of commercial broadcast station's antenna patterns?

Thanks,

Dave P.

There are undoubtedly official sources from the FCC, etc.

The best hand-held guide I could recommend would be the National
Radio
Club's "Night Pattern Book," a fantastic resource for MW DXers and
broadcast listeners. Basically it is a book of maps of North
America
for each domestic broadcast frequency, with dots representing
transmitting locations, and the night time radiation pattern around
each.

It's available from the following link:

http://www.nrcdxas.org/catalog/books/

The 5th edition is sold out, but the new 2005-06 edition is
scheduled to
be out soon.

Brent Taylor
VE1JH


Dave Pitzer wrote:
Richard,

Both you and Reg have given excellent answers and I thank you
both.

By the way, you mention directional patterns and nulls. Is there
any place I
can find polar graphs of commercial broadcast station's antenna
patterns?

Thanks,

======================================
Dave,

http://www.nrcdxas.org/catalog/books/
Sounds exactly what you are looking for but may take some time to
obtain.

In the meantime, the basic groundwave radiation patterns of mediumwave
broadcast antennas are either simple circles with the antenna at their
centres, or heart-shaped with the antenna at the null.

The first occurs when the antenna is a single vertical mast located
near the centre of a large populated area.

The second occurs when the antenna consists of a pair of masts, which
radiate a very broad heart-shaped beam, located on one side of the
populated area to be covered.

Contour Maps of actual measured field strengths are useful when the
basic groundwave patterns are distorted by the terrain, e.g., the
existence of mountains, forests, rivers, built-up areas, high-rise
cities, or seas, lakes or coastal regions.

Radio frequency Field Strengths are usually measured in terms of
"millivolts per meter" or in decibels relative to one volt per meter.
----
Reg.

snip
In the meantime, the basic groundwave radiation patterns of mediumwave
broadcast antennas are either simple circles with the antenna at their
centres, or heart-shaped with the antenna at the null.

The first occurs when the antenna is a single vertical mast located
near the centre of a large populated area.

The second occurs when the antenna consists of a pair of masts, which
radiate a very broad heart-shaped beam, located on one side of the
populated area to be covered.
snip
----
Reg.



Reg:

I'm afraid you're way behind the practice on this one. In the US, there are
many 4, 5 and 6 tower arrays providing as many nulls to protect co-channel
stations. The UK got off easy with nationalized broadcasting, where the
frequency and location was dictated by the government, and none of that
nasty capitalism interfered. Here, the commercial interests are still
fighting it out. Check and see if the KLIF website shows their pattern from
a linear array of 5 towers just outside Dallas.

--
Crazy George
W5VPQ
My real address is my ham call atARRL.NET The ATTGlobal is a SPAM trap.

"Crazy George" wrote
I'm afraid you're way behind the practice on this one. In the US,
there are
many 4, 5 and 6 tower arrays providing as many nulls to protect
co-channel
stations.

=================================

What proportion of US MF broadcasting stations have antennas
consisting of more than two towers ?
----
Reg.

Reg, G4FGQ wrote:
"What proportions of U.S. broadcasting stations have antennas consisting
of more than two towers?"

I don`t know but from my own experience, the number is large. A new
applicant for a station must show he will not interfere with existing
stations by limiting his radiation in the directions of the existing
stations while providing minimum field intensity, 0.5 to 50 mV,
depending on population, in the new service area. A two-tower array
cannot satisfy some complicated pattern requirements.

Most broadcasters want to provide more than 1 KW radiation in their
areas. Well over one hundred channels in the medium wave broadcast band
in North America allow that. There are well over 1000 regional medium
wave broadcasters in North America. It is difficult to fit a new
broadcaster in when he wants to use real power.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
I don`t know but from my own experience, the number is large.

I would guess that the majority of US AM antennas that I
have seen with my own eyes have more than one element.
--
73, Cecil http://www.qsl.net/w5dxp

Reg:

I don't have an accurate number for the entire country, but here in the
local area, there is one clear channel station with a single radiator, a
half dozen two tower arrays, two 3 tower arrays, five 4 tower arrays that I
can recall off the top of my head. If you had asked a dozen years ago, I
could have been more accurate. I don't think we have any 5 or 6 radiator
arrays here locally, if so, I don't remember them. If my math is correct,
that is a 50-50 split, so maybe half have more than 2 towers.

--
Crazy George
W5VPQ
My real address is my ham call atARRL.NET The ATTGlobal is a SPAM trap.
"Reg Edwards" wrote in message
...

"Crazy George" wrote
I'm afraid you're way behind the practice on this one. In the US,
there are
many 4, 5 and 6 tower arrays providing as many nulls to protect
co-channel
stations.

=================================

What proportion of US MF broadcasting stations have antennas
consisting of more than two towers ?
----
Reg.

"Crazy George" wrote
I don't have an accurate number for the entire country, but here in
the
local area, there is one clear channel station with a single
radiator, a
half dozen two tower arrays, two 3 tower arrays, five 4 tower arrays
that I
can recall off the top of my head. If you had asked a dozen years
ago, I
could have been more accurate. I don't think we have any 5 or 6
radiator
arrays here locally, if so, I don't remember them. If my math is
correct,
that is a 50-50 split, so maybe half have more than 2 towers.

=======================================

Thanks George,

I am amazed at the number of multi-tower MF antennas in the US. As you
say, they are necessary to prevent co-channel interference, day and
night, between a large number of broadcasters in the more densely
populated regions of your vast country. ( Antenna salesmen have had a
field day.)

It is also interesting that the whole system is technically regulated
by State and/or Central Government. It is not just a free-for-all for
newcomers.

I imagine the revenue comes solely from advertisers. Which makes me
wonder what percentage of program time is allocated to adverts. Are
such matters also regulated? Are any broadcast stations State or City
owned?

In this (UK) relatively densely populated country things settled down
about 20 years ago. Few MF antennas have more than one tower (or masts
as we call them). Although there is much broadcasting at MF for
individual cities, most broadcasting takes place at FM VHF where
'capture effects' reduce interference from co-channel transmitters.

It may be of interest that the BBC, still the World's finest
broadcasting system, including its overseas services, no longer owns
any transmitting stations, Mrs Thatcher quietly sold them off to a
private party. Do some Googles for who the eventual owners are?
----
Reg.

Dave P. wrote:
"Is there any place I can find polar graphs of commercial broadcast
stations?"

I have an old book, "Map Book, 540 kc to 1600 kc" published by
"Cleveland Insritute of Radio Electronics". In it, WABC is 50 KW
non-directional day and night. It shares 770 kc with KOB Albuquerque, 50
KW day and 25 KW night. Also, KUOM Minneapolis and WCAL Nortjhfield are
both 5 KW and share the frequency on some schedule between themselves.
WNEW St. Louis is on the frequency daytimes only, as is KWA Seattle, 1
KW. XEHB in San Francisco de Oro, Mexico is a 500 watt daytimer on the
frequency, as are XELM, 150 watts at Lagos de Morens and XEDI at
Queretaro, 1 KW. There is also CMDC, 1 KW at night when it could trouble
you in Holquin, Cuba. So, at night there is possible same-channel
interderence from New Mexico and Cuba. On 760 kc, you have WJR in
Detroit 50 KW non-directional at night and on 780 kc, you have WBBM in
Chicago 50 KW nondirectional at night. These non-directional 50KW
adjacent channel stations may exercise your AVC.

On 880 kc, WCBS has no same-channel night rivals but WLS (World`s
Largest Store, Sears in Chicago) on 890 kc, onetime home of "The
National Barn Dance", could work your AVC. Also, WWL in New Orleans
occupies 870 kc with 50 KW.
Good preselection will rid you of adjacent channel interference. I lived
in Portugal for years and listened to WCBS nightly. I would rock my
tuning from 880 to 870 for WWL during fades for my version of frequency
diversity. Both stations carried the same CBS programs. My antenna was a
Beverage aimed at New York. The receiver was a Hammarlund SP-600 which
had plenty of preselection to avoid adjacent channels. Ed Murrow came in
very well.

Best regards, Richard Harrison, KB5WZI