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Author Topic: What do the commercial radio stations use for an antenna?  (Read 2162 times)
WA9CFK
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Posts: 174




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« on: December 01, 2017, 03:51:13 PM »

I believe the commercial AM radio band starts a 530 KHz. I was wondering what they use for an antenna.

What kind of loading coil do us use for a clear channel 50,000 watt signal?

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K0OD
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Posts: 2991




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« Reply #1 on: December 01, 2017, 04:34:07 PM »

Do you mean clear channel or CLEAR CHANNEL?

A ham-oriented tour of WLW's old 500 KW (no typo) AM transmitter in Ohio.

https://www.youtube.com/watch?v=CbHjcwIoTiY
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G8YMW
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Posts: 661




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« Reply #2 on: December 02, 2017, 06:46:04 AM »

If you have a single frequency transmitter and you are not restricted (well, not much) on aerial size, what do you want with a loading coil?
They only add in losses.
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73 de Tony
Windows 10:  Making me profane since March 2017
WB6BYU
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Posts: 17192




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« Reply #3 on: December 02, 2017, 07:33:07 AM »

While 0.528 wavelengths is considered an optimum tower height to
minimize fading problems at night, many smaller stations with a lower
budget get by with a 60 degree ( 1/6 wavelength ) "stick" matched
with a physically large loading coil in the tuning shack at the base of
the antenna.

Few hams are going to put up a 300+ foot tower just for one band.

Probably a more apt comparison would be NDBs (aircraft beacons) with
a much lower budget:  they are very often "T" antennas with moderate
dimensions, even below 300 kHz.  There are a number of photos available
from NDB enthusiasts on line.

A good reference is Laporte's Radio Antenna Engineering, available
for free on line.  Jasik's Antenna Engineering Handbook is another
source, though not as comprehensive (but covers a lot of other antenna
types as well) if you have access to a copy.
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WA9CFK
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Posts: 174




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« Reply #4 on: December 02, 2017, 08:31:49 AM »

A half wave at 530 KHz is over 900 ft.  Not too many of those around.

Even at the high end of the AM band, the tower would be over 250 ft. at a half wave. Doable but not cheap.

I would like to see one of those low end loading coils.  Shocked
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WB6BYU
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Posts: 17192




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« Reply #5 on: December 02, 2017, 01:44:43 PM »

Quote from: WA9CFK

I would like to see one of those low end loading coils.


Check page 189.  For scale, the window with the feed-through insulator is probably around
2 feet square, so the coils on the right probably stand at least 6' high.
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KC2QYM
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Posts: 860




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« Reply #6 on: December 07, 2017, 08:02:15 AM »

I visited a local radio station in my area and had a chance to see the antennas in use up front.  Apparently the entire steel tower structure for the 50KW station is the antenna.  It's insulated from the cement base and the cable guy wires that I always thought were the antenna are actually just the supporting guy wires.  There was a coil house adjacent to the antenna which was actually the coupler (tuned circuit) used to keep the SWR low and balance the load.  OK so I'm not the most technical in this description but I must admit I had some preconceived notions of what an AM broadcast antenna looked like but came away from the tour with a different perspective.
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KC2QYM
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Posts: 860




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« Reply #7 on: December 07, 2017, 08:05:57 AM »

Addendum to my last...The coil house was actually a house that you walked into.  The huge coils were enclosed in a metal meshed Faraday cage to keep stray RF from getting out.  The coils appeared to be made from half inch copper tubing...not wire.
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N0NB
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Posts: 143


WWW

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« Reply #8 on: December 07, 2017, 04:58:37 PM »

In addition to stations with a single vertical radiator for an omni-directional pattern, other stations are required to have irregular patterns so multiple towers are phased to the main radiator to obtain the specified pattern.  Some stations must change patterns multiple times per day as well as adjust power.

Wikipedia has some information on the antennas of AM broadcast stations:  https://en.wikipedia.org/wiki/Mast_radiator
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73, de Nate
Bremen, KS

SKCC 6225
AA2UK
Member

Posts: 380




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« Reply #9 on: December 13, 2017, 10:37:59 AM »

Not a commercial AM station but 50kw longwave station in Sweden.
http://home4.swipnet.se/~w-41522/lfpics/lfpics.html
Bill, AA2UK
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KM1H
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Posts: 2644




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« Reply #10 on: December 13, 2017, 06:14:25 PM »

Quote
In addition to stations with a single vertical radiator for an omni-directional pattern, other stations are required to have irregular patterns so multiple towers are phased to the main radiator to obtain the specified pattern.  Some stations must change patterns multiple times per day as well as adjust power.

Some used a passive reflector to steer a null, see the WFEA info on
http://j-hawkins.com/blaw-knox.html#Blaw-Knox%20Ads

http://www.manfrommars.com/wfeatowers.html

That station is still on the air with those towers. The old AM TX with a pair of 4-1000A's each in the RF and modulator was my primary source of tubes when I was building ham amps on the side,  Long been all SS now.

Also note only 64 radials are used on the Blaw-Knox radiator and not the so called 128 FCC mandate.

Carl

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KB2IUA
Member

Posts: 70




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« Reply #11 on: December 14, 2017, 04:44:16 AM »

I was the Engineer for a few AM stations over the years. While there are changes to the plan at many build-outs, there is a basic plan. For you basic non-directional station the whole tower is the vertical radiator. The tower is not grounded but sits on one or more insulators depending on the tower design. Just underground at the base of the tower is a mesh made of heavy copper and a copper strap ring. from the copper ring 120 copper radial wires go out ward from the tower for 1/4 wavelength or to the property line, whichever comes first. There is a matching network at the base of the tower to match the feed line to the tower so that the transmitter sees 50 ohms with no reactance. The tower is normally 1/4 wavelength but can be 5/8 or less then a 1/4 at low frequencies.

AM stations with more then one tower are either protecting another station from interference or pushing the signal to the population center, or both. Here is a link to an article on the basics of AM directional stations:
  http://www.radiomagonline.com/deep-dig/0005/directional-antenna-basics/33911
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KB2IUA
Member

Posts: 70




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« Reply #12 on: December 14, 2017, 05:03:54 AM »

Here is a link the the next article. It covers the splitting of the RF to the towers and matching:
http://www.radiomagonline.com/misc/0082/understanding-tee-networks/34082
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K7AAT
Member

Posts: 22




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« Reply #13 on: December 14, 2017, 07:00:09 AM »

A newer type of antenna is starting to be used by AM broadcasters:

http://www.nab.org/xert/scitech/pdfs/rd020909.pdf

We got the very first one in 2009 in Florence, Oregon.

K7AAT
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W9IQ
Member

Posts: 1715




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« Reply #14 on: December 14, 2017, 07:32:50 AM »

These antennas are 4 short, top loaded verticals fed with 1/4 wave, paralleled phasing sections to yield a 50 ohm feedpoint impedance. With proper construction techniques the efficiency can be quite high. Due to the close proximity of the vertical elements, it acts like a cage antenna.

Something to think about for 630 meters, et al when you are height limited and looking for an omni antenna.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
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