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Author Topic: What do the commercial radio stations use for an antenna?  (Read 1235 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
Member

Posts: 2982




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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
Member

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
Member

Posts: 17170




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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
Member

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: 17170




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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
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