[BC] WSM Tower

[Dana Puopolo]

850 in Boston had 1/2 wave towers that also had 10 foot faces. I used to hear
significant selective fading due to high angle radiation in a zone between 35
and 50 miles from the transmitter at night. During the fades, KOA was clearly
reciiveable. The preiod fote se fades was (is!) about 1/2 hour and they peak
out around Midnight.

I'm willing to bet that shortening their towers by 60 feet or so would make a
big difference!

-D
 

------ Original Message ------
Received: Thu, 28 Sep 2006 11:48:26 PM EDT
From: PeterH5322 <peterh5322 at rattlebrain.com>
To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
Subject: Re: [BC] WSM Tower


>The best overall AM antenna (I believe) is a half wave tower. Decent gain on
>the horizon and no high angle radiation. Unfortunately, many half waves wind
>up being more then they were designed to be, due to their girth...

Of course, for any given physical height, the electrical height will be 
taller, for a radiator which is thicker than "infinitely thin".

5/8-wave (225 degrees) is just about the WORST choice for a Class A 
station, yet at least two were built that way, both Blaw-Knoxes, although 
they were later truncated to under 200 degrees [ * ] .

The best-performing ND vertical is 195 degrees tall, which is slightly 
shorter than 5/9-wave (200 degrees).

The average electrical height over all ND-U Class As is also 195 degrees.

195 degrees produces the maximum radiation in the horizontal plane 
without also producing significant radiation in the vertical plane (which 
causes fading). About 400 mV/m/kW at 1 km.


[ * ] One Class A is still significantly over 200 degrees, but it is 
grandfathered at 362.10 mV/m/kW at 1 km, which means it has to dissipate 
about 15 kW into a resistor in order to reduce the efficiency of that 
tall radiator to the equivalent of about 170 degrees.

And, one Class A has a 225 degree radiator in its day and night DAs, but 
that radiator is a hold-over from a now co-owned Class B with a frequency 
which is about one-half of the Class A's frequency. It is already tall at 
790, so it is super tall at 1510.
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