Re: [IRCA] 1130 - interference reported by CKWX

["HASCALL, DAVID CIV DFAS" <DAVID.HASCALL@xxxxxxxx>]

Thanks Deane (I removed the extra "n" that I put in your first name,
sorry),

I have created an excel template for my figuring these easily.

I think that I may see an error in your 240 degree example - I think
that it is 64.6 watts, not 646.  Is that right?  79.86/1571.07 =
0.050832.  0.050832 squared is 0.00258. 0.00258 X 25000 =64.596.

Thanks again and 73,

Dave in Indy

-----Original Message-----
From: Deane McIntyre [mailto:dmcintyr@xxxxxxxxxxx] 
Sent: Thursday, September 30, 2010 5:27 PM
To: HASCALL, DAVID CIV DFAS; Mailing list for the International Radio
Club of America
Subject: Re: 1130 - interference reported by CKWX


On 2010-09-29, at 3:49 PM, HASCALL, DAVID CIV DFAS wrote:

> Thanks Deanne.  Inquiring minds want to know (maybe just me, hi), how
> can you figure ERP from power (25 Kw) and the numbers from the plot
> info?  That'd be good info to know (i.e. I know that my local WNDE
1260
> is stronger at night and want to know what the ERP is versus the
actual
> 5 Kw, transmitted).
> 

The values in the FCC tables and polar graphs are the electric
field strength in millivolts per meter at one kilometer from the antenna
array. The RMS value is what the field would be from a non directional
antenna. Two sets of values are given, std (standard) and theo (theory);
in some cases  third value is given, aug (augmented) which does not
concern
us here.

As the field strength is proportional to the square root of the
transmitter
power, if follows that the power is proportional to the square of the
field strength.

Thus to get an estimate of the effective radiated power alone one
azimuth one takes
the ratio of the field strength along that azimuth (from the chart or
estimated from
the polar plot) to the RMS value and squares that number, which in turn
is multiplied
by the transmitter power to the array. It does not really matter if one
uses the std or theo
values as we are looking for an estimate only.

For example:

KPWX-1130 operating at 25 kW the RMS std value is 1571.07 mV/m at 1 km

At an azimuth of zero (straight north, towards CKWX) the std field is
2248.79 mV/m at 1 km

(2248.79/1571.07) squared is 2.0488 x 25 kW = 51.2 kW ERP to QRM CKWX

At a azimuth of 240 (middle of the null)  std is 79.86 mV/m at 1 km

(79.86/1571.07) squared = 0.00258 x 25 kW = 0.646 kW = 646 watts

73,

Deane McIntyre VE6BPO





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