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Re: [Amps] power into 2 MHz antenna

To: jtml@lanl.gov, amps@contesting.com
Subject: Re: [Amps] power into 2 MHz antenna
From: G3rzp@aol.com
Date: Wed, 23 Mar 2005 02:12:28 EST
List-post: <mailto:amps@contesting.com>
 
In a message dated 22/03/2005 19:58:05 GMT Standard Time, jtml@lanl.gov  
writes:

Use a  Delta Electronics TCT current transformer to get the current 
component. Or  use a Pearson current transformer. Use a voltage 
divider (like the  Jennings glass vacuum capacitor unit) and a meter 
with 2 MHz accuracy  (such as a good old Hp 400C) to get the voltage 
component at the base.  These would give you the VA. Use a network 
analyzer, impedance meter,  antenna analyzer, or the knowledge of 10 
ohms in series with 250 pF if  that is accurately determined)  to get 
the phase angle, and use that  to correct the calculation for real 
watts.



But the question is: what accuracy?
 
Now this wasn't such a hypothetical case. For Type Approving marine  
transmitters, that was the load into which you measured the transmitter.  
Remember 
it's also several kV of RF, and you have to be very careful with  capacitive 
currents. The usual method was to use a thermo-ammeter, but when you  had a 
customer (as we did) who wanted 400 watts indicated and not 390 (!), it  got a 
bit 
fraught.
 
For a more practical view, the UK law (Merchant Vessels Radio Regulations)  
said that if you had an antenna where the horizontal top was more than twice 
the  vertical, then if the product of the length of the vertical in meters 
times 
the  antenna current in amps exceeded 7.5, you could communicate over 75 
miles by day  on 2182kHz! If the top was shorter, then the product had to be 
12.5.
 
But as we said earlier, for us, does it matter? And  the answer in  most 
cases is 'probably not'. Just as I don't worry about SWR provided it's  a) 
below 
2:1 (there's the LP filter to think of), and more importantly b)  hasn't 
changed appreciably since last time I tuned up. 
 
A well calibrated power meter in the milliwatt range would be nice, though,  
for confirming the signal generator calibrations. Admittedly, all 6 of them 
are  within a 2dB window, and they tie up in that respect with the spectrum 
analyser  calibrator, but still, it would be nice.....
 
73
 
Peter G3RZP
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