Topband: EIRP Measurement

W2XJ w2xj at
Tue Feb 21 13:17:04 PST 2012

In broadcast work the antenna impedance is first determined by 
calculation (now modeling) then the actual impedance are measured when 
the station is tuned up. Loading coils are not used at least not by that 
name. The current is measured after the antenna matching unit which is 
often a T network but could easily be an L or even PI.. In broadcast 
rules a loss resistance of one ohm is assumed and the maximum allowed 
but all losses attributed to matching are eliminated due to the 
measurement location.. This will be an issue in amateur operation since 
the actual losses would be higher and more difficult to determine. A 
short vertical will show a gain of about 4.78 dbi regardless of height. 
Tapering and top loading are taken into account when the feed point 
impedance is measured.

At least in the US and probably Canada the FCC and Industry Canada might 
possibly used a simplified version of the broadcast methods they have 
used for over 80 years. It could be something like assuming a 4.78 db 
gain over isotropic, base impedance measurements and ignoring ground and 
other system losses. This would assure that no station would actually 
exceed EIRP and is easy to measure upon inspection.  It would be 
necessary for each amateur to develop a chart that equates allowable 
antenna current to frequency as the drive impedance could change 
drastically on such a short radiator.

On 2/21/12 3:31 PM, Rick Karlquist wrote:
> W2XJ wrote:
>> This method is virtually universal for MW power measurement. There are
>> calibrated RF ammeters available since they are required at each AM
>> broadcast station (directional stations may use as many as a dozen
>> depending on the array). Alternatively the voltage can be measured. In
> Why is this method bulletproof?  So you measure the RF current, which
> by the way is different below the loading coil than above the loading
> coil.  How do you determine the radiation resistance of the antenna?
> By modeling?  You have to correctly take into account tapering,
> top loading, etc.  How do you know the counterpoise isn't radiating?
> Rick N6RK
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK

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