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
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|