Topband: EIRP Measurement

[Joe Subich, W4TV]

On 2/21/2012 4:17 PM, W2XJ wrote:
> 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 methods used in broadcast work are not applicable to amateur use
for a lot or reasons.  First, broadcast towers are generally a far
greater fraction of a wavelength than 0.05 wave where an amateur
antenna for 630 meters will almost certainly be no more than 0.05
wave.  Second, broadcast ground systems are extensive - 120 half
wave radials ... an amateur ground system for 630 meters is likely
to be no more than .04-.05 wave!  Losses in the ground system alone
- not counting losses in the matching system are going to be very
high.

The only way an amateur is going to get *accurate* EIRP numbers is
to measure the field strength at 1 KM or other known distance well
beyond the near field/far field transition range - that means 1 km
or more and compare that to the theoretical field intensity for 5W
EIRP.

Anyone who does not have the capability to make accurate field
intensity measurements will simply be guessing.

73,

    ... Joe, W4TV


On 2/21/2012 4:17 PM, W2XJ wrote:
> 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
>>
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