Topband: EIRP Measurement

[W2XJ]

Ahh but I think the FCC will see things much differently. I base this on 
the existing 60M regs where an ERP is specified:

(i) No station may transmit with an effective radiated power (ERP) 
exceed- ing 50 W PEP on the 60 m band. For the purpose of computing ERP, 
the trans- mitter PEP will be multiplied by the antenna gain relative to 
a dipole or the equivalent calculation in decibels. A half-wave dipole 
antenna will be pre- sumed to have a gain of 1. Licensees using other 
antennas must maintain in their station records either the an- tenna 
manufacturer data on the an- tenna gain or calculations of the an- tenna 
gain.

This can give a pretty good indication of how the FCC thinks. I think  
600M installations might be allowed to assume a certain amount of losses 
and a means to measure power into the radiating element. They will not 
create a regulatory nightmare. Notice in the case of 60M the feedline is 
not considered. Any losses their are at the amateur's expense. The same 
will likely be the case for 600 M ground loss. There are possibly a 
handful of amateurs in the world that will be able to install a full 
ground system and will gain an advantage but the rest will just have to 
be as clever as possible in reducing losses.

The height of the tower is not that important as the radiation pattern 
does not change from 1/4 wave for a short element. What does happen is a 
dramatically lower resistance resulting in higher losses. Matching 
systems losses are unimportant since they would precede the point of 
measurement. One would expect transmitter powers of at least 100 to 200 
watts in order to achieve to EIRP limit.

Many broadcasters would gladly operate with much shorter towers and less 
ground if the FCC would allow. But the FCC is and always has been all 
about what makes it easier for them to administrate.

On 2/21/12 7:36 PM, Joe Subich, W4TV wrote:
> 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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