ERP and ERP

[Andre' Kesteloot]

WarmSpgs@aol.com wrote:

> In a message dated 5/18/00 8:52:58 AM Eastern Daylight Time,
> j.r.moritz@herts.ac.uk writes:
>
> << ERP will be calculated on the basis of estimated radiation
>  resistance and measured antenna current for each antenna. For
>  the inv. L this should be about 20 milliohms and 2A, for the main
>  mast about 0.7 ohms and 0.34A, both giving about 80mW radiated
>  power in theory.
>
>  By definition, two transmitters/antennas giving the same ERP will
>  yield the same signal strengths at equal distances. But a number of
>  things may modify the actual ERP obtained ... >>
>
> Please don't misunderstand my earlier inquiry, Jim.  I think this experiment
> is worthwhile and may yield much useful experience.  I'm just concerned about
> the seemingly rather casual use of the term ERP, and the tendency to equate
> it with power dispersed in radiation resistance.  These are very different
> things.
>
> ERP _is_ actual ERP, and can only be actual ERP.  That is, ERP is determined
> by actual field strength.  It therefore already includes antenna and nearby
> environmental losses; hence, equal field strength is the only way to actually
> say the ERP of two systems are equal, not the power in the radiation
> resistance.  (Measuring the signal out of two different systems with equal
> powers in the radiation resistance is instructive, of course, because it does
> permit one to compare the interaction of two different radiators with their
> environments.  I take this to be your fundamental objective.)
>
> Someonce commented earlier--I was thinking it was Rik, although I can't find
> the post now--that there were those who believe that one watt ERP from one
> kind of antenna is better than one watt ERP from another kind.  I got a good
> chuckle from that, but actually there is a certain grain of truth to the
> concept...particularly when the radio authorities set a maximum ERP limit on
> transmissions.
>
> This means that the field strength in ANY direction or elevation cannot
> exceed so many mv/m at so many km from the antenna.  (Different
> administrations may choose to define the distance differently for their own
> regulatory purposes, and the corresponding field strength will vary inversely
> with that part of the definition.)  The key words in the definition of
> maximum ERP, though, are usually "maximum field" in "any direction" from the
> antenna.  I supervise a number of FM broadcast stations, by way of showing
> why this is significant.  One has a maximum ERP of 100kw, yet covers less
> than half the geographical area of another one that also has a maximum ERP of
> 100kw, over similar terrain.  Why?  Because the former uses a directional
> antenna.  The _maximum_ ERP is indeed 100kw in the main lobe, but is less in
> all other directions around the antenna than its omnidirectional counterpart.
>
> If any two given antennas have the same 1w ERP in the same direction, and it
> happens to be the direction you need to reach, then all is well.  You have
> equal field strength...in that one direction.  But, if that maximum 1w ERP
> happens to be in a less useful direction from one antenna (say, straight up)
> than the other, then yes, 1w ERP produced by one antenna can be different
> from 1w ERP produced by another!
>
> Neither any theory nor practice that I can find indicates two electrically
> short verticals (one roughly .004 wavelength and the other .04 wavelength,
> for instance) would produce any different radiation pattern.  If you can
> force enough power through them to achieve the same ERP, it will be achieved
> in all directions, barring local obstructions.
>
> Your experiment, however, is not so much about big versus small, but between
> antennas of differing shapes.  That's what I think is interesting: can
> amateurs make up for terrible inefficiency by directing their energy with
> other geometries?  And if so, can it be done predictably, or will we always
> be imprisoned by trial and error?  I'll be watching with keen interest.  Good
> luck.
>
> 73,
> John KD4IDY