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Re: LF: Puckeridge Experiments

To: [email protected]
Subject: Re: LF: Puckeridge Experiments
From: [email protected]
Date: Thu, 18 May 2000 23:31:18 EDT
Reply-to: [email protected]
Sender: <[email protected]>
In a message dated 5/18/00 8:52:58 AM Eastern Daylight Time, [email protected] 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


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