Thanks for your input, Marcus. You usefully considered the issue from a number
of perspectives.
73 John, G3WKL
[email protected] wrote :
> Hi John and LF,
>
> having never transmitted and only sparadically monitored 500 kHz, I am a
> little reluctant to state "outsider" opinions in the ongoing UK power limit
> discussion. Anyway from a more technical perspective, I tend to very much
> support Jim's views and favour an ERP based limit, if possible on the order
> of one to ten watts.
>
> - Innovation: In my opinion, many of the new concepts for LF communication
> have been centered around optimizing the receive side. FFT-based and coherent
> reception techniques are now widely employed by amateurs, which would not
> have been the case if signal strength was not so much limited. Unidirectional
> receive antennas and noise cancellation approaches are being explored and
> used. On the other side, maximizing antenna size is not a new art - large and
> efficient antennas have been the standard method for broadcasting and
> commercial communications.
>
> - Equal opportunities: As Jim pointed out, a transmitter power based limit
> would put many of us who have limited space available practically out of
> business. This discrimination is probably more severe in the low and medium
> frequency range, compared to HF where even a tiny or makeshift antenna can
> have reasonable efficiency.
>
> - Learning: It has been stated that the requirement of calibrating one's
> antenna efficiency adds an additional burdon on the operator, compared to
> simply measuring TX power - agreed. On the other hand, I think this is a good
> thing: having to learn these techniques, we are also led to understand a lot
> more of how an antenna works, and what the basic physical limits are.
>
> - Compatibility and supervision: There is no way of measuring TX power from a
> distance, and all potential spectrum compatibility issues in the far field
> would have to be based on ERP assumptions anyway. Limiting transmitter power
> would seem a bit like imposing a speed limit based on horse power rather than
> miles per hour... However I have to concede that in close proximity of a
> small antenna driven by high power, significantly higher reactive nearfields
> will be present, which may impose additional limits from electromagnetic
> compatibility issues.
>
> Best wishes to all
> Markus, DF6NM
>
>
> In einer eMail vom 24.05.2007 20:02:55 GMT-Normalzeit schreibt
> [email protected]:
>
> Dear John, Rik, LF Group,
>
> I would agree with Rik's comments. Experimentation on 136k (and 73k) over
> the last several years has given us a pretty good idea of how to optimise
> the efficiency of small LF antennas, but it has also shown that, once "best
> practice" has been employed, one quickly reaches a point of diminishing
> returns where further improvements have negligible effect on the radiated
> signal level. Beyond this, significant improvement in antenna performance
> can only really be achieved by increasing the physical dimensions of the
> antenna, which is usually not feasible for non-technical reasons. The same
> is certainly true for 500kHz, although the antennas are effectively a bit
> better to begin with. So however much one may strive to improve antenna
> efficiency, and whatever incentives there are, beyond a certain point it
> just isn't possible without discovery of some revolutionary and unknown new
> antenna design principle, which one cannot rely on ever happening (I
> hesitate to mention "Poynting vector synthesis"...). The 1W ERP limit on
> 136k has given us a fair amount of freedom and flexibility and has led to
> some interesting technical challenges and solutions (e.g. how to
> economically generate a lot of TX power and feed it into a tiny antenna
> without something going up in smoke :-) ) - I don't think the last several
> years on LF would have been nearly so rewarding if TX power had been limited
> to 10 or 20 watts as is currently being suggested for 500kHz.
>
> Cheers, Jim Moritz
> 73 de M0BMU
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