Re: LF: 500kHz ERP

[MarkusVester@aol.com]

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 james.moritz@btopenworld.com: 

> 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