Re: NEC-LIST: Why coil-load?

Something which is often overlooked is the fact that short monopoles
often are placed where the ground resistance is high compared to the
radiation resistance, as on a vehicle. I believe it's often the case
that the ground resistance is substantially greater than the resistance
of even a mediocre coil. In those cases, it makes no sense to devote any
effort to improving coil Q, since the inductor resistance is only a
fraction of the total loss resistance.

To answer your question about doubling the efficiency of a short mobile
whip, I believe it can perhaps be done by mounting the whip on a larger
vehicle, but not by improving inductor Q. And although I'm not an expert
at it, I believe that the most significant improvements in the
performance of short mobile whips, other than mounting them on large
vehicles, has been done by raising the radiation resistance rather than
trying to lower the loss resistance. Top loading gives the most
improvement, with results from moving the coil upward from the base
typically bringing only small improvement as far as I know.

One further comment -- the 10% improvement in efficiency that you noted
is only 0.4 dB, insignificant for any purpose I know. Considerably
larger improvements have to be made to make any significant difference
for transmitting or VHF/UHF receiving, and won't likely make any
difference at all for HF receiving.

Roy Lewallen

D. B. Miron wrote:
> Good day all,
>
> The reason I asked about maximum practical Q is that I've
> done a couple of examples of coil-loaded whips, and of
> course the higher the Q the better the performance. The
> first example was a 0.038 wave monoppole on perfect ground.
> I chose this because I have a 1973 paper by C. W. Harrison
> that gives a table of computed values based on analytical
> models. With a Q of 450, base-tuning has an efficiency of
> 11 % and center-loading has an efficiency of 12.3 %, about a
> 10 % improvement.
>
> The second example was a wh8ip on a car. The model meets
> NEC guidelines and has a good APG values in free space and
> over perfect ground. The antenna is 1 m tall, 12.7 mm
> diameter, and sits in front of the forward window post. I
> intended to tune the antenna for 30 MHz and 90 MHz to cover
> the Citizens' and public radio FM bands. The whip has 11
> segments and the source is in segment 2. Using just Q=450
> coils and tuning for 30 MHz gives an efficiency of 91.6 % at
> the source and 91.82 % at segment 5. Using a parallel LC to
> tune for both frequencies gives an efficiency of 79.7 % at
> the source and 80.13 % at segment 6.
>
> I suppose the 1 m whip is not sufficiently electrically
> short that the coil-loading improves its radiation
> resistance much. I should try the no-loss case for
> interest. Anyway, I conclude from these experiments that
> coil-loading is not enough better than base-tuning to
> justify the extra fabrication time and cost. Is there a
> practical case where the efficiency of a short mobile whip
> has been doubled, for instance?
>
> Regards,
> Doug Miron
>

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