Re: NEC-LIST: Why coil-load?

Hello to All,

I do not know where we are going on this one. The Q-factor of the
coil is very difficult to measure, and if you succeed, then from a
measured base impedance one can deduce the ground induced loss
resistance, but not easily done with present day equipment.

In the 1950s I used a battery operated signal generator and receiver,
and a General Radio Impedance Bridge. Nowadays we have super
equipment to measure impedance of antennas, but impedance is
referenced to the instrument/vehicle ground --- and so you have to
have a DC-AC converter, and sit inside the vehicle to make the
measurement (which introduces other problems, see below) so the
ground reference is the vehicle ground --- isolated from the AC Mains
ground.

In my ARRL Antenna Compandium Volume 4 article, I address the subject
of ground loss resistance. The impedance measurements were made
right at the antenna (no feed through insulator) using a HP Impedance
Analyzer Model 4194A, powered by the AC Mains, and so we used a HP
balun to isolate instrument ground from vehicle ground --- but I have
never been all that certain about wide-band baluns. Note the
departure (comparing theory (NEC) with measurement) at the low end of
the band (my Figure 3). As Roy says there will not be much
improvement is achieving a coil Q of 500 compared with 300.

But the main purpose of this note to speak to the subject of
centre-loading. Comparing centre-loading with base-loading, we find
the radiation resistance is increased significantly, more current
area on the antenna, but the gain difference is not all that great
because a larger coil must be used for centre-loading.

But please use centre loading for a very different reason. The base
impedance with centre-loading is a pure resistance, a low resistance,
and the reactance of the capacity of the feed through insulator is
very high.

With base loading and tuning done inside the vehicle, a significant
current flows through the capacity of the feed through insulator,
this is non-radiating current, and so measuring field strength we
find the centre-loaded antenna much more efficient compared with base
loading --- but for a reason overlooked by most.

73, Jack, VE2CV

>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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