Re: NEC-LIST: Why coil-load?

 
For the 0.038 wavelength monopole on perfect ground I get these numbers:

Base loaded: Rr = 0.62 ohms, Coil XL = 1150 ohms, Rad Eff = 19.5% with
a coil Q = 450
Center loaded: Rr = 1.42 ohms, Coil XL = 2083 ohms, Rad Eff = 21.1% with
a coil Q = 450

Lets take the case of something a bit shorter- an 8' whip at 1.8 MHz.
That is a 0.015 wavelength monopole.

Base loaded: Rad Eff = 1.22% with a coil Q = 450
Center loaded: Rad Eff = 1.34% with a coil Q = 450 (good luck building
such a coil)

Not much difference, is there? Only 0.4 dB. Let's add in a ground loss
of 10 ohms at the base.

Base loaded: Rad Eff = 0.52%
Center loaded: Rad Eff = 0.82%

   Now we have the significant difference of 2.0 dB.

Add a top hat that has three 2' spokes and the base-loaded Radiation
Efficiency soars to 0.97% while the coil reactance drops to 2400 ohms,
making a practical inductor. This antenna is 2.7 dB above the simple
base-loaded design.

And while 1% efficiency sounds low, the ERP will be 1 watt with a 100
watt rig. I normally radiate barely one 1 watt at home when I run 5W on
160 meters and that is enough to work the East Coast from Idaho.

For your second example the base loaded design sounds much better from a
mechanical and aesthetic view. Sounds like an interesting project. I
recall seeing a CB/AM/FM antenna for sale 25 years ago.

-----Original Message-----
From: nec-list-bounces+drcuthbert=micron.com_at_robomod.net
[mailto:nec-list-bounces+drcuthbert=micron.com_at_robomod.net] On Behalf Of
Roy Lewallen
Sent: Tuesday, February 15, 2005 7:51 PM
To: nec-list_at_robomod.net
Subject: 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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