Re: NEC-LIST: Why coil-load?

Doug,

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.

I loaded my auto antenna on 20 meters by building a loading coil with a
30" piano wire whip. This was hose clamped to the auto antenna and the
thing was guyed with fishing twine. It was a last minute project so that
I could operate the CWSS contest while driving from Colorado to Idaho.
It was fed through the hi-Z coax that was already there. With a little
help from an antenna tuner it worked well enough for some contest work
and some rag chewing. I suppose the auto contributed about as much
radiation as the "antenna" as the effective length of the whip was 44"
while the effective length of the auto was about 30". 40% from the auto
and 60% from the "antenna" is what it works out to.

     Dave Cuthbert
     Micron Technology

-----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
D. B. Miron
Sent: Tuesday, February 15, 2005 5:22 PM
To: nec-list_at_robomod.net
Subject: NEC-LIST: Why coil-load?

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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