Re: NEC-LIST: Why coil-load?

Doug,

A good question on the current distribution in the ground. I don't have
a definitive answer- perhaps someone else does.

I would approach the problem of the ground loss like this: Build a wire
grid model of the vehicle and place it over Sommerfield-Norton ground.
Then experiment with the loading coil Q and placement. I only have NEC-2
and that will give optimistic results and will not have absolute
accuracy. But it might be good enough for relative comparisons. If you
have NEC-4 then you are all set. The vehicle model will be good for
exploring the effect of nearby structures. For example, the center
loaded whip mounted on the fender will have a higher E-field than the
base loaded whip and will couple more current into the roof. This then
travels down the roof pillars thereby lowering the radiation resistance.

     Dave Cuthbert

-----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: Wednesday, February 16, 2005 6:38 AM
To: nec-list_at_robomod.net
Subject: Re: NEC-LIST: Why coil-load?

Dave,

Sorry, I misremembered the height. Here is the listing for
Harrison's monopole.

CM Coil-loaded monopole.
CE
GW 1 41 0 0 0 0 0 31.83 0.122895
GS 0 0 0.001
GE 1
GN 1
LD 0 1 21 21 5.5858 1.33347e-6 0
LD 5 0 0 0 26e6
EX 0 1 1 1 100 0 1
FR 0 1 0 0 300 2
PT -1
RP 0 46 46 1002 0 0 2 2
EN

You can see it's a little shorter than I said. Adding a 10
ohm resistor to the base yields the following efficiencies:
No tuning=3.64 %, base tuning=2.83 %, center-loading=5.51 %,
both with Q=450. I am a little suspicious about
representing ground loss by a fixed resistor. Doesn't
ground loss depend on the antenna's current distribution?

Regards,
Doug Miron

<drcuthbert_at_micron.com> writes:

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

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