Re: NEC-LIST: Coil-loading and ground resistance.

Miron,

I ran a simulation using a cone-like thing to represent the auto and
found:

1) Almost no difference in the radiation resistance between an 8' whip
on the bumper and a 3' whip on top of the 5' roof.
2) The shorter roof whip needs +j3000 ohms to resonate while the bumper
whip needs +j800 ohms. This is not the way to go.

I then added a six spoke, 2.5' spoke top hat to the 3' whip on the car
roof. The radiation resistance increased by 1.5X and the loading coil XL
is only 400 ohms. This would fit on a car nicely and the top hat does
not overhang the car. This looks like the best for a given height. More
bandwidth too with less loading reactance. I might build one and give it
a try. I have a big magnetic mount (with four magnets) that should hold
something like this.

Dan, thanks for the automobile models. I did not use them for this but I
did try one at VHF and it looked good.

     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
drcuthbert
Sent: Thursday, February 17, 2005 3:24 PM
To: nec-list_at_robomod.net
Subject: Re: NEC-LIST: Coil-loading and ground resistance.

Miron,

I wonder if a short whip on the roof is equal to or better than a long
whip on the bumper. If the car and whip are functioning as a vertical
dipole then the distance from the bottom of the auto to the top of the
whip might be all that matters. I will run a simulation.

  Dave

-----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: Thursday, February 17, 2005 1:21 PM
To: nec-list_at_robomod.net
Subject: NEC-LIST: Coil-loading and ground resistance.

Good day,

I just translated the Harrison monopole to 3 MHz (3.183 m
tall), put 8 5-m radials around it, and put the system 0.1 m
above average ground (sigma=5 mS/m, epser=13). The ground
resistance was only 1.6 ohms, not enough to give
center-loading a big advantage over base-tuning. I guess
I'm just going to have to put an HF whip on the back of my
hatchback and see what happens.

Doug Miron

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