Thanks for all of the great replies! I think one of these antennas is
going to work for my "stealth" antenna. I have read the various web
sites that offer "screwdriver" antennas until my eyes are crossing.
Everyone seems to be trashing eveyone else. Very hard to determine who
is straight and who is bull-------. Can anyone offer some insight into
stuff like coil diameter vs coil length vs "radiation efficiency" vs
anything?

I know that I want full band coverage 10-160, and I only have 100 watts
from my transmitter. So, I can't have an antenna that "eats" 90 percent
of my transmitter ourput. So what should I look for in a "screwdriver"
antenna that will give me the best performance with my specifications
and limitations?

Thanks for any advice, jimbo

On 9 Feb 2005 17:33:28 -0800, "jimbo" wrote:

I know that I want full band coverage 10-160, and I only have 100 watts
from my transmitter. So, I can't have an antenna that "eats" 90 percent
of my transmitter ourput.

Hi Jimbo,

The simple numbers are radiation resistance and Ohmic resistance.
Which is larger? Answer that and you have a simple answer. What are
their proportions? Answer that and you have a more complex answer.
Think of a voltage divider (now a power divider).

Now ask yourself, what is the radiation resistance? What is the Ohmic
resistance? For quarterwave and smaller antennas, simple formulas
abound (or tables are available) for radiation resistance. For Ohmic
resistance, this is a function of materials and construction. In this
last regard, it only takes a loose wire, or screw and this resistance
climbs through the roof.

Let's put that into context. For a 12' vertical radiator operating on
160M, you have a 4/160th's wavelength radiator. Radiation resistance
is running on the order of 1/4th Ohm. If you manage to hold all Ohmic
resistance to that value, or lower (everywhere - in TOTAL) then you
verge on 50% efficiency, or better.

Your Mileage May Vary.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
.. Let's put that into context. For a 12' vertical radiator operating on
160M, you have a 4/160th's wavelength radiator. Radiation resistance
is running on the order of 1/4th Ohm. If you manage to hold all Ohmic
resistance to that value, or lower (everywhere - in TOTAL) then you
verge on 50% efficiency, or better.

And ground losses are likely to be in the neighborhood of ten ohms
or about 2.5% efficiency.
--
73, Cecil http://www.qsl.net/w5dxp


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