Now if any one could come up with a formula that includes all of
these factors then we would be able to answer your question. There
is no formula. However we can say this.
The best mobile antenna is a thick stiff nine foot whip that is
silver coated and mounted on top of the roof with multiple radial
grounding points.
No one is going to use the antenna above in the real word, so one
might draw the conclusion that in the real world we might substitute a
102" stainless whip for the best real world performance. WRONG.
The 102" is made of stainless. Stainless has only 3% the
conductivity of copper. This alone will stop the 102" stainless from
being the top performer. Subtract also the fact that the 102"
stainless bends over in the wind and is also a relatively thin
antenna.
So what is the best choice? It appears that for a realistic
antenna, it is a mildly loaded top or center loaded antenna made
of thick conductive materials. There are a few of these ugly looking
antennas around. They do work marginally better than the
102" stainless and have the benefit of being shorter. These
antennas can get as short as five foot and still equal or
outperform the 102" stainless.
A common choice that was overlooked was the 102"
whip made out of fiberglass with copper wire embedded
in it. Due to the lower velocity factor (how fast radio
frequency energy travels though a substance), the
fiberglass version of the 102" whip can actually be 6"
or so shorter than its stainless counterpart. Due to the
shorter length and thicker outer material, it's also
more rigid (stays straighter when the vehicle is in motion).
In practical terms, it doesn't need a spring to achieve
resonance in the CB band, and can usually be shortened
an additional 4" or more depending on where in the
spectum the operator most often operates. For example,
the fiberglass whip (Radio Shack part number 21-905)
is usually 2 to 6 inches shorter than the stainless version
to begin with. It doesn't need a spring. And when
shooting for resonance (lowest SWR with highest field
strength) on Channel 40, it can usually be shortened
an additional 4 inches. If you shorten it from the bottom
instead of the top, the rigidity is improved. So, the end
product very often ends up a full foot shorter than the
108" stainless whip and spring combination as well as
being cheaper because there's no need to pay for a
spring. Not having a spring makes the setup look
nicer and stay straighter too.
Over the years I've taken a number of the 21-905
whips, shortened them 4 inches or so from the bottom,
and coated them with a thin coat of Varathane to
prevent splintering of the fiberglass over time. The
end result looks great, lasts until it gets smacked
low enough and hard enough to break it, and per-
forms very, very well. To outperform it, an antenna
would have to be a closer match to 50 ohms resistive
and have a lower angle of radiation. Few are capable
of that--and none are cheaper or easier to obtain.
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-=[Bill Eitner]=-
http://www.cbtricks.com/~kd6tas
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