SpamLover wrote:
Purdon my deep egnorance, but, why do we tend to use thin conductors
for antennas? I know of only few counter-examples, all at VHF and
above:
- a discone can be made of sheet metal, rather than thin radials
- a log-periodic broadband dipole can be made as an etched spiral
pattern on PCB (and I guess it can't be made with constant section
conductors at all...)

I know that things get more complicated at higher frequencies, what
with slot antennas & suchlike.


A connected curiosity is regarding short active receiving antennas on
HF. My concern is putting together an efficient, mobile mounted,
all-band HF receiving antenna. If we use short whips, they show an
extremely high impedance and require a carefully designed matching
amplifier. At such high impedance levels, a broad 3-30MHz bandpass
filter may not be easy to design - so I have been told.

But I wonder - why not use 1-2 sqft of conductive surface instead, e.g
PCB or big bore copper rainpipe, worked against the car body?

I just ran an unscientific experiment. I grounded a pocket HF
receiver to a steel-topped table, and balanced a steel pot lid on the
top of its collapsed whip antenna. Signals were booming. A 10" PCB
disk, placed 5" above the steel roof of a car shouldn't be much
different.

Comments?

Wire is thin, light, cheap, easy to support, and it works.

Fat and solid things have high wind loadings, are heavy, and are difficult
to support, especially in the wind.

Short mobile antennas are used because tall mobile antennas hit things. Do
they have parking structures where you come from?

Try holding your pot lid out the window of your car while doing 65 mph
and the answers becomes pretty obvious.

BTW, a disk on the top of a vertical is called a top hat, is pretty common,
and a disk made of thin radials works just about as well as a solid disk
with a small fraction of the weight and wind loading.

All real antennas are a compromise between electrical and mechanical
concerns.

For example, I have a 40M vertical in the back yard made of aluminum
tubing. The bottom start out at 1 1/4 inch and tapers down to about 1/2
inch at the top. It is supported by a piece of water pipe driven a couple
of feet into the ground and guyed with cheap UV resistant dacron line tied
to eye screws screwed into existing structures. It has been there for 15
years and I've replaced the guys once.

While a vertical made of 12 inch pipe might show a slightly greater bandwidth,
it would take machinery to get such a beast into the vertical position,
a concrete base to support it, and heroic guying and associated attachement
points to keep it vertical when the winds hit 70 mph.

If I wanted such an antenna, a cage made of thin wire would work just
as well as a solid vertical without all the mechanical support problems.

--
Jim Pennino

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