Why do we use thin antennas?
 

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?

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

Remove -spam-sux to reply.

Thickness is mainly a matter of making them self-supporting.

Thank you everyone!

65 mph
We have enough problms with all those flying saucers annoying people
from time to time. I promise I won't create more.


So, it seems that high capacitance is not a no-no for electrical
reasons. If you look at naval radio masts, sometime you'll see
manufacturers boasting of their "high capacitance" - like that
double-drainpipe-thick-mast stainless steel monster that I guess won't
cost less than $20k - but takes 40 kW, and won't stretch an automated
ATU to seek an impossible match.
(http://www.valcom-ottawa.com/Guelph/...415_photo.html
et al.)

From a mechanical standpoint I'd have no problem putting up a 1ft.
diameter wire "disk" or a similar structure. I have been
experimenting with a bizarre material: MIG copperflashed steelwire.
Dirt cheap, springy, solders extremely well, can be hand-shaped. It
can be used to put together small, super-light, bouncy, 3-dimensional,
lattice structures. Rusts like hell, but nothing that can't be dealt
with using an ordinary nitric acid paint and laquer.

I had already planned to use that stuff in my next VHF discone, now I
am tempted to use a high capacitance disk as pickup in a mobile
broadband HF active antenna.

Is there some good soul here who can help me figure out the impedance
I need to match?

I would like to interpose either a T + Pi network or a Chebyscheff
between the antenna and a classic FET gate, to kill off RF below 3 MHz
and above 30 MHz.

My first try would be either a 1 ft disk with 16 braced 1mm radials
about 8" above a car roof, or a cone of the same dimensions pointing
down into the amplifier case. Hmmm... that would be a structure
similar to those monster HF broadband designs - just 50 times smaller!

SpamLover wrote:
Thank you everyone!

65 mph
We have enough problms with all those flying saucers annoying people
from time to time. I promise I won't create more.


So, it seems that high capacitance is not a no-no for electrical
reasons. If you look at naval radio masts, sometime you'll see
manufacturers boasting of their "high capacitance" - like that
double-drainpipe-thick-mast stainless steel monster that I guess won't
cost less than $20k - but takes 40 kW, and won't stretch an automated
ATU to seek an impossible match.
(http://www.valcom-ottawa.com/Guelph/...415_photo.html
et al.)

From a mechanical standpoint I'd have no problem putting up a 1ft.
diameter wire "disk" or a similar structure. I have been
experimenting with a bizarre material: MIG copperflashed steelwire.
Dirt cheap, springy, solders extremely well, can be hand-shaped. It
can be used to put together small, super-light, bouncy, 3-dimensional,
lattice structures. Rusts like hell, but nothing that can't be dealt
with using an ordinary nitric acid paint and laquer.

I had already planned to use that stuff in my next VHF discone, now I
am tempted to use a high capacitance disk as pickup in a mobile
broadband HF active antenna.

Is there some good soul here who can help me figure out the impedance
I need to match?

I would like to interpose either a T + Pi network or a Chebyscheff
between the antenna and a classic FET gate, to kill off RF below 3 MHz
and above 30 MHz.

My first try would be either a 1 ft disk with 16 braced 1mm radials
about 8" above a car roof, or a cone of the same dimensions pointing
down into the amplifier case. Hmmm... that would be a structure
similar to those monster HF broadband designs - just 50 times smaller!

Not that I would discourge experimentation, but it would be worthwhile
to get a copy of something like the ARRL Antenna Book and read up on
top loading to learn what those capacitve hats actually do.

You will also find how to figure out the impedance of a top loaded system.

BTW, brazing rod is a bit more expensive but doesn't have the rust
problem. Use the plated MIG stuff for the prototypes, brazing rod for
the final build.

--
Jim Pennino

Remove -spam-sux to reply.

Generally,

Because there is a good agreement between
theoretical [analysis/design] computations
and measured [explanation/prediction] data.

Sincerely,

pez
SV7BAX
TheDAG

"SpamLover" wrote in message om...
| 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?