Light,Lazers and HF
 

What is the main factor that prevents HF radiation from focussing
for extra gain?

Light,Lazers and HF
 

Art Unwin wrote:
What is the main factor that prevents HF radiation from focussing
for extra gain?

Money.

If you can afford to build a 20m parabola about 2,000 feet in diameter
and the place to mount it, you'll get lots of gain.


--
Jim Pennino

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Light,Lazers and HF
 

On Sep 10, 8:45*pm, wrote:
Art Unwin wrote:
What is the main factor that prevents HF radiation from focussing
for extra gain?

Money.

If you can afford to build a 20m parabola about 2,000 feet in diameter
and the place to mount it, you'll get lots of gain.

--
Jim Pennino

Remove .spam.sux to reply.

Then are you saying it is the antenna size that is the main factor?.
So my antenna which is physically small
can be focussed on a dish which would provide straight line radiation
or a radiation beam?
Working on a single element on the ground with a optimizer instead of
a half sphere I got a
straight vertical line at the sides which suggested a gun barrel
radiation with a perfect earth as the reflector.
Gain was around 8db vertical which is why the question regarding
focussing! If it was properly focussed the gain should be more.
2000 foot dish seems somewhat odd, probably based on a "straight"
wavelength and not a small volume in equilibriumas the directer
right?
Art

Light,Lazers and HF
 

On Sep 10, 9:23*pm, Art Unwin wrote:
On Sep 10, 8:45*pm, wrote:

Art Unwin wrote:
What is the main factor that prevents HF radiation from focussing
for extra gain?

Money.

If you can afford to build a 20m parabola about 2,000 feet in diameter
and the place to mount it, you'll get lots of gain.

--
Jim Pennino

Remove .spam.sux to reply.

Then are you saying it is the antenna size that is the main factor?.
So my antenna which is physically small
can be focussed on a dish which would provide straight line radiation
or a radiation beam?
Working on a single element on the ground with a optimizer instead of
a half sphere I got a
straight vertical line at the sides which suggested a gun barrel
radiation with a perfect earth as the reflector.
Gain was around 8db vertical which is why the question regarding
focussing! If it was properly focussed the gain should be more.
2000 foot dish seems somewhat odd, probably based on a "straight"
wavelength and not a small volume in equilibriumas the directer
right?
Art

Let me ask the question another way. Whether it is believed or not,
if a 80 Metre antenna was compressed to the size of a couple of shoe
boxes
would the dish be reduced in size accordingly?
Regagards
Art

Light,Lazers and HF
 

"Art Unwin" wrote in message
...
What is the main factor that prevents HF radiation from focussing
for extra gain?

HF can be focused for gain. Main factor is that it takes a big antenna to
focus a signal on 3.5 MHz or so. Even a 2 or 3 element beam is too large
for most people , but it has been done.
To get any gain from a dish at that frequency would take a dish larger than
400 to 500 feet in diameter, maybe much larger. Hard to put that up a
tower.

Light,Lazers and HF
 

Art Unwin wrote:
On Sep 10, 8:45?pm, wrote:
Art Unwin wrote:
What is the main factor that prevents HF radiation from focussing
for extra gain?

Money.

If you can afford to build a 20m parabola about 2,000 feet in diameter
and the place to mount it, you'll get lots of gain.

--
Jim Pennino

Remove .spam.sux to reply.

Then are you saying it is the antenna size that is the main factor?.

Of course.

Everyone knows the gain of a parabola is directly proportional to
the size in wavelengths, or:

G=10*log k(pi*D/L)^2

Where G= gain in DB over an isotropic, k ~ .55 for most real parabolas,
D is the diameter, and L is the wavelength (wavelength and diameter
in the same units.

So a 2,000 foot parabola on 20m would have just about 58db gain.

--
Jim Pennino

Remove .spam.sux to reply.

Light,Lazers and HF
 

Light,Lazers and HF
 

On Sep 10, 10:29*pm, Art Unwin wrote:
On Sep 10, 9:23*pm, Art Unwin wrote:





On Sep 10, 8:45*pm, wrote:

Art Unwin wrote:
What is the main factor that prevents HF radiation from focussing
for extra gain?

Money.

If you can afford to build a 20m parabola about 2,000 feet in diameter
and the place to mount it, you'll get lots of gain.

--
Jim Pennino

Remove .spam.sux to reply.

Then are you saying it is the antenna size that is the main factor?.
So my antenna which is physically small
can be focussed on a dish which would provide straight line radiation
or a radiation beam?
Working on a single element on the ground with a optimizer instead of
a half sphere I got a
straight vertical line at the sides which suggested a gun barrel
radiation with a perfect earth as the reflector.
Gain was around 8db vertical which is why the question regarding
focussing! If it was properly focussed the gain should be more.
2000 foot dish seems somewhat odd, probably based on a "straight"
wavelength and not a small volume in equilibriumas the directer
right?
Art

Let me ask the question another way. Whether it is believed or not,
if a 80 Metre antenna was compressed to the size of a couple of shoe
boxes
would the dish be reduced in size accordingly?
Regagards
Art- Hide quoted text -

- Show quoted text -

No. The shoebox size antenna would approximate an isotropic if it did
radiate. It would still have to be placed at the focal point of a very
large parabola due to the size of the wave length. Such an antenna, I
believe, on the island of Puerto Rico (the SETI antenna) although it
is currently used primarily as a receiving antenna. That parabola is
positioned to have a very high radiation angle and might not be be
that good for terrestrial DX.

Light,Lazers and HF
 

Roy Lewallen wrote:
wrote:

Of course.

Everyone knows the gain of a parabola is directly proportional to
the size in wavelengths, or:

G=10*log k(pi*D/L)^2

Where G= gain in DB over an isotropic, k ~ .55 for most real parabolas,
D is the diameter, and L is the wavelength (wavelength and diameter
in the same units.

So a 2,000 foot parabola on 20m would have just about 58db gain.


Hm. I get 47.

Roy Lewallen, W7EL

Hmm, when I use 14 Mhz and 6 decimal places I get 37; must have fat
fingered it the first time.

Working backward from 47 I get a wavelength of 21 feet.


--
Jim Pennino

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Light,Lazers and HF