Hi All,

Modern engineering text indicates that for the same frequency, a large
fully illuminated dish will provide more gain than it’s smaller
equivalent. Why is this ?

Consider the following scenario:

======

Take a 10GHz RF source, and attach an appropriate feed horn to fully
illuminate a 2m dia dish.

Adjust focus so that the wave front out of the dish is parallel (2m
diameter ?)

At a distance, arrange the same setup, except the RF source is
replaced by a detector.

Make a note of received signal strength.

======

Repeat the same experiment at the same frequency, but with a
3mdiameter dish at each end, and appropriate feed horns .

Make a note of received signal strength.

======

Also, see http://www.idesignz.org/misc/dishproblem.pdf

In both cases all of the TX energy is transmitted in a parallel beam,
whose diameter is the same as the respective dish.

It would seem that if you can fully convert all of the TX RF into
parallel waves, at some point the diameter of the dishes should not
make any difference to the transfer efficiency of the system.

So, why would the received signal for the 3m + 3m dish be greater than
that of the 2m + 2m dish experiment ?

Thoughts please.

Mark

http://www.idesignz.org/AMPS/AMPS_BS.html – Experimental AMPS micro
Base Station project

http://www.idesignz.org/UAV/index.html – Early work on the four rotor
FlyingThingy

On Mon, 12 Oct 2009 21:45:39 -0700 (PDT), MarkAren
wrote:

Hi All,

Modern engineering text indicates that for the same frequency, a large
fully illuminated dish will provide more gain than it’s smaller
equivalent. Why is this ?

Consider the following scenario:

======

Take a 10GHz RF source, and attach an appropriate feed horn to fully
illuminate a 2m dia dish.

Adjust focus so that the wave front out of the dish is parallel (2m
diameter ?)

At a distance, arrange the same setup, except the RF source is
replaced by a detector.

Make a note of received signal strength.

======

Repeat the same experiment at the same frequency, but with a
3mdiameter dish at each end, and appropriate feed horns .

Make a note of received signal strength.

======

Also, see http://www.idesignz.org/misc/dishproblem.pdf

In both cases all of the TX energy is transmitted in a parallel beam,
whose diameter is the same as the respective dish.

A parallel beam is not formed, rather a slightly expanding beam due to
diffraction.

Diffraction also limits the resolving power of an astronomical
telescope, so when looking at some binary stars, a small telescope
will show only a single blob, a slightly larger telescope will show an
elongated figure (e.g. figure of 8) and an even larger telescope will
show two separate stars.

The diffraction limit is defined as 1.22*wavelength/diemeter radians
or about 70*wavelength/diameter degrees.

Diffraction also controls the beam spreading from a parabolic disk or
laser. For this reason, a laser (with an aperture less than 1 cm) can
not be used to illuminate the reflectors on the moon, but typically
the laser beam is transmitted through a telescope with typically 1 m
diameter. The beam is 100-1000 times narrower than the beam from the
laser alone. The area illuminated on the moon is 10,000-1,000,000
times smaller and hence reflected power that much stronger than with a
bare laser.

Paul OH3LWR

Hey OM:

Yes how is it? Bigger is better.
I mean you are using the same source for both antennas but how does
the larger one have all that extra power, being emitted and where does
all that extra power come from?

So instead of using that 400 watt microwave oven I just get me a 30
foot dish and a 20 milliwatt emitter and save all that money I would
have wasted using the microwave oven, cooking my turkey.

73 OM
de n8zu




On Oct 13, 12:45 am, MarkAren wrote:



Hi All,

Modern engineering text indicates that for the same frequency, a large
fully illuminated dish will provide more gain than it’s smaller
equivalent. Why is this ?

Consider the following scenario:

======

Take a 10GHz RF source, and attach an appropriate feed horn to fully
illuminate a 2m dia dish.

Adjust focus so that the wave front out of the dish is parallel (2m
diameter ?)

At a distance, arrange the same setup, except the RF source is
replaced by a detector.

Make a note of received signal strength.

======

Repeat the same experiment at the same frequency, but with a
3mdiameter dish at each end, and appropriate feed horns .

Make a note of received signal strength.

======

Also, seehttp://www.idesignz.org/misc/dishproblem.pdf

In both cases all of the TX energy is transmitted in a parallel beam,
whose diameter is the same as the respective dish.

It would seem that if you can fully convert all of the TX RF into
parallel waves, at some point the diameter of the dishes should not
make any difference to the transfer efficiency of the system.

So, why would the received signal for the 3m + 3m dish be greater than
that of the 2m + 2m dish experiment ?

Thoughts please.

Mark

http://www.idesignz.org/AMPS/AMPS_BS.html– Experimental AMPS micro
Base Station project

http://www.idesignz.org/UAV/index.html– Early work on the four rotor
FlyingThingy

Paul Keinanen wrote:
On Mon, 12 Oct 2009 21:45:39 -0700 (PDT), MarkAren
wrote:

Hi All,

Modern engineering text indicates that for the same frequency, a large
fully illuminated dish will provide more gain than it’s smaller
equivalent. Why is this ?
...

In both cases all of the TX energy is transmitted in a parallel beam,
whose diameter is the same as the respective dish.
****



A parallel beam is not formed, rather a slightly expanding beam due to
diffraction.

Diffraction also limits the resolving power of an astronomical
telescope, so when looking at some binary stars, a small telescope
will show only a single blob, a slightly larger telescope will show an
elongated figure (e.g. figure of 8) and an even larger telescope will
show two separate stars.

The diffraction limit is defined as 1.22*wavelength/diemeter radians
or about 70*wavelength/diameter degrees.

Diffraction also controls the beam spreading from a parabolic disk or
laser. For this reason, a laser (with an aperture less than 1 cm) can
not be used to illuminate the reflectors on the moon, but typically
the laser beam is transmitted through a telescope with typically 1 m
diameter. The beam is 100-1000 times narrower than the beam from the
laser alone. The area illuminated on the moon is 10,000-1,000,000
times smaller and hence reflected power that much stronger than with a
bare laser.

Paul OH3LWR

Great response!

Brian W

raypsi wrote:
Hey OM:

Yes how is it? Bigger is better.
I mean you are using the same source for both antennas but how does
the larger one have all that extra power, being emitted and where does
all that extra power come from?

So instead of using that 400 watt microwave oven I just get me a 30
foot dish and a 20 milliwatt emitter and save all that money I would
have wasted using the microwave oven, cooking my turkey.

73 OM
de n8zu

It comes from "focusing" the existing energy. No new energy is created.

Yes, you *could* do that with a turkey. Just like you *could* cook it
with a magnifying glass out in the sun.

-Bill

Hey OM:

Take a 60db gain dish illuminate that with 20 milliwatts of power.
Your ERP is now 20,000 watts.

My question raises the bar, how can you focus 20 milliwatts of EM
particles and waves, to end up with 20,000 watts of EM particles and
waves? And I can bet that it's all those extra particles that are
going to be cooking my turkey.

There's only one way that can happen: the dish is a storage device:
the size and frequency of which determine the storage capacity in this
case the storage capacity gives me a 60db gain. And will always give
you 60db more than you put in.

73 OM

de n8zu

It comes from "focusing" the existing energy. No new energy is created.

Yes, you *could* do that with a turkey. Just like you *could* cook it
with a magnifying glass out in the sun.

-Bill

On Oct 15, 3:02*pm, raypsi wrote:
Hey OM:

Take a 60db gain dish illuminate that with *20 milliwatts of power.
Your ERP is now 20,000 watts.

My question raises the bar, how can you focus 20 milliwatts of EM
particles and waves, to end up with 20,000 watts of EM particles and
waves? And I can bet that it's all those extra particles that are
going to be cooking my turkey.

There's only one way that can happen: the dish is a storage device:
the size and frequency of which determine the storage capacity in this
case the storage capacity gives me a 60db gain. And will always give
you 60db more than you put in.

73 OM

de n8zu


60db in one specific direction more than if same amount of RF was
dissipated equally in all directions. Correct?
Would not matter if starting with one milliwatt, one watt, one
megawatt.
The beaming effect will concentrate the available energy from all
directions in a specific one.

raypsi writes:

Hey OM:

Take a 60db gain dish illuminate that with 20 milliwatts of power.
Your ERP is now 20,000 watts.

EIRP, actually ... Radiated power, relative to a (abstract) isotropic
radiator.

My question raises the bar, how can you focus 20 milliwatts of EM
particles and waves, to end up with 20,000 watts of EM particles and
waves?

You can't. You're just focusing that 20 mW of power in the "useful
direction"

Imagine an incandescent light bulb hanging from a wire -- this is a
reasonable metaphor for an isotropic radiator - it puts out (nearly) the
same amount of light in all directions ... If the light isn't bright
enough to read a book, you might put a reflector behind it - there is no
more light being generated, but the page just got brighter, because some
of the energy that was illuminating the garage roof, your head, the
walls and the junk on the shelves is now illuminating the page.

And I can bet that it's all those extra particles that are
going to be cooking my turkey.

There are no particles, and there are no "extra" particles.

de n8zu

It comes from "focusing" the existing energy. No new energy is created.

Yes, you *could* do that with a turkey. Just like you *could* cook it
with a magnifying glass out in the sun.

Indeed - first you heat one molecule of water, then refocus the beam,
heat the next, repeat 6 x 10**23 times for each mole of turkey.


-Bill

Hey OM

What you are saying is that I can take a 20 mW emitter and with an
infinite size dish I can get infinite power from a 20 mW emitter?

No way you can get infinite power from a 20 mW source unless the dish
is supplying the extra power. In other words it's not possible to get
infinite power focused from a 20 mW source, and that all that power is
coming from the 20 mW emitter.

Take that 20 mW emitters' isotropic pattern, there is a potential of
an infinite amount of power in that pattern because it can be
focused?

Frying that turkey only takes 20 mW of total power period. And it may
or may not be from focusing the 20 mW. The turkey gets cooked with 20
mW is still a fact that can't be refutted.

73 OM
de n8zu




You can't. *You're just focusing that 20 mW of power in the "useful
direction" *




-Bill- Hide quoted text -

- Show quoted text -

raypsi wrote:
Hey OM

What you are saying is that I can take a 20 mW emitter and with an
infinite size dish I can get infinite power from a 20 mW emitter?

You can get infinite *Effective Isotropic Radiated Power* from 20 mW
with an infinite dish. Not practical, but theoretically correct.

You're misunderstanding EIRP, Effective Isotropic Radiated Power. If
you feed a 60 dB parabolic antenna with 20 mW you get the equivalent of
feeding a theoretical *isotropic* radiator with 20 kW, the key word
being 'isotropic,' meaning that 20 kW is being radiated in all
directions. Your 60 dB dish directs your 20 mW *in a specific
direction* with a *specific* beam width. *In that beam* the field
strength is the same as it would be from 20 kW *going in every
direction*. The ratio of every direction to your dish's beam width is
60 dB.

No way you can get infinite power from a 20 mW source unless the dish
is supplying the extra power. In other words it's not possible to get
infinite power focused from a 20 mW source, and that all that power is
coming from the 20 mW emitter.

You're not getting infinite power, you're getting 20 mW focused into a
finite beam width and in that beam width it has the same field strength
that 20 kW going in all directions would give you.

Take that 20 mW emitters' isotropic pattern, there is a potential of
an infinite amount of power in that pattern because it can be
focused?

Only if it can be focused into an infinitely small point which you won't
get from any finite gain antenna.

Frying that turkey only takes 20 mW of total power period. And it may
or may not be from focusing the 20 mW. The turkey gets cooked with 20
mW is still a fact that can't be refutted.

Part of that fact is it will take a long, long time to cook it with only
20 mW because you can only cook (defined as raising the temperature to
about 160 F) a very, very small portion of the turkey at a time with
only 20 mW.

- W8LNA



73 OM
de n8zu




You can't. You're just focusing that 20 mW of power in the "useful
direction"


-Bill- Hide quoted text -
- Show quoted text -