Is it possible to build an omnidirectional receiving antenna with
gain?

Let us assume an ideal dipole Call it 0dBd

Now let us build two ideal dipole antennas and combine their outputs
we have an antenna with possibly twice the received energy or 3dBd
gain.

Now for signals from certain orientations we signals canceling in
phase for less than 0dBd gain....



Is it possible to build an antenna with more capture area, giving
gain,
without adding directivity?


Thought experiment saying it is possible......
Cover a 1 meter sphere with solar cells......

Cover a 100 mete sphere with solar cells......
It's clear that the 100 meter sphere would gther more energy than the
1 m sphere.... thus it seams it would be possible,

No what forms would an isotropic (Or at lease hemispherical) pattern
antenna with gain 0 dBi look like?


Paul (Kl7JG)

wrote:
Is it possible to build an omnidirectional receiving antenna with
gain?

Of course, the Aug. 2003 QST has some of them on page 19 & 108. The
Comet GP-24 for 2.4GHz is twenty end-to-end 1/2WL antennas.
--
73, Cecil http://www.qsl.net/w5dxp



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Sometimes "omnidirectional" means "equally radiating in all azimuth
directions", which is the interpretation Cecil has used. In that case,
yes, of course, you can achieve gain by concentrating the power in the
vertical direction, i.e., increasing the power density at some elevation
angles at the expense of others. A collinear antenna (linear antennas
placed end-to-end as Cecil describes) is a common way of achieving that.

I inferred from the original posting that the intended meaning was truly
omnidirectional, as in isotropic, the same in all directions in three
dimensional space. If that's correct, the answer is no.

The explanation in my response was in terms of a transmitting antenna.
It's well established (the reciprocity principle) that the gain of an
antenna is the same for transmitting and receiving, so the conclusion is
equally valid for a receiving antenna.

Roy Lewallen, W7EL

W5DXP wrote:
wrote:

Is it possible to build an omnidirectional receiving antenna with
gain?


Of course, the Aug. 2003 QST has some of them on page 19 & 108. The
Comet GP-24 for 2.4GHz is twenty end-to-end 1/2WL antennas.

Paul,
If you mean 'omnidirectional' in the commonly used
fassion of ham radio, then the answer is yes. If you
mean 'omnidirectional' as in an isotropic antenna, then
no. To date, and to the best of my knowledge, I have
never used an isotropic antenna, nor have I ever heard
one used. So, I guess the answer is yes...
'Doc

I was implying iostropic in 3 dimensions....

Now for a paradox.....


If RX==TX as for as antenna gain

And 100W is 100W.... then isotropic had dBi =0 be the laws of
conservation of energy.....


No suppose we have an antenna that captures all of the energy passing
though a 1M square.......

Now make that a 10M square.....


Clearly a 10M "Fishing net" can capture more than a 1M net, but

Then RXgain != TXgain


So what gives where is this wrong?

What relative gain do you think the two antennas have?

Are you still talking about isotropic antennas, or have you moved on to
accommodate other patterns?

How do you conclude that the receiving gain of your hypothetical antenna
is different from the transmitting gain?

Roy Lewallen, W7EL

wrote:
I was implying iostropic in 3 dimensions....

Now for a paradox.....


If RX==TX as for as antenna gain

And 100W is 100W.... then isotropic had dBi =0 be the laws of
conservation of energy.....


No suppose we have an antenna that captures all of the energy passing
though a 1M square.......

Now make that a 10M square.....


Clearly a 10M "Fishing net" can capture more than a 1M net, but

Then RXgain != TXgain


So what gives where is this wrong?

Paul,
Never happen unless the isotropic antenna is
made with red wire...
'Doc

Paul@Rasdoc wrote:

I was implying iostropic in 3 dimensions....
Now for a paradox.....
If RX==TX as for as antenna gain
And 100W is 100W.... then isotropic had dBi =0 be the laws of
conservation of energy.....
No suppose we have an antenna that captures all of the energy passing
though a 1M square.......
Now make that a 10M square.....
Clearly a 10M "Fishing net" can capture more than a 1M net, but
Then RXgain != TXgain
So what gives where is this wrong?

The only application that I have seen in which this was true
was the so-called Rectenna - an array of low-gain dipoles,
each of which had a diode detector at its feedpoint terminals.
The direct-current diode outputs were summed to produce the
"signal" which was more-or-less proportional to the area of
the array. But the array itself had very low directivity -
pretty much that of a single dipole element - making it
unnecessary to beam-steer it.

This was proposed as the Earthside end of a space-based power
satellite system that would convert sunlight in geosynchronous
orbit to microwave energy and beam it down to the rectenna. There
it would be "detected" to DC which would then be converted to
60-Hertz HV sine waves for injection into the power grid.

The thing that makes this work was, of course, the non-linear,
non-coherent rectification process at each array element. And
it only works for DC. If a higher-bandwidth modulation is
imposed on the RF energy, a point is reached due to travel-time
delay where the detected signal at one end of the array is out
of phase with that detected at the opposite end. If synchronous
detection is attempted, the highly undesireable array directivity
is immediately restored - much to the chagrin of the system designers.


--
Jim Bromley, K7JEB k7jeb(at)arrl(dot)net
Glendale, Arizona

"K7JEB" skrev i meddelandet
...

Paul@Rasdoc wrote:

I was implying iostropic in 3 dimensions....
Now for a paradox.....
If RX==TX as for as antenna gain
And 100W is 100W.... then isotropic had dBi =0 be the laws of
conservation of energy.....
No suppose we have an antenna that captures all of the energy passing
though a 1M square.......
Now make that a 10M square.....
Clearly a 10M "Fishing net" can capture more than a 1M net, but
Then RXgain != TXgain
So what gives where is this wrong?

The only application that I have seen in which this was true
was the so-called Rectenna - an array of low-gain dipoles,
each of which had a diode detector at its feedpoint terminals.
The direct-current diode outputs were summed to produce the
"signal" which was more-or-less proportional to the area of
the array. But the array itself had very low directivity -
pretty much that of a single dipole element - making it
unnecessary to beam-steer it.

This was proposed as the Earthside end of a space-based power
satellite system that would convert sunlight in geosynchronous
orbit to microwave energy and beam it down to the rectenna. There
it would be "detected" to DC which would then be converted to
60-Hertz HV sine waves for injection into the power grid.

The thing that makes this work was, of course, the non-linear,
non-coherent rectification process at each array element. And
it only works for DC. If a higher-bandwidth modulation is
imposed on the RF energy, a point is reached due to travel-time
delay where the detected signal at one end of the array is out
of phase with that detected at the opposite end. If synchronous
detection is attempted, the highly undesireable array directivity
is immediately restored - much to the chagrin of the system designers.


--
Jim Bromley, K7JEB k7jeb(at)arrl(dot)net
Glendale, Arizona


Thats in the right direction. The original question was about energy (DC)
and this is the answer. Modulated energy is another issue, not applicable
for the original question.
--
/ME (first time, checking my signature)

wrote in message
...
Is it possible to build an omnidirectional receiving antenna with
gain?
TV transmitting antennas are generally omnidirectional, and have gain. They
do this by concentrating all energy along the horizon. In fact, some work so
well that they put up a secondary antenna at 100ft or so to take care of
people who live close by.

If you want to talk about practicality, it would be easy to build one at 2
meters. Forget about it on 160m.

Tam/WB2TT