On 10/13/2014 4:37 PM, Jeff Liebermann wrote:
On Mon, 13 Oct 2014 12:26:23 -0500, Lostgallifreyan
wrote:

If any given directional antenna can radiate at its best to one particular
direction, is it safe to assume that it will be at its best similarly aimed
when receiving?

Yes. For purposes of calculations and under most conditions, the
pattern is the same for transmit and receive for most antennas.

However, there are plenty of confusing exceptions. A common exception
is putting a 2.4GHz USB Wi-Fi dongle at the focus of a dish or corner
reflector. The USB dongle is almost an isotropic radiator, which
spews RF in all directions. If you transmit from the USB dongle, most
of the RF will never hit the dish antenna and wander off to parts
unknown. Only the part that hits the dish eventually ends up going
towards the other end of the wireless link. However, in receive,
almost all of the signal that hits the dish, gets reflected to the USB
dongle. Therefore the gain is higher in receive, than in transmit.


A common misconception. The radiation pattern is the same for both
transmit and receive.

True that most of the received signal is reflected back to the antenna.
But the antenna still receives in an almost isotropic pattern, also.
It has the same amount of gain in the direction of the dish in both cases.

If the USB dongle were replaced with a proper dish feed, where the
bulk of the transmit RF hits the dish, the dish becomes more
"efficient". About 50% to 70% efficiencies are typical. However, it
is also possible to mess that up in the opposite direction. Instead
of a very non-directional feed, suppose I use as a feed, a high gain
directional antenna with a very narrow beamwidth. Instead of spraying
RF outside of the dish edge (over-spray), It puts all of it into a
narrow diameter spot somewhere on the dish surface. This time, the
symmetry is in the opposite direction. Transmit is fine, because all
of the power produced by the feed hits the spot and is radiated in the
direction of the other end of the link. However, receive is now a
problem because none of the RF seen by the dish OUTSIDE the area of
the spot is "seen" by the spot. Therefore, the gain is higher in
transmit, than in receive.

Gone to move some firewood...


The same is true on receive. The dish feed also has gain, and more of
the signal will be received by a proper dish feed than by an isotropic
antenna.

Look at it this way. The antenna is a strictly passive component. It
doesn't know or care whether it is transmitting or receiving. And it
doesn't care.

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