On 12 abr, 23:40, "Yuri Blanarovich" wrote:
"Wimpie" wrote in message

ups.com...

On 12 abr, 00:47, "Yuri Blanarovich" wrote:
"Wimpie" wrote in message

oups.com...

So if you suspended another dipole above your NVIS dipole and oriented
90
degrees to each other, the difference would be insignificant?
Then move it up into the "clouds", then move down to earth at the
distance
and you will see "insignificant" difference in signal levels?

Seems that direction finders should not work according to this
"verdict",
Eh?

One thing is the direction of the signals (maximum) another one is the
polarization. Based on the orientation of antennas, one can orient the
antenna to find the minimum signal.

Yuri, K3BU.us

Hi Yuri,

As the wave pass through the ionosphere, strange things happen. You
could google on Faraday rotation, ordinary and extraordinary waves to
find out that at low frequency, the change in polarization is
significant.

Change does not mean that polarization dissapears.

Based on the down coming wave, you cannot determine the orientation of
the transmitting antenna, neither the position with reasonable
accuracy (for NVIS propagation).

When you place the receiving antenna just above the transmitting
antenna you are right, but we were discussing NVIS propagation.

Best Regards,

Wim
PA3DJS

So when signal is reflected, the polarization disappears?

No, mostly the down comming wave has a circular component (eliptical
polarization), therefore the orientation is not of significant
importance. Wim, PA3DJS.

73 Yuri, K3BU

So if there is ANY polarization left how can you claim that is not of
significant importance.
The proof in the pudding is that on any signal one can with any antenna,
especially directional, find the spot where signal is minimal or nulled out.
Nulls are sharp vs. broad pattern, but one has to be aware of them and
understand the difference between the pattern and polarization.

Yuri, K3BU.us

High Yuri,

I try again.

Ratiation pattern
As NVIS radiation comes from near 90 degrees elevation, the antenna
receiving the incoming wave will operate at near maximum gain, no
matter the orientation (in horizontal plane) of the dipole. The gain
of a dipole under 75 degrees is less then 1 dB below the maximum.

To orient your antenna to get a nul, you have to use a vertical dipole
(or vertical monopole). So radiation pattern is not of importance.

Polarization,
If the ionosphere would act as a pure mirror reflector, and there was
no free charge, you are right, orientation of both antennas would
matter. As you do not know the amount of polarization change, you
cannot predict the optimum orientation.

Prove it yourself by using a hand held antenna (for example small
vertical loop, or horizontal small dipole). Listen to an NVIS station
(at about 50..100 mile from you) and rotate the antenna around the Z
as. You should do this fast enough to distinguish between fading.
You will find certain orientations with less reception, but the
orientation (with minimum reception) will vary (that can be within a
minute because of the change in overall faraday rotation). You will
seldom find a real nul. This is because of the existence of a more or
less circular component.

If you want to be sure and also want to cover situations in which the
refracted wave is almost linear polarized, you may consider two
(small) antennas (perpendicular to each other) and switch between the
two.

During last year JOTA I had plenty of space to use two half wave
dipoles, but it wasn't worth to do. I checked it with a simple hand
held vertical tuned loop antenna by rotating the antenna round the Z-
axis.

Best regards,

Wim