I'll echo those comments 100%. Lately I've been trying to monitor
arrival angles by feeding the signals from two different horizontally
polarized antennas on my tower into the two identical receivers of my
K3, which are phase locked when Diversity Mode is activated. Since
relative phase is preserved in the translation from RF to AF, I feed the
audio from the rig into the stereo input of my computer sound card and
monitor both channels using a sound card based oscilloscope application
(Zelscope). I'm not yet set up for measuring absolute phase so I can't
translate to absolute arrival angle, but I can see the relative phase in
the two waveforms when I use the same trigger for both ... and therefore
I can monitor real time CHANGES in arrival angle.
It's amazing to see how quickly and how significantly the arrival angle
changes, to the point that anything other than a completely real time
comparison between two antennas now seems critically flawed to me.
Here was another surprise. The two antenna I've been using are a 40m
yagi at 83 feet and a single feedline tribander ( 4 elements on 20m, 4
elements on15m and 8 elements on 10m) at 73 feet. I've been using 40m
BC stations around 7250 for signal sources, so naturally the 40m yagi
generally provides the stronger signal ... although I don't really know
where the signals are coming from so quite possibly signals from the 40m
yagi might be weaker for some paths. However, I was listening to a BC
station the other evening where the signal from the 40m yagi varied in
strength with fading as you would expect, while the signal from the
tribander varied from being roughly one third the strength of the 40m
signal to being three to four times stronger than the 40m signal
(linear scale) ... all within a couple of seconds. The signal from the
40m yagi was strongest the great majority of the time, but since we
don't really know for certain what propagation mechanism was causing the
signal from the 20m yagi to pop up like that it could have had a greater
influence on any comparison. Any way you look at it, that's a LOT of
variability for two antennas only ten feet apart.
73,
Dave AB7E
On 9/16/2010 9:44 AM, Jim Brown wrote:
> As to comaparing antennas -- MANY smart engineers among us have correctly
> observed that the difference between two antennas is virtually ALWAYS
> obscured by variations in propagation between two points, and by selective
> fading (multi-path) over that path. I've just built two 160M antennas that
> are wires sloping away from my 120 ft tower, driven against radials. One
> antenna slopes to the east, the other to the west. In this configuration,
> the tower acts as a reflector, providing roughly 6dB front to back. NEC
> predicts a 3dB advantage as compared to my 86 ft tall top-loaded vertical
> with 70 radials. NEC predicts that the antennas will have the same vertical
> pattern.
>
> For the last two weeks, I've been attempting to confirm that advantage. I
> can CLEARLY hear the F/B by switching between the new E and W antennas, but
> QSB is nearly always 15-20 dB over any skywave path, so I've not yet been
> able to confirmed that predicted advantage by switching between new and old
> antennas.
>
> On the other hand, when I compared my 40M vertical dipole with my 100 ft
> high horizontal 40M dipole, the advantage of the high horizontal was QUITE
> obvious, no matter how many times I made the comparison.
>
> I'm also in the process of hanging a 3-el inv vee wire yagi for 40M in my
> tall redwoods at about 100 ft. It's wired so I can instanataneously switch
> between it and a horizontal flat dipole that faces the same direction but
> is about 20 ft higher. So far I have the driven element and the reflector
> rigged, and it's got some gain. It's aimed to about 70 degrees. These
> antennas DO have different vertical patterns.
>
> Last night I was hearing stations from AF at a distance of about 7,800
> miles at 60 degrees. Some signals are louder on one antenna, some on the
> other, I'd estimate by 3dB or more. One was stronger on the Yagi, the other
> on the dipole. I worked one of them.
>
> 73, Jim K9YC
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|