Your 40M monitoring using the tribander may have your tower
acting as a top-loaded vertical.
I have not played much at 40M, however when I listen on 160M
using my 10 el. log periodic on top of a 75' tower/mast, I get
better signals (better S/N, less fade, better DX) than with a 250' Zepp
fed with 600 ohm line.
Am in the midst of tower repairs right now, so that configuration is
temporarily QRT, however I plan to do more experimenting in that arena.
YMMV
73
Dan
K0DAN
----- Original Message -----
From: "David Gilbert" <xdavid@cis-broadband.com>
To: <towertalk@contesting.com>
Sent: Thursday, September 16, 2010 12:43 PM
Subject: Re: [TowerTalk] 80-m. Inverted Vee vs. Dipole Performance
>
> 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
>>
>>
>> _______________________________________________
>>
>>
>>
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