That's a good observation. Although both antennas (an Optibeam OB2-40
and an OB16-3) have all their elements isolated from the boom, I do have
the shields of all my coax runs grounded to both the top and the bottom
of the tower. I suppose it is possible that the combination of the
tribander and the tower is not acting with horizontal polarization, and
that would certainly make a big difference compared with horizontal
polarization of the 40m yagi.
Eventually I want to install two identical non-resonant horizontal
dipoles insulated from the tower (spaced about 10 or 12 feet apart) and
fed with equal lengths of identical coax. That should be a much better
test vehicle.
Still, the significant variability within such a brief period of time
that I'm observing suggests that how we hams typically compare antennas
is not very reliable.
73,
Dave AB7E
On 9/16/2010 12:07 PM, K0DAN wrote:
> 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
>>>
>>>
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>> _______________________________________________
>>
>>
>>
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