Hi Jim,
The antenna I used was a ta33 and I mounted a vertical dipole out on the
end of the boom near the director. I realize it was kind of a crude way
of doing it as the beam had more gain than the vertical dipole but the
basic properties of vertical and horizontal were there.
I ran equal length cables and made an additional length of cable the
proper electrical length to compensate for the physical spacing
difference between the vertical element and the driven element of the
beam. That put them both electrically at the same point in time.
I used an additional 90 degree length of line to switch into either feed
line to get either right hand or left hand circular polarization.
I found that when signals were fading rapidly, 5 to 10 seconds apart,
switching from right hand to left hand made a tremendous difference.
When the signal went into a deep fade on right-hand polarization I
switched to left hand and the signal jumped up out of the noise.
Same when the signal was in a fade on left-hand polarization and I
switched to right-hand it again jumped out of the noise.
This seemed to confirm that the signals were rapidly changing from one
mode of circular polarization to the other. If that was not what was
happening then switching from left to right would not have produced a
null at one while enhancing at the other sense.
When I fed the antennas in phase, they were electrically at the same
point in time via the proper length delay line as above. Only the 90
degree line was not used.
This smoothed out the fades. No deep nulls. It was a pretty constant
signal strength. However signal strength was down a little with both
antennas as compared to one alone on average.
I could also switch to either just the vertical or just the horizontal
antenna. At times one or the other would be best. But it required
constant switching as the 5 to 10 second fades happened.
The two antennas in phase were the smoothest. No deep fades but with a
sacrifice of some gain.
It was not multi path. The two antennas were separated by 6 or 8 feet
but in line with the other station and the delay line put the two at the
same point in time electrically.
The path was about 2000 miles to a friend. We met on several days and
always found the same results.
Now supposedly by having the vertical and horizontal in phase should
give polarization at 45 degrees? Whatever I got sure worked well on
those signals.
73
Gary K4FMX
Jim Jarvis wrote:
> Gary (TT):
>
> FM Stations transmit circular polarization,
> which results in signals being 3dB down in
> any single plane, but less susceptible to
> selective fading/interference problems,
> particularly with subcarrier content.
>
> Not to forget, they're only concerned with line
> of sight, which eliminates the problem of
> ionospheric scrambling of the polarization.
>
> As to your 10m experience, you didn't indicate
> what the spatial relationship of the two antennas
> might be. I assume they were the CB type, on a common
> boom, if you actually fed them circularly.
>
> My belief is that you experienced the diversity effect,
> where the sum is down 3dB from peaks on either antenna,
> but the signal dips are filled in. It's more likely
> due to differences in arrival angles, rather than
> polarization, by the way. Most stuff is coming in
> horizontally polarized, once it goes through the ionosphere,
> or so I've read.
>
> N2EA
>
> -----Original Message-----
> From: Gary Schafer [mailto:garyschafer@comcast.net]
> Sent: Monday, April 04, 2005 12:38
> To: Tom Rauch
> Cc: jimjarvis@ieee.org; towertalk@contesting.com
> Subject: Re: [TowerTalk] simultaneous Horizontal and Vertical antennas
>
>
>
>
> Tom Rauch wrote:
>
>>All of the time a practice like this severely increases
>>fading where signal levels from the two antennas are close.
>>It may reduce the number of people operating on or near the
>>TX frequency, but everyone should consider it also increases
>>fading in a broad unpredictable area.
>>
>> > Secondly....with respect to the original concept of
>>running parallel
>>
>>
>>>vertical and horizontal antennas to get multiple
>>
>>polarizations...
>>
>>
>>>twaddle!
>>
>>
>>Doesn't work. What you get is a skewed pattern with wider
>>beamwidth and less defined polarization. In every direction
>>the resulting pattern is a single polarization that is the
>>vector sum of the two fields in that direction.
>>
>
> How do FM stations manage to transmit dual polarity?
>
>
>
>>The ionosphere will scramble the polarization, anyway...what
>>
>>
>>>he's doing is simultaneously exciting high and low
>>
>>angles...(assuming
>>
>>
>>>the vertical has a decent ground system.) He's also
>>
>>destroying the dipole
>>
>>
>>>pattern.
>>
>>
>>As the above statement points out, the concept of thinking
>>of this as two totally isolated and independent signals is
>>wrong.
>>The actual result is it simply tips or tilts the
>>polarization of the pattern in every area where each antenna
>>has significant radiation.
>>
>
> What do you think I was seeing when I phased vertical and horizontal
> antennas together on 10 meters? I found that with severe rapid fading on
> either vertical or horizontal alone, when I switched to both phased
> together that the fading smoothed out. No more deep fades into the noise.
>
> I did find that by phasing the two as right hand or left hand circular
> that the signal was rapidly shifting between left hand and right hand as
> confirmed by swapping sense. But with the antennas in phase at those
> times it worked quite well.
>
>
>
>>Sometimes we want to transmit a broad beamwidth signal, but
>>the best solution is to just design an antenna that does
>>that and understand doing so severely increases fading in
>>the wide area (angle and direction) where both antennas have
>>significant FS.
>>
>>
>>
>>>I'd bet he'd get better results by switching between them
>>
>>and picking
>>
>>
>>>the better antenna at any given moment.
>>
>>
>>I bet that also. The reason broadcast stations quickly
>>abandoned 5/8th wave verticals is the combination of high
>>and low angles caused by the high angle lobe that appears
>>with the 5/8th led rise to severe fading outside of the
>>strong groundwave area of the antenna. This effect was so
>>severe they actually called shorter verticals "non-fading
>>verticals".
>
>
>
> But here you are talking about multipath propagation.
>
>
>>How you tune and load each amplifier plus other less
>>esoteric things like feedline length control where the new
>>nulls are as well as where the new main lobes are formed.
>>
>>73 Tom
>>
>
>
> 73
> Gary K4FMX
>
>
>
>
>
>
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