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RE: Cross Yagi with folded dipoles.



Ib,
I have experimented a bit with folded dipole feed points in 70 cm Yagi-Uda
antennas and have seen similar results to yours, but too a much lesser
degree.  

I have found an SWR above 1.5:1 will cause significant 2 m spurs that find
their way into my 70 cm preamp with ease.  I suspect the culprit is
common-mode current flowing on the coax, but cannot say with certainty
anything other than eliminating the feedpoint imbalance cures the problem
(chokes did not help).  I have always been able to get a good match by
adjusting both the overall length of the dipole and the length-width ratio
of the driven element (this latter may not be an option with a commercially
manufactured antenna depending on the stiffness of the material).  I tune my
"crossed dipole feedpoint" antennas without the phasing line by connecting
both feedpoints in parallel to 50 Ohm cable.

If you are using a simple 1/4 wl delay line and coaxial relay to impose the
90 degree phasing, it is virtually impossible to get the two circularity
patterns exactly the same due to the physical offset of the driven elements.
Just 1 cm at 436 mHz is more than a 3 percent change in resonance.
Ellipticity would be the expected outcome from even slight mis-phasing.  The
designs utilizing 1/4 wl axially-separated elements and a 1/2 wl phasing
line for switching are probably superior in terms of circularity, but the
penalty is a longer boom length for no increase in gain.

My modeling results from NEC4Win conflict with your findings:  if the
dipoles are perpendicular and the length-width ratio is high (>= 10:1), the
interaction is practically nil. 

With regards your experience in listening to the different sats, I sometimes
observe a greater than 6 dB difference just in background noise between the
opposing circularities--but with no repeatable pattern.  There is no
apparent correlation between background noise and the stronger signal from
the satellite, thus S/N ratio is how I pick the downlink circularity
du-jour.

73,
Jerry, K5OE


 > Hi all,
 > I have just changed antenne to a 2x19 element Tonna
 > cross yagi for 70 cm.
 > Up to now I have used first a 16 turn helix, model G3RUH, then
 > a linear 19 element Tonna.
 > The Tonna 2x19 have folded dipoles which seems to give a small
 > problem if they are not exactly at resonance.
 > I have made a left hand - right hand switch which is in line
 > as well.
 > 
 > It seems that the rear driven element changes impedance compared
 > to the forward driven dipole.
 > First I had trouble getting the VSWR down. It is 1.7 now.
 > Second I was not convinced that the pattern is really circular.
 > This was when listening to different satellites.
 > 
 > After the live tests I simulated the antenna using EZNEC 2. 
 > My findings are that the folded dipoles have great influence
 > on one another, when the antenna is not in perfect resonance.
 > The change in impedance is big enough to give a 3 dB difference
 > between the vertical and the horizontal field.
 > 
 > I know there are a lot of you using cross yagis with folded dipoles.
 > Have any of you seen anything like this ??
 > 
 > Thanks in advance.
 > 
 > 73 OZ1MY/Ib
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