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Re: mounting crossed yagis



At 04:39 PM 8/7/00 -0500, Jon Ogden wrote:
>You never want to have a metal object in the same plane as your antenna
>elements.  That's why for a vertically polarized yagi, you don't want to use
>a metalic mast ... However, by putting the
>elements at 45 degrees to the boom, I think you minimize any coupling.
>Ideal decoupling is to have the metal object orthogonal to the antenna
>element, but in a CP antenna that's not possible, so you pick the option
>that minimizes the damage.

By putting a wire (or mast or boom) at 45 degrees instead of 0 degrees, you 
multiply the coupling by cos^2(45 degrees) ie 1/2, or -3dB.  However, when 
rotating the antenna from + to X configuration you have to consider the 
other Yagi as well.  You have increased its coupling from zero( ie cos^2(90 
degrees) ) to 1/2 (ie cos^2(45 degrees) ).

So in one case you have a strong coupling to one Yagi, and no coupling to 
the other, and in the X case you have reduced the coupling to one Yagi but 
INCREASED the coupling to the other Yagi.

cos^2(theta) + cos^2(theta+90degrees) = 1

In other words, you can't really get away from the boom by rotating.

Now there are 2nd order effects.  I'll admit that.  For example, the two 
Yagis are fed by a splitter, and the power distribution to the two antennas 
depends on the impedances seen by the splitter.  At least in the X 
configuration the amount that the boom screws up the impedance is equal in 
the two Yagis, thus the splitter is still splitting the power 
equally.  This means that the resulting pattern distortion will be 
different.  Is one case better than the other?  Pretty complex problem to 
figure out unless you simulate or test.

Finally, someone commented on a paper that claims the boom doesn't 
matter.  The theory behind that paper was that along the Yagi's boom (in 
the near field) the E field is not constant, so if you put a crossboom at a 
place where the E field is small, then not much current will flow in the 
crossboom, so it will have little effect.  Sounds good, but unless someone 
can tell me where those magic places are for my particular Yagi, I won't 
know how to take advantage of this magic.  For the typical person who 
doesn't know, and just bolts the Yagi to the metal boom 'cause somebody 
says it's ok, he'll have a screwed up antenna pattern, and never know 
it.  I think it's better to avoid this situation.  By the way, if you're 
trying to use this magic, I would avoid the X configuration, because the 
typical crossed-Yagi has one Yagi mounted 1/4 wavelength behind the 
other.  That means that if you position the boom optimally for this low E 
field interaction the other Yagi will surely be positioned poorly for this 
interaction.

If we all had antenna ranges, and could measure our antennas as installed, 
then all this would be moot.  However, most of us don't have that luxury.

On a related subject, I think antenna vendors should tell people how far to 
keep other antennas away from a long Yagi.  I suspect that many hams who 
mount multiple long Yagis to the same crossboom put 'em too close 
together.  It would be great to see simulation data on gain loss due to 
nearby antennas.

This will be an issue as many of us add some higher frequency antennas to 
our crossbooms for P3D.  Because Yagis are long and skinny, many are 
visually fooled into believing that the space next to the Yagi is 
available.  However, I think it wise to consider first order that we ought 
to keep anything conductive outside the effective aperture, which is 
roughly speaking the size of a dish of the same gain at the same frequency.

If I apply this to a 20dB Yagi at 70cm, I get a radius of 1.1 meters.
If I apply this to a 10dB Yagi at 2m, I get a radius of 1 meter.

Formulas:
Effective aperture area = lamda^2 x Gain / (4 x pi)
Radius = sqrt ( area / pi )

Now if I start with a 10ft fiberglass crossboom, with a rotor in the 
center, and a 2m Yagi at one end and a 70cm Yagi at the other end, that 
only leaves me 5ft on each side.  If the big Yagis would like me to stay 
away from them by about 3ft, then there is only 2ft usable on each side of 
the rotor for the new stuff!  How do I fit antennas for 3 new bands in there?

As I said above, this is all first order thinking.  I'm sure there are 
tricks that can be played.



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