[TowerTalk] Raibeam's

[Tom Rauch]

>   I'd like to hear from any and all that may have any experience
> directly, indirectly, (even hearsay) with Raibeams.

I've use almost identical phasing systems since the 70's on 
receiving antennas and on a few transmitting antennas. My 40 
meter 2 element beam in South Amherst Ohio had cross-fire 
elements. 

> defecting from the multi-band 2 element yagi camp, lured by thoughts of
> high f/b ratio, "quiet as a quad" reports, etc. I don't put much faith in
> high gain as these appear to be ZL Special's/HB9CV's and reports of 3-4
> dbd are all i've ever seen attributed to them. BUT, 20-30 db f/b is a
> powerful incentive.

Quiet as a quad, eh? That would be true in most situations, since a 
quad isn't the least bit "quieter" than a yagi unless the antenna 
itself is involved in the generation of corona discharge. 

If the antenna was directly involved in the generation of corona, 
then the quad could be quieter than an antenna with pointed 
elements whose high-impedance ends stick-out in space.

High F/B ratio is true, since the F/B or F/Null depth of any multiple 
driven element array has in theory infinite null depth when properly 
adjusted. That was a good point for my cross-fire 40 meter 
antenna, it had over 50 dB of F/N ratio. F/N ratio, when the nulls 
are properly placed, is much more important than anything else 
when the antenna is used for receiving on a band crowded with 
QRM from a direction different than the desired signals.

You have less freedom to place nulls at a useful angle while 
making them extremely deep when you have parasitic elements.

The gain essentially is no higher than any other combination of two 
similar elements on the same size boom, there is no magic 
involved. Maximum gain would occur with the back-nulls set 
somewhere between 25 degrees and 50 degrees off the exact rear.

It's easy to visualize how gain works by squeezing a slightly 
inflated balloon with your hands and watching the nose. If you 
poked a "null" straight off the back, a large area of the balloon gets 
the extra air and so it swells less than with other squeezes.

If you squeezed a null in the form of a circle not directly on the rear 
axis of the balloon, you have more air (energy) to expand the 
balloon. Maximum forward swelling (gain) would occur when the 
null occupies the largest surface area possible. 

That's why no antenna, despite anyone's claims, can have 
maximum gain and maximum F/B at the same time.

The balloon thing comes from live demonstrations I used in 
"lectures" on antenna patterns. Please don't patent it!


73, Tom W8JI
w8ji@contesting.com

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