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backfire helix questions answered as best I can



I have received several questions on my backfire helix:

1. [K0VTY] can you adjust the pitch to change the Helical Beam width in 
order to accommodate a f/d change?
Somewhat.  From Nakano's results, you can vary from maybe 95 degree -10dB 
beamwidth (pitch=18deg) to about 135 degree -10dB beamwidth (6 degree 
pitch) and still maintain a -15dB front-to-back ratio.  Maximum 
front-to-back however is clustered fairly well between the 125-135 degree 
-10dB beamwidth region (f/b~20dB).
2. [K0VTY] The insulation in the matching stub that prevents the shorting 
of the helix to the reflector ?
Yes, but the ground-plane to "matching stub" spacing is sufficient that 
insulation shouldn't be necessary.  The honest truth is that I was too lazy 
to strip the THHN bare :-)
3. [K0VTY] Looks hard to be able to support the monofilar helix.
I did not come up with a decent solution to this.  A thin plexiglas rod or 
thin fibreglass sheet would probably do the trick.
4. [K0VTY] The number of turns ? Needed for the good axial ratio or beam 
width? Or both?
More for axial ratio than beamwidth, I think.  I don't have it handy but I 
believe other work by Nakano indicates between 5 and 8 turns.  Fewer than 5 
makes for poor axial ratio, and more than 8 don't change anything.  The 
current has diminished so much that there is no additional "antenna" effect.
5. [K0VTY] The "N" connector center pin turns normally . how was this 
captured ?
None of mine do.  Never have.  They all have epoxied center pins.  Maybe 
because they're UG-numbered, not imports?
Joe's keeping me busy...
6. [N4HY] Can you see any reason not to support the helix with some small 
threaded Plexiglas rods? This would involve
a modification of the ground plane to allow for the mounting of the 
Plexiglas and probable change in its dimensions. Comments?
No reason not to support it so, but the metallic dimensions of the ground 
plane are *critical* and should not be blindly messed with.  Going from 
just 0.35wl to 0.29wl in diameter changes the front-to-back by 10 dB.
I have never come up with an easy way to support my helices...at least not 
with material on-hand

And thanks to all who have passed along their comments!






Scott Townley NX7U
Gilbert, AZ  DM43di
http://members.cox.net/nx7u

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