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Re: backfire helix questions answered as best I can



Excellent response Scott (NX7U).

Thanks you sir.    You are right on top of the issue.

Joe  K0VTY
=========================
On Mon, 17 Nov 2003 21:32:26 -0700 Scott Townley <nx7u@arrl.net> writes:
> 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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> 

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