[time-nuts] DIY QFH

Jim Lux jimlux at earthlink.net
Fri Nov 29 09:55:57 EST 2013


On 11/28/13 10:37 AM, Attila Kinali wrote:
> On Wed, 27 Nov 2013 08:32:58 -0800
> Jim Lux <jimlux at earthlink.net> wrote:
>
>> What's interesting is that this is NOT a crossed dipole, at least from
>> what I can see. It looks like all 4 arms are connected to the same point
>> at the top. So it's some sort of funky folded monopole
>
> The generic helical antenna is just a funky folded monopole.
> I cannot exactly say how that works out, as i'm not an antenna
> guy, but judging from the amount of ham radio helical antennas
> the design seems quite robust.
>
>

Kraus's book describes helicals, and as you say, the design is very 
non-critical, particularly for an axial mode (end fire) helix.

A broadside or normal mode helix is a bit trickier to analyze and 
design, and for the quadrafilar helix even more so, especially if you 
want to have good axial ratio.

Attached is a modeled plot of a crossed drooped dipole. The color is the 
axial ratio, and while it's pretty good straight up, at the horizon, 
it's basically linear polarized.

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