[TowerTalk] lp V stepIR

[Steve, W3AHL]

Gene,

Once you go beyond 13 elements or so, a "formula" LPDA isn't too bad.  But 
the 8-10 element versions definitely benefit from optimization to improve 
gain, F/B ratio and SWR in the ham bands.  There simply aren't enough 
elements to ensure a smooth transition of phasing in the active region where 
the near-resonance elements are.  I haven't modeled a T-8.  But the T-10 is 
definitely not cut to a formula or from LPCAD.

The common approach is to circularize the tau ratio to improve gain at the 
two frequency extremes.  And then gently vary element spacing to maximize 
F/B ratio in the ham bands by keeping the shift around 92-88° between 
adjacent elements to maintain a backward firing wave.  If you reverse 
engineer the T-10's tau and sigma ratios,  Tennadyne took a little more 
creative approach.

When covering an octave range with 8 elements or less, there aren't enough 
active elements in any band to efficiently absorb the power from the 
transmission line and to maintain the phasing required to fully develop the 
pattern.   Yet I know there are many happy T-8 and T-6 owners.   But the 
T-10-class design is really where an LPDA starts settle down and work right.

I agree that an LPDA is certainly a worthy DIY project and I'm looking 
forward building one this spring.  The biggest challenge is translating the 
initial electrical design to the stepped-diameter tubing layout that meets 
the stress requirements without incurring excessive wind load and weight. 
Then modeling the final design before cutting metal.  DX Engineering's Yagi 
Mechanical software is an alternative to the classic Yagi Stress program. 
It had a few bugs initially, but the programmer was very responsive to 
issuing updates and works well now.

The commercial WARC yagis may have better numbers for a given boom length 
and wind load, but the LPDA is a more elegant design approach to true 
broadband performance, in my opinion.  And our local serious contester never 
hesitates to recommend a T-10 as a solid performer to those who can't 
justify stacked mono-banders....

Steve, W3AHL
----- Original Message ----- 
From: "Gene Fuller" <w2lu at rochester.rr.com>
To: <lists at subich.com>; "Tower and HF antenna construction topics." 
<towertalk at contesting.com>; "'Scott McClements'" <kc2pih at gmail.com>
Cc: <k2vi at cox.net>
Sent: Saturday, January 23, 2010 7:36 PM
Subject: Re: [TowerTalk] lp V stepIR


> Another side thought on LP's -
> Obviously not a consideration v the SteppIR, but the LP is pretty straight
> forward for ththe DYI guy who wants to build his own. Somewhat of a
> carryover from before there were as many good yagi design computer 
> programs.
> With the LP you don't have to decide tradeoffs betwen gain and F/B. You
> pretty much cut to formula and get what you expect. When I did mine on the
> PC I did fudge it a bit to try to favor the ham bands but I'm not sure 
> that
> it really bought much extra.
> Gene / W2LU
>
..snip..