[TowerTalk] lp V stepIR

Sat Jan 23 20:36:42 PST 2010

Hi Steve -
I'm replying just to you because much of the gang may be tired of reading 
this thread. I'm glad to hear you're thinking/planing to do an LP before 
long. I'm sure you'll be happy with it. A few notes on mine -
Frequency  7-60 MHz
Tau ..835
Sigma  .042
Boom 46 feet - 4" heavy wall irrigation tubing  30' +16'  with overhead guys 
from mast to 4'x4" channels .
15 elements
Longest element 46 feet - last three have linear loading from 10 feet out
small inductive short on low freq end.
Stretched the spacing to the longest element in  hopes of picking up some 
parasitic reflector action on 40 but still only have about 10 db on 40. It 
probably could be improved with some fine tuning.
Boom feeder is basically 500 ohms but tapered it a bit to account for the 
change in element diameter/freq ratio.
Feed point designed to be 200 ohms with 4:1 balun back at the mast.
Turned with small Prop Pitch motor with sylsens.
Used AO for design of everything including linear loading and boom feeder.
Can be seen on QRZ.com

Because of the compromise Tau and Sigma the SWR is not really good but under 
3:1. Amazing how close the low freq cut off came out . 7.03 about 2.5:1,7.0 
about 4:1, 6.9 about 20:1. Fortunately I'm not a CW operator, although I'm 
sure it would operate ok down at the band edge.  By virtue of being at 100 
feet, being fairly efficient and having at least some gain it works fairly 
well  on 40. Works the Aians at sunrise in the contests and is very 
competitive on 20. - with a SINGLE 8877 homemade amp.

I'm currently going thru some pain with the rotator. The Radio Shack diode 
bridge I was using to supply DC to the PPM shorted and burned out the 
transformer. This was all at the top of the tower. I'm trying to cut back on 
the tower climbing (now almost 75) . I am therefore moving the power supply 
down to the base of the tower and redoing the sylsen setup, since I found a 
good commercial gear to match up with the bevel gear on the PPM.  At any 
rate if you haven't guessed, I'm very happy with the LP.
Re stacking- I inherited a TH-6DX a number of years ago after our big ice 
storm. I totally rebuilt it, welded up a side gate and mounted it at about 
45 feet on the tower. Recognizing that the effective phase center on the LP 
changes with freq I buildt a variable "phase" box with 2,4,and 8 foot pieces 
of coax selectable with relays as 2-14 feet. I find little advantage in 
using the "stack". If the phasing is really wrong I find a notch that will 
nock signal down 10-20 db, but the peak is very broad and scarsely 
measurable given QSB. In contests however it's very nice just pointing the 
TH-6 at the Carib and letting the LP do the rest of the work. For the Carib, 
the TH-6 at 45 feet is literally about as good as the LP at 100. Down to VP8 
of course is a different story.

My conclusion is that a pair of LP's (about 20 elements each on 50 foot 
booms)  on a 100 foot tower with tipped booms giving something like halfwave 
stacking for 10-20 meters would be an absolute killer I think W1KW had 
something .like that back in the 90's, but not quite that long on the booms.

I guss that's more than enough. Good luck with your's. If you're not too 
busy working dx after it's up you might let me, or all of us, know how you 
made out.
73's    Gene / W2LU

----- Original Message ----- 
From: "Steve, W3AHL" <w3ahl at att.net>
To: "Tower and HF antenna construction topics." <towertalk at contesting.com>
Sent: Saturday, January 23, 2010 9:06 PM
Subject: Re: [TowerTalk] lp V stepIR

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..

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