[TowerTalk] lp V stepIR

[Joe Subich, W4TV]

> Your model of the 8-element LPDA was apparently done from a 
> formula and wasn't optimized at all. 

Absolutely correct.  However, it was compared element length 
and spacing data for both the Cushcraft ASL-2010 and the then 
current Tennadyne T8.  The Cushcraft and Tennadyne designs each 
had only minor departures from the "formula" design (which came 
from Roger Cox's LPDA software).   

It would be easy to optimize a 9 or 10 element LPDA on a 20 
foot boom and get closer to yagi performance ... there are 
tricks for improving performance at both ends of the spectrum 
for the log-periodic at the expense of performance some other 
place in the range.  However, a 20 foot boom 10 element LP 
is much larger and heavier than a 3 element SteppIR of Force 
12 XR5 and still does not match the performance of either 
of those antennas. 

73, 

   ... Joe, W4TV 
 


> -----Original Message-----
> From: towertalk-bounces at contesting.com 
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Steve, W3AHL
> Sent: Saturday, January 23, 2010 3:27 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] lp V stepIR
> 
> 
> Your model of the 8-element LPDA was apparently done from a 
> formula and wasn't optimized at all.  Although I do agree 
> that 6 & 8 element logs are sub-optimum.   But an optimized 
> design for a 9 element 14-30 Mhz LPDA on a 20' boom will 
> average closer to 6.5 dBi with 17 dB F/B, except at 20m, 
> where it drops to 5.7 dBi & 11.3 F/B in free space.  Add a 
> 10m director and you can get 7+ gain with 30 dB F/B on 10M 
> (with a 4' boom extension).
> 
> And stacking an LPDA does have unique issues.  Anything that 
> disturbs the current phasing in the active-element  region 
> will distort the pattern more than with a standard yagi design.
> 
> Steve, W3AHL
> 
> Message: 8
> Date: Sat, 23 Jan 2010 14:02:36 -0500
> From: "Joe Subich, W4TV" <lists at subich.com>
> Subject: Re: [TowerTalk] lp V stepIR
> To: "'Tower and HF antenna construction topics.'" 
> <towertalk at contesting.com>, <k2vi at cox.net>
> Message-ID: <8094CBED99BC4487B4337E12BE80281D at laptop>
> Content-Type: text/plain; charset="us-ascii"
> 
> 
> > 3 element stepir and the t8 will radiate almost the same
> > signal strength at the other end.
> 
> Both modeling and theory strongly disagree with this  ... 
> 
> The active portion of an 8 element 13-32 MHz LP on an 180 
> foot boom is a small portion of the total boom length.  
> Here are model data of an 8 element LP (18 foot boom), 
> the SteppIR 3 element yagi (16 foot boom) and Force 12 
> C3E (extra 10 meter director and 18 foot boom).  
> 
> The SteppIR/3 vs. 8 element LP vs. C3E (free space): 
> ---------------------------------------------------------
>                    LP            SteppIR            C3E
>     Band        Gain   F/R     Gain     F/R     Gain   F/R
>      20          5.0   9.2      7.4     25       6.6   10
>      17          4.9  12.3      8.3     25       3.0    2
>      15          4.8  13.0      8.5     20       6.2   11
>      12          5.0  14.9      8.8     15       4.4    5 
>      10          5.1  14.5      9.0     11       7.4   23 
>       6             n/a        10.1     20          n/a
> 
> The Log Periodic is a simplified model of an 8 element design 
> on an 18 foot boom.  K9LA's work with the T-6 shows about .5 
> dB less gain for a shorter (12 foot boom) antenna while   
> Preliminary (unverified) results for a 10 element model (24 
> foot boom) show about .8 dB additional gain and 1-2 dB better 
> F/B (model data from AO). 
> 
> The SteppIR/3 vs C3E numbers closely parallel the field test 
> data published by N0AX/K7LXC and SteppIR. 
> 
> One can see that the LP fails to come even close to the SteppIR 
> - falling short my more than 3 dB except on 20 meters.  Further, 
> falls one to two dB short of the typical gain of a two element 
> (Reflector/Driven with 50 Ohm feed).  
> 
> An LP is a poor trade-off unless you have excess power and 
> need to operate on any frequency instead of the relatively 
> narrow amateur bands. 
> 
> 73, 
> 
>    ... Joe, W4TV 
> _______________________________________________
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