[TowerTalk] lp V stepIR

[Steve, W3AHL]

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