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@subich.com>
Subject: Re: [TowerTalk] lp V stepIR
To: "'Tower and HF antenna construction topics.'"
<towertalk@contesting.com>, <k2vi@cox.net>
Message-ID: <8094CBED99BC4487B4337E12BE80281D@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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