Jim,
As it said -- it's free space gain. It's the same antenna I sent you the
direct reply about earlier. This antenna design is still being tweaked and is
an example of optimizing it for the higher bands and low SWR (below 1.4:1
across the 20-10m bands). Adding another element and going to a 24' boom
brings 20m performance up. Which is why the T-10 is so popular.
And contrary to an earlier post I saw -- the current Tennadyne T8 and T10 booms
seem to be 50 ohms, not the classic 200 ohm impedance split boom transmission
lines. The advantage is you only need a 1:1 balun at the feed point and the
boom is more rigid. The disadvantage is they seem to be a little prone to
pattern reversal above 24 MHz, although the rear stub can usually move it out
of the ham bands. A 150 ohm boom presents a feed point impedance of about 100
ohms, so a 2:1 balun works.
And to clarify my earlier post, stacking two identical LPDA's doesn't create
any unusual problems. And there is only a slight advantage to tilting the
booms in a V configuration. But putting a 15m yagi too near an LPDA isn't
good. Stacking a 40m yagi 20' below an LPDA didn't create too much pattern
distortion in EZNEC, but it probably depends upon where the elements are and
how large the boom is.
Steve, W3AHL
Date: Sat, 23 Jan 2010 13:14:47 -0800
From: jimlux <jimlux@earthlink.net>
Subject: Re: [TowerTalk] lp V stepIR
To: "Tower and HF antenna construction topics."
Steve, W3AHL wrote:
> 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).
Is that gain in free space, or over some assumed ground at some assumed
height?
>
> 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.
>
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