[TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please

[hasan Schiers]

Comments interspersed in the text below:

----- Original Message ----- 
From: "Pete Smith" <n4zr at contesting.com>
To: <keith at dutson.net>; "'Towertalk'" <Towertalk at contesting.com>
Sent: Monday, June 21, 2004 12:54 PM
Subject: RE: [TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please


> At 11:16 AM 6/21/2004, Keith Dutson wrote:
> >Stacking LPAs is rare to my knowledge.  The problem is in planning how
far
> >to separate them.  For best 20-15-10 performance the distance should be
> >about 33 feet.  But this will not work well for 17 and 12, so you will
get
> >better performance working each antenna separately (unphased) on these
> >bands.

Stacking LP's is a problem because the spacing is only going to be optimal
on one band. It can be done with decent results, but not like stacking a
pair of monobanders. Stacking tri-banders has the same problem. See the
Tennadyne web page for an example of stacked T-6's.

> I agree that the LPDA is not the right solution -- it is designed for
> acceptable swr and gain over a wide, continuous frequency range, at the
> cost of gain and F/B in the ham bands, weight and wind area.  However, if
> you chose to go that route, I think that modeling can answer the question
> about stack spacing.
>

Weight and wind area are not concerns with the Tennadynes I used. See my
other post. They are a concern with the monster (in size/weight/price) LP's
I've seen for sale by other vendors.

The only point I would contest, based on my own experience and measurements,
is the lumping in of F/B as an advantage for the typical tribander. I didn't
find this to be the case, even comparing it to a 203ba....3 el 20m
monobander. I found it to be 25 dB in a selected spot, but moving 150 kHz
away, saw the F/B deteriorate markedly.

Even well designed tri-banders do not do a good job of maintaining F/B
across the 20m band. They can have a very high F/B over a VERY narrow range
of frequency, but they do not maintain this front to back going, for
example, from 14.025 to 14.300.  These same antennas do a much better job of
maintaining forward gain across the 20m band, but in F/B, they did not fare
well. To be fair, a monobander can be designed for better front to back
across a wider range than the typical settings provide, but there is always
a sacrifice in gain when this kind of optimization is done.

With respect to the Tennadyne T-10, I made some specific measurements using
a step attenuator and found that the F/B was no worse than 17 dB at every
frequency I checked (sampled approximately ten points per band for all ham
bands from 13 to 30 mhz) It was greater than 22 dB at many of these points,
and greater than 25 dB in a few selected spots.  The tribanders I compared
this to were nowhere nearly as good, once one moved away from the frequency
of best f/b.

In general, the forward gain of the well designed tribander will be better
throughout the common frequency range of a similarly sized LP. The match
will probably not be nearly as good as the LP, and from my experience and
measurements, the F/B of the LP will stay consistently higher throughout the
common range of the two antennas, EXCEPT at one narrow band of frequencies
where the tri-bander or monobander will be considerably higher than the LP.

The advantages compared to tribanders of the Tennadyne LP's I have used:

1. Relatively lower cost.
2. Lower or equal weight to a tribander.
3. Lower wind loading.
4. Mechanically more sound.
5. No traps to fail, virtually no maintainence.
6. Better consistency in F/B
7. Better match to a wider range of frequencies.

Disadvantages:

1. Lower forward gain by at least 2 dB

Summary Comments:

The Tennadyne LP's are light, ruggedly designed and excellent performers.
They are not killer contester antennas, because they lack the ultimate
forward gain of a monobander or very well designed tri-banders.

The one thing I can say as far as "on air performance" is that I get
virtually everyone I call, in the first or second attempt, even in large
pileups. I do call when the band favors me. Kenwood TS-870s > ALS-600
TMOSFET PA (600w) > Tennadyne T-6 @ 52', rural area, excellent HAAT, low
noise.

Hopefully these observations will help someone in the market for a good, but
not perfect antenna that is a fine performer, mechanically sound and
reasonably priced.

...hasan, N0AN