At 08:59 AM 3/16/2005, K4SB wrote:
>Michael Tope wrote:
> > Steve,
>snipped....
> > to do for a fixed element length yagi. It could very well be that
> > a SteppIR can eek out an extra .5dB of forward gain when
> > you get rid of the bandwidth constraints.
> > 73 de Mike, W4EF................................
>
>I would tend to agree more with Steve and Dave on this matter. There
>is simply no way in the world ( short of an advanced antenna range )
>to determine how an individual element in the SteppIR might be changed
>to get additional gain.
There are two possible strategies:
Do modeling, and optimize for desired performance, running several models
with small variations to take care of tolerances. This will prevent you
from getting into configurations that are excessively "picky".
Adjust it on the fly for observed performance. I think some folks are
doing this by hand, but there's no real reason why you couldn't have a
computer start with a modeled starting point, then make small changes in
element lengths to increase a desired signal or reduce an undesired signal
(essentially doing what Excel's solver does). There are a significant
number of problems with this strategy (fading being but one), however, over
the long term, with good data recording, it could probably optimize your
antenna for your location.
(The strategy would be one like the optometrist does when checking your
correction. The computer would offer two choices and switch between them
and ask you whether you preferred "A" or "B". It would then log this
(along with azimuth and any other relevant data). Over some time span
(say, months, or years), you could arrive at an optimized
configuration. This is really an implementation of a "genetic" algorithm
in the optimization world.)
>As an example, the element length and spacing on my 5 element
>monobander on 20 * and its twin on 15 ) is so critical I had to use
>millimetres for the tips and element spacing. As it stands right now,
>the antenna models at 50 +/- j0, and shows 15.93 dbi of gain with 55.6
>db F/B. All this modelling was done with EZNEC. (DOS and Windows
>versions) I can vary the spacing of the 3rd director ( or length ) and
>that pattern goes to hell in a hand basket.
Which is an indication that the design is overly sensitive.
null depth (which is what you're really looking at here) is exceedingly
sensitive to small changes, particularly with nulls more than 30 dB
down. A deep null is really the result of all the radiation from all parts
of the antenna, in that particular direction, exactly cancelling. It
doesn't take much of a difference to destroy the null.
Think of it this way... if you're radiating a kilowatt in the desired
direction, inadvertently radiating 1 watt in the wrong direction makes your
sidelobe performance 30dB.
(People who design radar antennas with better than 50 dB sidelobes have
quite a challenge in front of them... Radiate a kilowatt, and the biggest
error can be 10 milliwatts..)
On typical amateur antennas (probably ANY HF antenna), a 50 dB null is
spurious and unrealistic.
>Admittedly, 55.6 db F/B is excessive, but I have always gone with the
>theory that you
>shoot the "juice" in the desired direction at all costs.
Maximum F/B is NOT maximum forward gain. As you say, on Tx, you want to
squirt the juice in the desired direction, but changing from a 40 dB F/B to
a 30 dB F/B is like radiating 1 watt more out of a kilowatt. That's the
same as your feedline loss changing by 0.004 dB.
I would venture to guess that a very high F/B design will have more
resistive loss in the elements (almost by definition, it's highly coupled
among the elements), and will actually radiate less power in the desired
direction than a lower F/B design.
What you want on TX is max foward gain. What you want on receive is good
F/B and good directivity (to suppress interference from the non-desired
direction). Since, on HF, most of the noise isn't from the receiver,
you're willing to give up some antenna efficiency, in exchange for looking
at a smaller piece of the sky. Not to mention suppressing that guy coming
in on the reverse path behind you.
I don't know that anyone is doing this with the SteppIR (I don't know that
the response time is fast enough), but it might be worthwhile switching
between a low loss, high efficiency mode on Tx, to a higher loss,
superdirective, narrow beam mode on Rx.
>As an after thought, I firmly believe that antenna height is the
>overall controlling factor. As some of the guys have pointed out, my
>TH-11 at 55' will whip the snot out of that big 20 under 5000 miles.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|