Al,
Your summary of how yagis work is substantially correct. If you want to
really understand and predict how antennas work, you need a lot more
than "trigonometry". It's one of the most complicated parts of
classical Electricity & Magnetism, in my experience. (As an ancient PhD
in physics.) The good news is that there are lots of rules of thumb and
design guides that get you the results you need for practical ham antennas.
I can ramble on a little...
Changing the element dimensions can change the patterns, in both planes
(vertical and horizontal). Just for example, if you adjust the director
& reflector lengths way off resonance, you are left with only the driven
element carrying significant current, which will produce the pattern of
a simple dipole. In free space, the patterns will be symmetrical above
and below horizontal. Realistic installations get reflections and
absorption with the ground and any nearby stuff. Changing the element
dimensions will broaden or narrow the free space pattern, but it will
always be symmetrical.
For a particular QSO path, the only "useful" power is the power radiated
at the particular takeoff angle that gets to the other end of the link.
You want to maximize that, and it's possible that detuning the vertical
pattern will help in some cases. Likewise, it may help to raise (or
lower) the antenna, if you can, to change the ground reflections.
When you twiddle the lengths of a SteppIR at a particular frequency, you
are mainly changing the phase of the currents flowing in the elements.
The antenna pattern you see in the far field (at the other guy's
antenna) will always be the vector sum of the fields produced by all the
yagi's element currents. The hard part of the calculation is to figure
out what those currents and phases are going to be, because everything
interacts with everything else...
Why does a dipole have gain and directivity? A simple perspective is
that "currents cause radiation". If you look at a dipole end-on from
some distance away, you will not "see" any current flow (projected on
the plane perpendicular to the line of sight) -- it's all toward and
away from you. If you look broadside, you see maximum current. So, you
might expect the fields would be proportional to the "projected length"
of the dipole at any angle - i.e., like the cosine of the angle off
broadside. Power goes like cosine**2, which is a pretty good fit to
reality.
The fact that radiation off the ends is reduced means that broadside
radiation is increased -- i.e., you have gain. It's easiest to think in
terms of a transmitter -- the power has to go somewhere.
Hope this helps.
73 Martin AA6E
Al Williams wrote:
> No one on the SteppIR yahoogroups forum has responded to my posting. Maybe
> some Towertalkians are interested and knowledgable?
>
> k7puc
> ----- Original Message -----
> From: Al Williams
> To: SteppIR@yahoogroups.com
> Sent: Thursday, July 27, 2006 1:10 PM
> Subject: Re: [SteppIR] 3El height
>
>
> Jerry, k3bz, sets forth an interesting concept that tuning the SteppIR
> elements may somehow change the vertical angle of maximum radiation for Yagi
> type arrays.
>
> For some time I have been trying to understand why/how Yagi antennas provide
> directional gain. In going through about 20 antenna books written for the ham
> community, only one offers what appears to be a technical explanation. This
> book is Lawson's "Yagi Antenna Design", but its explanation uses too much
> trigonometric formulas for me to wade into. All other books limit their
> explanation to two factors: 1. that the
> magnetic field caused by current in a antenna causes an induced current into
> adjacent wire/elements which then radiates its own electromagnetic wave. 2.
> The electromagnetic wave from this adjacent element adds to the
> electromagnetic wave from the driven element
> which results in a composite wave strength at distant receptions. The
> composite wave strength may be greater or less than that from the driven
> element alone depending on the phase difference between waves at the
> receiving site.
>
> The phase difference is caused by three factors: 1. The distance/time to the
> receiving site from the driven and the induced elements signal.
> 2. The phase difference in the induced element causes am 180 degree lag of
> induction plus a phase shift if the adjacent wire is not resonant.
> This non resonant adjacent wire appears to answer why a reflector is longer
> and a director is shorter than the driven element.
>
> 3. The third factor is the phase shift caused by a distance/time of the parts
> of the wave being (reflected/reradiated?) from the source on its journey to
> receiving site. Note that I haven't mentioned the angle of reflection here!
>
> Now, I think that Jerry is suggesting that this third factor can be
> compensated for by adjusting SteppIR's element lengths.
>
> Well, why not??????
>
> k7puc
>
> BTW Why does a dipole have a directional pattern?
>
>
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