At 09:12 AM 7/28/2006, Al Williams wrote:
>No one on the SteppIR yahoogroups forum has responded to my
>posting. Maybe some Towertalkians are interested and knowledgable?
>----- Original Message -----
>From: Al Williams
>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
That's a fairly good description
>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.
Exactly.. there's an effect just from distance (i.e. if you were a quarter
wavelength away, the incident field is going to be 90 degrees lagging) and
an effect from the reactive component of the element (which can be either
leading or lagging)
>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
>Now, I think that Jerry is suggesting that this third factor can be
>compensated for by adjusting SteppIR's element lengths.
Yes. Sort of. The usual Yagi design process tries to create element
currents that produce the desired gain in the "end fire" direction aligned
with the elements. However, nothing says that this HAS to be the
case. you can choose currents (or try to get currents) that cause the
steering angle to be end fire, broadside, or anywhere in between.
Start with a sort of simple case, 3 elements a a sixth of a wavelength
apart. Say you could magically get the currents to be phased 0, 60, 120...
That would make a beam off the end. If you could get the currents phased
0,0,0, then you get a beam at 90 degrees relative to the end (i.e.
broadside to the array). If you phased, say, 0, 30, 60: then you get a
beam at roughly 45 degrees off the end (or, really, two beams, one up and
Since you can change the relative phasing by changing the relative element
lengths, you can do this sort of thing (within limits). The real challenge
is that you get limited adjustment capability because the spacing is fixed,
and by changing lengths, you can't get arbitrary phases and amplitudes.
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