Nice response Jim. One comment regarding the statement that "The
composite wave strength may be greater or less than that from the driven
element alone. . ." This may be true in the sense that the field strength
is greater in some parts of the spherical spectrum and weaker in others.
There is still a total field strength that cannot be multiplied without
violating the first law of thermodynamics. The design of the antenna array
is to purposely distort that field so that there is a higher radiated
strength in a chosen direction or directions and the trick is to understand
how to do the distorting to achieve the desired goal. That is probably
what the books with all the tirg were saying. Understanding fields was on
of the tougher subjects, for me, at least, in engineering school and
translates into an incredibly complex subject when applied to antennas.
Fortunately finite element methods have in many ways superseded pure math in
analyzing antennas as evidenced by the popularity of the NEC type programs
and their ease of use for those not familiar with the complex math involved
in field analysis. The unfortunate part of finite element analysis is that
it does not convey the underlying principles involved. That is one of the
reasons why your cogent comments are so much appreciated by old retreads
like me.
73
Bill
W7VP
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "Al Williams" <alwilliams@olywa.net>; <towertalk@contesting.com>
Sent: Friday, July 28, 2006 2:11 PM
Subject: Re: [TowerTalk] Fw: [SteppIR] 3El height
> 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?
>>
>>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.
>
> 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
>>reflection here!
>>
>>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
> one down.
>
> 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.
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|