On Wed, 23 Jul 1997 20:03:33 EDT k1vr@juno.com (Fred Hopengarten) writes:
>From:
>Fred Hopengarten K1VR 617/259-0088
>Six Willarch Road
>Lincoln, MA 01773-5105
>permanent e-mail address: fhopengarten@mba1972.hbs.edu
>
>On Wed, 23 Jul 1997 10:40:48 -0700 (PDT) Larry Tyree
><n6tr@teleport.com> writes:
>
>>I have heard of at least one case where doing this destroyed the
>>magic on topband. I believe my antenna has lots of "magic"
>currently,
>
>
>While KM1H and some other guys have told me that matching a loaded
>tower is easy, and requires only "iterations" until you get it right,
>W1JZ has always taken the position that you should leave the tower
>bare, except for a single yagi on top. Finding the exact right
>feedpoint is, in my experience, a problem -- and I'm not sure I've
>found it yet. The heart of the problem is that it remains black
>magic. No one has ever written up a series of steps which, if
>followed, will ALWAYS result in a good match and an efficient antenna.
> If someone has written up such a step-by-step procedure for loading a
>tower, I've never seen it -- BUT I'D LOVE TO SEE IT! -- Fred K1VR
Fred, the John True articles in HRM will put you darn close. If you are
looking for an Appliance Operator solution, I am afraid you have come to
the wrong band. There are just too many variables and unknowns to let you
"Plug n Play".
It makes very little difference how many yagis are at the top of the
tower. The lowest one in that top stack, assuming it is also the lowest
band or physically larger, will predominate in setting the capacity to
ground and top loading effect.
At my old QTH, once I had the tower tuned with the 10/15/20 M top stack,
it worked fine no matter what I added down below later on. Those add-ons
were 10 and or 15M 4el monobanders. For the purist, a slight tweak on the
Omega match brought the VSWR back to 1:1 but if it only degenerated to
1.3:1 or so I never worried about it.
Adding many more radials and the ground screen had a more pronounced
effect on the tuning than some side mounted yagis. If those sidemounts
were monster 5 el 20M or a huge 40M then I would agree that the tuning
would probable be upset. But with the Omega it is no big deal to retune.
Unless you hit it just right the tower is simply a non reasonant
structure that you are forcing to accept power and radiate efficiently.
Structual reasonance is not a prerequisite to efficient radiation when we
are working with 80 to 150' towers on 160M.
Read True's articles, mine in CQ in 1986 sometime I think, and ON4UN's
book and you can get pretty close. I wrote mine without having read
Trues's but the results were very similar. Later we corresponded numerous
times, up to a few years before his death.
The trouble with you lawyers Fred, is that you expect everything in a
reference book in your library!! On top band you gotta get ur hands
dirty and feel the pain....
73...Carl KM1H BTW say hi to Betty and the kids.
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