Stacks are good things when you are over flat ground. As Lee noted, you
can select different antenna combinations to suit the band conditions.
The big contest stations have known this for a long time.
Less known is what happens when your ground is not flat. Until recently
hams have not had any way to analyze this affect. The 20th Edition of
the ARRL Antenna Book has included (HFTA (High Frequency Terrain
Analysis) as a free program. This program will allow you to look at
antenna launch angles from your QTH, or from any other place you desire.
To me this program, by itself, has been worth more than the price of the
book.
This program will also analyze stacks. If you are not on flat ground,
the optimum stacking arrangement will be different from what you expect.
You may be in for some surprises. I happen to have one of those
locations. I have sloping ground which produces some low angle signals
from a low antenna. When I try to construct a stack at my location
(using this program) I find that there is no acceptable solution. In
order to realize any gain from a stack the antennas must be located a
significant distance apart, maybe 0.6 to 0.9 wavelength (varies with
antenna gain). When I put two antennas that far apart, the top antenna
is too high and an unacceptably low angle null is created by the entire
stack. The conclusion is that there is no stack at my location that will
beat a single antenna. Your mileage will probably vary. This is a unique
solution for my location, but it could be duplicated at other selected
spots.
A side note: HFTA does not model close spaced antenna interaction, so
stacks placed closer than about 0.5 wavelength will produce optomistic
results.
No, I don't get any kick-back from N6BV. I just think it is a very good
program and fills in a big void in terms of antenna analysis.
I'll bet that if K3LR had had HFTA when he built his big 15 meter stack,
it wouldn't have resulted in performing the same as if it was on flat
ground. (He is on the top of a mountain.) See his website.
Jerry, K4SAV
Lee Hiers wrote:
>On 6/12/06, K4SAV <RadioIR at charter.net> wrote:
>
>
>
>>The arrival angle will vary depending on where you are located. Ga or Al
>>will not be the same as W3 land. It also varies with the band and the
>>MUF, which means it changes every hour. The probabilities are also
>>skewed by the sunspot cycle. When picking an optimum angle you have to
>>decide if you want the optimum angle over a short period of time or a
>>long period of time.
>>
>>
>
>That's correct. That's also why stacks work so well...you can modify
>the takeoff angle as condx warrant.
>
>A good resource for this stuff is N6BV's ARRL book "All The Right Angles".
>
>--
>Lee Hiers, AA4GA
>"Have Dobro Will Travel"
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>
>
>
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