Sergio PP5JR
Most of the U.S. stations will give too short a distance for
stacking 20 meter yagi's. The choice for PY may be different.
The height of the bottom antenna is a very important consideration.
If the height of the "low" antenna is at least a wavelength then
this antenna, when used by itself, will add to the overall
performance of the system. Quite often you can use the low antenna
split from the high during a contest if its "loud" to the target
area. A 5 element 20 meter yagi plays very well at 70+ feet with a
nice 10 degree takeoff angle depending on terrain.
The top antenna needs to be separated from the lower antenna by
enough height as to add a second set of different takeoff angles.
These angles will then fill in the pattern of the stack. The
maximum gain reached at 5/8 spacing is for dipoles not 5 element
yagi's. A better distance is a wavelength between the antennas or
two above ground. This spacing also helps keep the antenna
interaction to a minimum.
A well behaved stack at one and two wavelengths above ground is a
pleasure to use in a contest. The signals will often "jump up" when
selecting "both" and the fading stops. During the slow times or as
the band changes the system can be split by spraying different
areas. For more front to back the high antenna can be selected to
null near channel QRM.
The excellent software that is now available can predict correct
heights for your 20 meter stack. I found a small change in
topography made some large changes in angles so you need to be
accurate. Stations in high density areas (U.S.) that work both
domestic and DX contests could use lower stacking heights although
the very low heights seem wrong to me.
The stacking distance for my 20 meter system was set empirically
years ago using a reference 100 foot high (excellent 20 meter
height) antenna on a second tower. The "low" antenna was raised
until it competed well with the reference. Then the "high" antenna
was raised 10 feet at a time above the low yagi until the swr plots
went normal. Each antenna was then set to 50 ohms for an equal
power split.
For my QTH (Alaska arctic circle) to Ja/Europe/US the height was
77 and 150 feet.
Rich KL7RA
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