[50mhz] 5 over 5 stacking recommendations

[Bill W5WVO]

Todd,

I'm running two modified (see http://www.6mt.com/A50-5S_w5wvo.pdf) Cushcraft 
A50-5S yagis spaced just about ten feet apart, which is 0.5 wavelengths 
approximately. This spacing was dictated entirely by the limits of what I 
could put up reasonably on top of a flat-roof house using a small 4-ft roof 
tower and guy lines at the ten-foot level. In other words, there is ten feet 
of mast above the guy lines, with the lower antenna just above the thrust 
bearing that secures the guys, and the upper antenna ten feet above that.

Many would say that this represents a severe compromise in stacking 
distance, but the truth is, anything other than actual elevation path 
modeling based on your individual surrounding terrain (ARRL's HFTA software 
is a good start) is, at best, anecdotal "evidence" from one guy's setup and 
terrain environment, or at worst, simple belief without empirical 
measurements to back it up. You can get HFTA off the CD-ROM that comes with 
any current ARRL Antenna Book. Setting it up and getting your terrain info 
into it is fairly involved, but well worth it.

That said, half-wavelength spacing generally works extremely well if your 
antenna is not too high off the ground, as mine is not -- about 40 feet for 
the top antenna. I recommend it because it's easy to do, and it probably 
won't come down in the wind. Remember that without a guy point BETWEEN the 
antennas, which most modest antenna setups won't permit because of lack of 
available space, your top antenna is going to be exerting a substantial side 
moment on the mast just above the thrust bearing when the wind blows, 
because of the mast extended 10 ft above that point and the wind load 
(antenna) on top of it.

If you want to go 0.6 wavelengths (12 feet) or even more, and you think your 
mast can take it (or you actually CAN guy the mast between the yagis), then 
go for it! But model it first using HFTA. It's not going to make much 
PREDICTABLE difference, because the actual elevation pattern you get is 
going to depend much more on your terrain than on any generic theory of 
stacking distance. Using HFTA, you can see exactly what your elevation 
pattern is going to look like in various directions.

Good luck! And consider doing my modification before putting those A50-5s 
yagis up there. You'll be glad you did. In just over three years on 6M, I've 
worked 420 grids and 49 states using the non-F2 propagation modes (mostly 
sporadic-E) available on six meters during a declining solar cycle.

Bill / W5WVO


WD0T wrote:
> Greetings all, first time posting here!
>
> I have a question on recommended stack spacing, for using 2 cushcraft
> 5 element 6m yagis, looking to put 2 up on a short crank up tower for
> the June VHF contest.  By short tower I mean a 30ft to the lower 6m
> antenna. I am talking about stacking one above the other.
>
> What would you recommend for stack spacing using 2 of the CC 5
> element, stock standard yagis?  Max gain would be nice, but also I
> need to not get crazy in terms of spacing distance.  I have seen in
> some places recommendations for .6 wavelength spacing for lower
> sidelobes, however I have also read that each type of antenna and
> height needs to be modeled to arrive at optimum spacing.
>
> Any help, thoughts certainly appreciated.  Thanks so much.
>
> 73 God Bless! Todd WD0T
> DN94, Pierre SD.
> Moderator: Ray Brown, KB0STN
> _______________________________________________
> 50mhz mailing list
> Home: http://mailman.qth.net/mailman/listinfo/50mhz
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:50mhz at mailman.qth.net