[TowerTalk] Stacking dissimilar antennas

[T A RUSSELL]

N4KG  comments intersperced below:

On Tue, 21 Jul 1998 18:15:07 -0700 "Robert L. Shohet" <kq2m@eci.com>
writes:
>
>Tnx to all who responded to my guy wire spacing question.
>
>I have wondered how important it is to stack similar antennas or if it
>does not matter at all.
>
>If I wish to stack 3 15 meter beams, how advantageous is it to make sure
>that all three antennas are the same design, boom lenght, manufacturer,
>match etc.?
>
>For example, does it matter if the bottom one is a widespaced 4L
>Cushcraft on a 27' boom with a gamma match, the middle is a 5L Hygain
>155CA and the top is a homebrewed 4L on a short boom with gamma match?

	Let me answer your questions with a question.
	What would be the relative phase between say
	a gamma matched Yagi and a Beta matched Yagi?
	
	Give up?  I don't have a clue either, and therein lies
	the problem with dissimilar antennas.  You MAY be
	lucky and have them close to being in phase, but
	most likely they will NOT be in phase and there will
	be little stacking gain when aimed in the same direction.

	If all you want to do is aim them in DIFFERENT directions,
	then it does NOT matter what antennas you use.  They
	can even be on separate towers if aimed differently.

	BTW, precise phasing down to a few degrees is NOT
	necessary.  Table 6-3 on page 6-14 of the W2PV Yagi
	Antenna Design book shows that with a 30 degree
	phase differential (and identical Yagi's), the maximum
	degradation in gain is 0.36 dB.  Physically, 30 degrees
	represents 1/12 of a wavelength (WL) so you could
	have a 0.08 WL variation in coax lengths or a 0.08 WL
	physical displacement from the same axis and suffer
	no more than 1/3 of a dB performance degradation.
	(This amounts to about 4 ft of displacement on 15M
	therefore close works in horseshoes AND stacking.)
	de  Tom  N4KG
>
>Ideally I would like the amplifier loading to be roughly the same for
>all combinations of antennas so do I need to make sure that all are
>identical or is the most important thing to make sure that they all
>resonate about the same frequency?
>
>What happens if I wish to have two resonant about 21.05 and the other
>resonant at 21.25?  Does the difference in resonant frequency screw-up
>the overall pattern and loading when in a stack?

	You would need to evaluate each case, but my guess
	is that you will be much better off if the antennas are
	IDENTICAL in matching, tuning, and geometry.  N4KG
>
>I know that these sound like basic questions to many but I feel it is
>better to ask and give the project lots of thought BEFORE anything 
>gets bought, built and put up!
>
>Thanks for your help.
>
>73
>
>Bob KQ2M

I still think you should model your proposed 4 stack vs. my suggested
3 stack.  My guess is that there is little benefit on 10M from a beam at
24 ft.    As a stand-alone, that antenna would have a main lobe over 20
degrees.  Not very useful on 28 MHz, even during very high sunspots.
Same for the 15M antenna at 30 ft.     35 to 40 ft covers all of the
highest
angles supported by the ionosphere when the MUF is 50 MHz.  Anything
lower is just sending energy to outer space.  (ASSUMING  FLAT  LAND.)

de  Tom  N4KG


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