Interesting reading re: stacking for those of you not on the VHF list<br><br><div class="gmail_quote">---------- Forwarded message ----------<br>From: <b class="gmail_sendername">Ev Tupis</b> <span dir="ltr"><<a href="mailto:w2ev@yahoo.com">w2ev@yahoo.com</a>></span><br>
Date: Sun, Dec 19, 2010 at 7:02 AM<br>Subject: [VHF] Close Vertical stacking distances [was: Horizontal stacking distance question]<br>To: Stanford VHF email Remailer <<a href="mailto:VHF@w6yx.stanford.edu">VHF@w6yx.stanford.edu</a>><br>
<br><br>I've taken the liberty of re-presenting Jay's informative plots. Thanks to Jay,<br>
we can study data rather than shooting from the hip.<br>
<br>
I can clearly see that the text attributed to Kent's observation held true:<br>
<br>
------ Original Message ----<br>
<br>
> > Kent WA5VJB did some experiments that were presented at Central States this<br>
> > year. He found that when single Yagis for different bands were stacked on<br>
> > one mast, they could be practically on top of one another (as little as 5<br>
> > inches apart, I think) and had virtually no effect on one another. When two<br>
> > Yagis for one band were used on the same mast, this did not hold true.<br>
----------------------------<br>
<br>
The "purist" will say "I told you that there would be interaction!" And, of<br>
course there is. The "practicalist" will say, "so what? it is so insignificant<br>
I don't care."<br>
<br>
See what I mean, below...<br>
<br>
I've restated Jay's post a little differently below...<br>
<br>
Note: "ascii figures" are NOT to scale and are left-justified. The assumption is<br>
that all antennas are centered on the mast instead. I'm including this as a<br>
visual aid only.<br>
<br>
Antenna Scenario #1<br>
<br>
<a href="http://s509.photobucket.com/albums/s339/K0GU/K0GU%20MISC/?action=view¤t=222-5wl.jpg" target="_blank">http://s509.photobucket.com/albums/s339/K0GU/K0GU%20MISC/?action=view¤t=222-5wl.jpg</a><br>
<br>
<br>
The stack lays out like this:<br>
<br>
----------------------- (222-5wl w/23' boom)<br>
two feet<br>
spacing<br>
------------------------------- (17b2 w/31' boom)<br>
two feet<br>
spacing<br>
------------------------------- (6m7jhv w/31' boom)<br>
<br>
Jay's modeling shows a slight "upward" skew to the 222 beam's main elevation<br>
pattern. However this is only when compared to the same pattern in "free space"<br>
w/o any ground interaction at all. Even so, the difference is only fractions of<br>
a dB between the 222 antenna in free space vs within 24" of a 17b2 "ground".<br>
<br>
Overall 222 MHz gain in elevation is -0.18 dB vs. free space<br>
Overall 222 MHz gain in azimuth is +0.04 dB vs. free space<br>
<br>
Mast needed above the tower: 4'<br>
<br>
Of significant note: The wind survivability of a 4' mast is greater than that<br>
of a mast that is long enough to conform to "perfect" stacking distances for<br>
these antennas.<br>
<br>
Cheers,<br>
Ev, W2EV<br>
------<br>
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