Re: [TowerTalk] stacking monobanders

[David Gilbert <xdavid@cis-broadband.com>]

Hi, Rick.

Those are interesting comments, so I modeled it up with EZNEC with 
approximately resonant antennas to see what it looked like.  Your 
message says "inverted vee's" (plural), so I'm making the leap to assume 
you had all three inverted vee's up at the same time and were able to 
switch between them.  Please correct me if I'm wrong ... and if I am 
wrong you and everyone else will probably want to ignore that which follows.

Here's what EZNEC says about inverted vee's individually at the various 
heights:

30 ft ...   max lobe straight up (90 degrees) of 6.4 dbi --- gain at 20 
degrees (arbitrary mid angle) of 0.2 dbi
60 ft ...   max lobe at 35 degrees of 5.8 dbi --- gain at 20 degrees of 
4.0 dbi
90 ft ...   max lobe at 23 degrees of 8.3 dbi --- gain at 20 degrees of 
8.2 dbi

It gets more interesting when you look at the three antennas all 
together on the same tower, but only one being fed.

30 ft antenna fed ...   max lobe at 90 degrees of 7.6 dbi --- gain at 20 
degrees of 2.7 dbi
60 ft antenna fed ...   max lobe at 26 degrees of 5.5 dbi --- gain at 20 
degrees of 5.1 dbi
90 ft antenna fed ...   max lobe at 26 degrees of 6.4 dbi --- gain at 20 
degrees of 6.4 dbi

None of this data should be taken too literally, of course, but the 
model implies a lot of parasitic coupling between the three antennas 
that affects the pattern even when only one of the antennas is being 
fed.  Individually, the signal level at 20 degrees varies by 8 db 
depending upon whether the antenna is at 30 feet or 90 feet.  
Collectively, the signal level of the stack of three antennas varies by 
less than half that (3.7 db in this arbitrary case) no matter which of 
the antennas is fed.  In real life the difference across the stack might 
be even less.  If I had my choice, I'd prefer to have only the upper 
antenna on the tower ... unless of course, as you say, someone wanted to 
optimize the close-in performance.  For longer DX, takeoff angles as low 
as 10 degrees are useful and there the difference according to the model 
jumps to 10 db.

It would be interesting to see someone hang an inverted vee from a pully 
and rope and take signal strength readings at different heights.  I 
don't have my tower up yet at this new QTH, but if nobody has done so by 
the time I get the tower up I'll promise to give it a try.

73,
Dave  AB7E






Richard (Rick) Karlquist wrote:
> > Anecdotal results from anywhere are irrelevant ... that was my point.  I
> > don't trust software analyses implicitly, but I trust them more than
> > opinions that aren't backed by direct comparison of some sort (like an A
> > vs B test of two antennas at the same height at the same time).
> >
> > Yup ... well, close anyway.  I used a fixed 2 element 40m wire yagi at
> > 70 feet for a while.  It worked great and I had a lot of fun with it.
> > It would have worked even better at 90 feet, and it would have worked a
> > whole lot worse at 45 feet like the original message from NY6DX discussed.
> >
> > Dave  AB7E
> >     
>
> Interesting that you should mention A/B'ing.  I did a lot of A/B'ing of 40
> meter
> inverted vee's at 30, 60, and 90 ft.  I thought the 90 ft one would have a
> substantial
> advantage over the lower ones, but in actual operation they were very hard
> to tell apart.  I listened to foreign broadcast stations and ham DX stations
> as
> much as I could and looked for S-meter changes.  On local stations (<100
> miles),
> there was a substantial difference which agreed with conventional wisdom of
> the lower the better for locals.  YMMV.
>
> Rick N6RK
>
>   
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