[TowerTalk] stacking monobanders

Wed Nov 29 19:29:23 EST 2006

I said "imply"because" I didn't really know if Rick's Inverted-V's were 
resonant or not, and I was referring to his situation.  I can guarantee 
that EZNEC will show currents being induced in the other elements 
without even looking at them ... that's what makes the patterns shift.  
I'll be happy to send you the simple file I used if you want to play 
with it.  I don't see the point in it, though, since it wasn't meant to 
be a rigorous model ... only an illustration of what might have been 
behind Rick's observations..

73,
Dave  AB7E

Al Williams wrote:
> EZNEC does more than "imply" parasitic coupling between the different 
> inverted vee antennas.  Take a look at the current tables to see the 
> actual parasitic coupling.  The degree of coupling depends on the 
> orientation of the parasitic to the driven, and the separation 
> distance.  I would suspect that driving the mid height inverted vee 
> would have the most coupling as it is closes to both the upper and 
> lower inverted vee antennas.  It would be interesting if you still 
> have the files to analyze the currents to determine why the lobe gains 
> changed.
>
> k7puc
>
>
> ----- Original Message ----- From: "David Gilbert" 
> <xdavid at cis-broadband.com>
> To: "Richard (Rick) Karlquist" <richard at karlquist.com>
> Cc: <towertalk at contesting.com>
> Sent: Tuesday, November 28, 2006 10:00 PM
> Subject: Re: [TowerTalk] stacking monobanders
>
>
>>
>> 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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>>>
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>>
>> -- 
>> save the cheerleader ... save the world
>>
>>
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save the cheerleader ... save the world