[TowerTalk] stacking monobanders

Wed Nov 29 10:37:19 EST 2006

You overlooked one other complicating factor, feedline length. The 
length of the feedline on the unused antenna determines if the unused 
antenna looks resonant or not.

Considering only two of the antennas for the moment, if the upper 
antenna is unused and has a multiple of a half wavelength of feedline 
attached and the source end open, the lower antenna would look much the 
same as any other inverted vee at the same height.  If the top feedline 
happened to be an odd multiple of 1/4 wavelength and the source end 
open, nearly all the radiation from the lower antenna goes straight up. 
Other feedline lengths cause all sorts of radiation pattern changes, and 
everything reverses if the unused end is shorted instead of open.

Experimental A/B testing of closely mounted antennas with uncontrolled 
parameters of feedline length and source impedance can produce some 
really weird results.

On the other hand, using controlled feedline lengths and switching 
source impedance can produce different patterns which may be useful.  
More useful patterns can be obtained by feeding two or more antennas. 
You can get some gain, or for the ultimate NVIS antenna (if you really 
want that) try feeding two of the antennas 180 degrees out of phase.

Jerry, K4SAV

David Gilbert wrote:

>
> 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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