[TowerTalk] stacking monobanders

Wed Nov 29 11:30:06 EST 2006

Your are absolutely correct.  I didn't try to include that in my quick 
model because I was only trying to show that parasitic coupling can 
affect what Rick observed for the three antennas fed separately.  I 
didn't know what feedline lengths he used or if the unused ones were 
shorted or left open.  I do take the feedlines into account when I model 
stuff for myself, and as you say they have a very big effect.   I didn't 
even know if Rick's Inverted-V's were resonant or not ... I assumed they 
were but the results would be significantly different if they weren't.  
To top it off, I assumed that the three Inverted-V's had the same apex 
angle.  There would be more even coupling between the three antennas if 
instead the ends were all tied off at common points.  I guess it's no 
wonder everyone has different experiences and different impressions of 
what works and what doesn't on the subject of antennas, with so many 
influences on real life results.

73,
Dave  AB7E

K4SAV wrote:
> 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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