[TowerTalk] stacking monobanders

[David Gilbert]

Hi, Rick.

I think K4SAV was referring to me and the model I ran when he talked 
about overlooking the possible feedline impact ... I don't think he was 
being critical of you.

But for sure, if you had relays to disconnect both halves of the unfed 
Inverted-V's from their respective feedlines there should have been no 
parasitic coupling.  Your comments on the 80m tests are, as you say, 
exactly what would have been expected.  Now I'm curious as heck why it 
would have been different for 40m.  Any thoughts?

I'm also planning to finally put up a modest tower and reasonable 
antennas next spring at this new QTH and I'd be very interested to hear 
how your tests go with the MonstIR at different heights on 40m, 
especially since you'll be able to easily retune at each height to 
eliminate the detuning effects of the ground.  HFTA is telling me that 
I'd gain roughly a db at 10 degrees takeoff for every additional 10 feet 
of height up to at least 100 feet, and I'd be really interested to see 
if your measurements verify that.  At this point I'm planning to top out 
around 85 feet.

73,
Dave  AB7E



Rick Karlquist wrote:
> 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.
>>     
>
> No I didn't overlook that.  I installed remote relays that disconnected
> both halves of the unused inverted vee from the coax.
>
>   
>> Experimental A/B testing of closely mounted antennas with uncontrolled
>> parameters of feedline length and source impedance can produce some
>> really weird results.
>>     
>
> The antenna under test was terminated in 50 ohms at all times.
>
>   
>>> 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
>>>       
>
> Yes I did that.
>
>   
>>> 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
>>>       
>
> An inverted vee with its feed point open circuited has virtually
> no parasitic coupling effects (you can easily model this).
>
>   
>>> 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
>>>       
>
> Did that too.  We put an 80m inverted vee on a 115 ft crank up tower
> and A/B'ed it with a ground mounted vertical.
> At full height, it was on a par with the vertical for DX.  For
> locals, it was always better than the vertical, but the difference
> was 10 dB more at low heights.  At low heights, the vertical beat
> the vee by 10 dB on DX.  This all agrees with modeling.
> For some reason, we don't see this neat relationship on 40 meters.
>
> Hopefully, next spring, the MonstIR will go up on the crank up and
> we can see how it works at various heights on 40 meters.
>
> Rick N6RK
>
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