[TowerTalk] Sigma 180-S / 280-S

[TexasRF at aol.com]

Jim, harmonic resonance of a loaded radiator is no longer related to  
wavelengths like a full sized radiator. The loading inductance is lumped and has  
a reactance that is linear with frequency. The equivalent radiator length 
in  degrees that is replaced by the inductance is more of a tangent function 
of  the number of degrees than being linear.
 
I have not studied this but seems like something that could be modeled with 
 proper software.
 
A direct measurement like you are asking for will give some very quick  
answers and hopefully someone will do it and report the results.
 
73,
Gerald K5GW
 
 
 
 
In a message dated 10/11/2010 9:11:41 A.M. Central Daylight Time,  
jim.thom at telus.net writes:

If  anybody owns either  a F-12  Sigma 180-S or  280-S.[ both 80m  ants].. 
or knows somebody who does own 
either one,  perhaps you could  stick a few watts into  it on 20m, and see  
what kind of swr
you  have on 20m.    If you have the time, perhaps sweep it with a  
MFJ/etc,  and see where it
does resonate.  Not all these  ants  will resonate on their 3/5th 
harmonics.   The F-12   340-N  [3-el 40m yagi]
does not resonate on 15m.... it resonates   just below the 12m band. 

What baffles me is  3.5 x 3 =  10.5  mhz.     10.5/14.2 =  .74       The  
80m ant, on it's 3rd harmonic is 
26%  lower in freq than 20m.   I have heard several horror stories now of 
80m arrays   completely
screwing up 20m yagi's that are in close proximity.   Rotating the 80 + 20m 
arrays , so their booms are 
90 deg to each other  does not always work.    The 35' booms  used on both 
the   280-S and 340N   will screw
up 20m real fast. 

I noticed  that Optibeam uses decoupling rods on their 4-el 40m  yagis.  
They  won't screw up a 15m yagi, nor
another HF array.   Does anybody  know how this is done ?   Can this 
technique be adapted to an 80m  array ? 

Jim    VE7RF
_______________________________________________



_______________________________________________
TowerTalk  mailing  list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk