TopBand: vertical and inverted vee interaction

[Tom Rauch]

> K9RJ's resonant balun is 31 single layer turns (~39 uH) of RG-8A (~13"
> long) on ~20" length of 4.5" OD PVC pipe.  He uses 81" of RG-8A for the
> required C (~200 pF) to resonate the balun at 1830, with the stub folded
> inside the PVC being careful not to create a situation where the shield of
> the stub could arc through the outer insulation to another conducting
> surface.  This system worked for a 160 inverted-vee mounted at the 80 foot
> level on Jim's 100 foot shunt-fed tower and I would imagine it should work
> for most typical shunt-fed tower installations on 160. 

There is a guideline that will give you a rough idea of the balun 
stress in this application.

If you have a tower with a big HF yagi some distance above the 
dipole, voltage across the choke should be fairly low. If you don't 
have anything of substantial distributed capacitance above the 
dipole, or if the dipole has a 160 meter self-resonance like mine 
did, voltages can be amazingly high (and trap bandwidth low)! 
Many, many, kilovolts at 1500 watts.

It is an impedance problem more than anything. If you 
disconnected the dipole feedline and brought it near the tower at 
that point, how far will it arc? That is the question. Obviously near 
the open end of any antenna, voltage can be very high.

Keep that in mind, especially if the components used are 
flammable and the grass (or house) below them is dry.

73, Tom W8JI
w8ji@contesting.com


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