TopBand: Re: Rohn Tower on 160m

Earl W Cunningham k6se@juno.com
Thu, 14 Aug 1997 13:38:53 EDT


Dave, K1ZG writes:

I READ YOUR INFO RE THE ROHN 25 TOWER SPECS FOR 160M >VERTICAL.  I
WASN'T SURE THAT A 12.5" DIAMETER IS THE PROPER >EFFECTIVE DIAMETER FOR
THE ROHN 25.  I THOUGHT MOST PEOPLE >USED 8" FOR MODELLING PURPOSES.  HOW
WOULD THE CHANGE IN
EFFECTIVE DIAMETER AFFECT YOUR CALCULATED VALUES FOR A >SHUNT-FED TOWER?
========================

Decreasing the models from 12.5" diameter to 8" yields:

   # of           Tower         Gamma    Gamma     Capacitor  
Sections      Height          Height     Capacity      Voltage
------------   -----------------     ------------   -------------   
--------------
      9          87'  4-1/2"       83'           77 pF          5044
    10          97' 1"              72'         105 pF          3712
    11         106' 9-1/2"       57' 6"'     155 pF          2515
    12         116' 6"            39' 9"      263 pF          1477
    13         126' 2-1/2"       26' 10     497 pF           783
    14         135' 11"           35' 6"      465 pF           837
    15         145' 7-1/2"       50' 3"      320 pF         1214

Compared to the 12.5" models, the most significant fact is that the gamma
height must be reduced by about 4 feet in the 8" models.  Reducing the
gamma spacing from 24" to 14" in the 8" models yielded very close to the
same values obtained in the 12.5" models.

My two 72' tower widths taper from 30" at the base to 18" at the top. 
Modeling them (along with the HF beams at 75') using the actual tower
width as the conductor diameter yields exactly the same gamma height &
spacing and capacitance values oblained in the real world (I've never
measured the voltage).

If anyone ever uses the values I've presented, I'm sure they'll initially
be in the ballpark.

73 es DX, de Earl, K6SE


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