SUBJECT: Radio Shack Design - Part 4

RF Distribution.


    Your RF distribution system should be designed as simple as
possible. For an example, I will use my antenna system. These will be
antennas existing and to be installed (if I get the time to climb.)

    Inventory, top down: 

    o (50 feet) 2m MARS Digi Co-linear vertical antenna.
    o (48 feet) 500-1300 MHz LPDA, horz. 37 elements
    o (46 feet) 67cm 10 element yagi, vert.
    o (44 feet) On cross-bar; left: 2m LPA 10 element, vert. 
                             right: 2m Yagi 9 element, horz.
    o (43 feet) Rotor: Aliance
    o (42 feet) HF LPDA 13.5-28.5 MHz 10 element, horz. (Shaft drive)
    o (40 feet) tower platform; Alpha Delta DXA (40m,80m,160m)
    o (36 feet) On cross-bars; left: UHF TV yagi with corner 
                                     reflector, Radio Shack Rotor
                              right: Discone 140-1400
    o (30 feet) 49 MHz vertical dipole
    o (25 feet) On cross-bar, on service tower; left: 2m J-Pole, 
                                                      vert.
                                               right: 1/4 wave 67cm,
                                                      vert.
    o (23 feet) On service tower; (88-108)FM 300 ohm dipole, horz.
    o (8 feet) 2S1 Mobile or 96" whip or A-99 at S.E. corner of house
               with Roof Flashing as Groundplane.
    o (3 feet) Tail-Twister Rotor
    o Central Ground

    There are 18 separate input lines to the shack!  A Wiring 
Nightmare, Batman!

    All is not lost. Use good rotor wire like Belden, good coax
like 9913 and RG-214/RG213 and there won't be any problems. RG-8 is 
good also for the low bands and can handle the high power. I would 
use RG-8m for the low power apllications and RG-58 for jumpers and 
receive only. RG-59 with pre-amp or RG-6 for the UHF TV. The ground 
should be in the range of 6 to 8 gage solid conductor copper wire. 
It should be attached from the base of the tower to the ground rod
and run into the shack for a common ground. This makes a common 
ground. The ground rod should be next to the tower (previous 
posting).

    OK, you have 18 lines to consider. This is not the norm; my tower 
configuration is anything but normal, but with this insight, your 
RF distribution system may be much easier to handle or maybe more 
difficult <gulp>.

    Looking at what I have to deal with I must first consider the 
ground. I'll run that low out of the shack, directly to the ground 
rod.

    Next, I need to consider the 17 other cable coming into the 
shack. Three of these lines are Rotor control cables. The will be 
going directly to the rotor with one exception. Each will have a 
MOLEX connector installed approximately four feet after the entry to 
the shack, at the RF panel. Why?  Disconnect it and the lightning has 
no path to your rotor controller or AC power via the rotor cable.
It also makes for easy service or relocation.

    There are 14 other cables to consider. These are all coaxial.
Let's eliminate one; the UHF TV coax. It's under the big LPDA and
static dispersion will handle most of the protection from lightning.
Run the RG-59 or such to the TV. Disconnect when lightning is in the 
area or the TV is not in use.

Continued next posting.


-WS