[TowerTalk] Liberal Arts Major - feed point impedance

[Terry Conboy]

At 02:01 PM 2006-06-01, Keith NM5G wrote:
>Let's say you have an 80M dipole made out of 12 AWG insulated copper wire
>supported at 80 feet on a piece of PVC pipe sticking out two feet from a
>Rohn 45G tower.  The ends have insulators with non-metallic line (e.g.
>Dacron) attached to reach to ground supports.  Bring the ends out on
>opposite sides and tie off to supports (e.g. trees or stakes) 120 feet from
>the tower base.  This would provide an inverted V with approx 56 degrees for
>each end with respect to the tower (112 degrees total apex).  How would one
>go about calculating the feed point impedance?

EZNEC+ gives 69.1 ohms over average ground (Real/High Accuracy) at 3.6 MHz.
It's resonant with each leg 66.65 feet long (bare #12 copper, 66 
segments per leg).

>Regardless of the feed point impedance, what difference would be noticed at
>the rig if 75 ohm coax is chosen as the transmission line versus 50 ohm
>coax?

It depends on the line length.  Pick a multiple of 1/2 wl and you'll 
hardly notice any difference.  At resonance, both would show about 
69.1 ohms at the transmitter, a 1.38 SWR referenced to 50 ohms.

If you have line that is an odd multiple of 1/4 wl, your transmitter 
will see 81.4 ohms with 75 ohm line (1.63 SWR referenced to 50 ohms) 
or 36.2 ohms with 50 ohm line (SWR 1.38).

A series matching section would easily bring it to 1:1.  From 
feedpoint, 16.37 feet of 50 ohm line (VF=0.66), then 11.5 feet of 75 
ohm line (VF=0.66), then any length of 50 ohm line to TX.  System 
bandwidth is 200 kHz (50 ohm SWR <2:1).

(E-mail off list if you want the .EZ file or an Excel Spreadsheet to 
calculate a series matching section.)

73, Terry N6RY

[Not a liberal arts major - although I will admit I took one poetry 
course at Oregon State, but sadly, the girls were ugly.]