[TowerTalk] Liberal Arts Major - feed point impedance

[Keith Dutson]

Jim Brown wrote:
>As a general rule of thumb, I use 75 ohm coax for high dipoles and 50 ohm
coax for lower ones. The generally accepted definition of "high" is more
than a wavelength; "low" is less than a half wavelength.

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?

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?

73, Keith NM5G

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Brown
Sent: Thursday, June 01, 2006 3:02 PM
To: TowerTalk Reflector
Subject: Re: [TowerTalk] Liberal Arts Major - feed point impedance

On Thu, 1 Jun 2006 11:55:57 -0700 (PDT), Lee Buller wrote:

>I forgot that a dipole is 72 ohms
>.....and I am feeding it with 50 ohms.  

Sort of, but not exactly. The impedance at resonance of a dipole will vary
with its height above ground, as well as soil conditions. So there's no one
good number. In general, a dipole closer to earth will TEND to have a lower
Z at resonance than a higher one. There are some nice graphs of this in
ON4UN's "Low Band DXing." 

Some other points. 

1) The mismatch between 72 ohms and 50 ohms is on the order of 1.5:1. 

2) Any imbalance in the length of the antenna, or the capacitance to
surrounding objects of the two legs of the antenna will unbalance it, which
can increase the SWR at resonance. 

3) In general, an SWR of less than 2:1 is fine as long as it does not cause
the transmitter any problems (it usually won't) or cause the transmitter to
reduce power. 

4) The increase in loss caused by a mismatch of less than about
4:1 is quite small. See the ARRL Antenna Book on this. 

As a general rule of thumb, I use 75 ohm coax for high dipoles and 50 ohm
coax for lower ones. The generally accepted definition of "high" is more
than a wavelength; "low" is less than a half wavelength. 

Jim Brown  K9YC


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