[TowerTalk] Liberal Arts Major - feed point impedance

Jim Brown jim at audiosystemsgroup.com
Thu Jun 1 16:02:04 EDT 2006

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 

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 

Jim Brown  K9YC

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