AntennaSmith Tech Tips

TTTZ-02  Vertical Antenna Radials

Description:  

How Many Radials?

Problem:   How many radials do I need on my quarter-wave vertical?  I don’t want to make any more than necessary, but I want to be sure I have enough. 

Procedure & analysis: 

Use the TZ-900 to measure the antenna impedance at the feed point at the antenna base as you add radials.  A resonant ¼ wave vertical will be about 35 ohms with zero reactance at resonance with an excellent ground plane.  With a poor ground plane, the antenna will have a higher impedance and a lower SWR (when measured relative to 50 ohms).  Unfortunately the higher impedance is the result of ground losses in the poor ground plane.  That means the radiated signal will be weaker.    The resistance measured at resonance is the sum of the ground loss and the radiation resistance.   You want to minimize ground loss and maximize radiation resistance.    In the case of the full size ¼ wave vertical we can’t change the radiation resistance but we can certainly strive to minimize the ground loss.

A perfect ground plane would be a disk under the antenna with the radius equal to the height of the antenna.    This is what we are trying to approximate by adding radials to the vertical.    The closer we can approximate the perfect ground plane the better the efficiency of our antenna system.

To verify this effect

   First adjust the length of the quarter-wave (0.25 l) antenna to the desired resonant frequency using the TZ-900.  Start with eight 0.25 l radials lying on the ground and double the number between each measurement.    Note that the feed point impedance will start off high and start to approach 35 ohm as radials are added (120 radials is usually considered an almost perfect ground plane).  The impedance changes will be smaller and smaller as the 35 ohm point is reached. (Note  that indicates your ground plane losses are getting smaller and your antenna is becoming more efficient.  Also note the SWR increase as you get closer to 35 ohms.  (This may require some means of matching) 

When the changes are smaller than a few ohms, adding more radials will not decrease the ground losses significantly so the antenna efficiency is approaching a practical maximum.   

Ground conductivity will affect the number of radials vs. impedance, as will the raising the radials above the ground.  Every installation is likely to be different, so don’t expect precisely the same results from one location to the next. 

Use low loss transmission line for the frequency of your antenna.   The ARRL Handbook has a chart of feedline loss vs. frequency for many times of popular transmission lines.

Check the ARRL Antenna Book for an excellent discussion of vertical antenna design.  The ARRL Handbook has a useful condensed version of the vertical antenna design discussion. 

References:

The ARRL Antenna Book

LOW Band DXING by John Devoldere, ON4UN, published by the ARRL.

Jerry Sevick’s shortened vertical antenna book


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