K1TTT Technical Reference

Date: 13 Apr 1995 21:59:52 -0400
From: w8jitom@aol.com (W8JI  Tom)
Subject: 1/4 WAVE Radial Lengths

This is the result of my measurements in one direction at ground level. Field strength measurements  
were made with an old Texscan FSM. I checked the calibration on it with a good attenuator pad, so I 
am sure it is very linear. I used my 80 meter mobile antenna and drove to a clear area about 1/2 mile 
from my location and parked in the same spot. The antenna tested was a 1/4 wave vertical wire about 
200 feet from my other antennas and my house in an area with no buried or overhead wires.  My soil 
is clay and rock, probably very poor conductivity.

I measured the impedance of the elevated ground wires with a GR bridge. To measure the  resistance 
of the wire as a counterpoise, I used a 10 foot base loaded antenna with a very good coil  (Q>300) as 
my reference. By measuring the change in impedance of the feedpoint with this antenna from over a 
standard ground, I believe a good estimate of ground resistance can be made. I accurately know the 
resistance of this short antenna when worked against 83 surface mounted 1/4 wave radials and a small 
10 foot square ground screen. The resistance of the radials can be assumed to certainly less than two 
or three ohms of the system resistance. 

While my measurements can be open to some error, they are certainly more accurate than an on the 
air" impression. Here are my measurements:

Resistance

Single 16 ga wire 8 feet high fed like dipole showed resonance when each side was 232 / F . That was 
62 - 3/4  feet each side at the test frequency of 3.7 MHz. The resistance was 60 ohms, the 3:1 SWR 
bandwidth of 170 kHz up and down was nearly all due to reactance.

Two crossed half wave wires fed against each other in an X were 22.7 ohms. There were no mutual 
coupling effects noticed in this test, so the wires were not "talking" to each other. 

The same two wires fed like this  > <  each leg 1/4 wl and still a 90 degree angle were 38.5 ohms. In 
this case mutual coupling effects were noticed. The mutual coupling lessened the effect of the second 
wire. 

Four 1/4 wave wires in 90 deg  X with short base loaded vertical in center added 12 ohms to base 
impedance, so the X arrangement should certainly be around 10 to 14 ohms of "push" resistance.

Eight wires added 7 ohms to the base impedance from over the reference radial ground.

Field strength measurements 

Four radials systems were tested, 4, 8, 16, and 60 wires. The highest reading came from 60 surface 
radials, so all other systems are dB down from that system, or percent of the 60 radial surface system. 

8 foot high counterpoise system:
	4 wires  		  37 percent 	-4.30 dB	20.1 mV
	8 wires 		  58 percent 	-2.38 dB 	25.0 mV
	16 wires 		  86 percent 	-0.63 dB 	28.7 mV 
	60 wires 		  96 percent 	-0.18 dB 	32.5 mV

Ground mounted radials:
	4 wires 		  28 percent 	-5.50 dB 	17.5 mV
	8 wires 		  53 percent 	-2.73 dB 	24.0 mV
	16 wires 		  74 percent     	-1.30 dB 	28.4 mV
	60 wires (reference)100 percent 	 0.00 dB 	33.0 mV        

Looks like elevated radials work better if only a few wires are used. At my location I would need at least 
16 for decent eff with a 1/4 wave vert, and certainly many more in a phased array or with a shorter 
radiator. With 16 wires the difference is only around 1/2 dB between either system. It looks like if a l
arge system is used the systems work about the same. 

Has anyone else found any FS references yet? I am supposed to be getting some soon. 

It's amazing how fast a day goes by when your having fun.