[Antennas] radial lengths for 102 inch cb whip antenna

Wed Sep 1 09:25:38 EDT 2010

Summary of responses regarding radial lengths for a 102 inch cb whip aerial
Thanks to all for your rapid responses to my question. Below are a summary 
of the responses.
K3VW and AF6AS suggest 1/4 wave long for the frequency desired which is 234 
divided by the frequency.
AF6AS also suggests 3 of them sloping down at a 45 degree angle
W2OVP suggests to match the radials to 50 ohm coax they should be at least 
1/2 the length of the vertical (102"), he also suggests referencing the ARRL 
Antenna Handbook and related texts noting that antenna design is as much art 
as science.
Barry S suggests 234/Fmhz= 234/27.185= 8.607 feet
Jim-K4SQR suggests using the formula :240/f(mhz) = 240/28.500=8.42 feet long 
for calculating radials.
and to use the following formula for calculating spacing
246/f(MHz)
Dave (W1NR) suggests since the antenna is 1/4 wave to make the radials the 
same length (102")
Alex 4Z5KS and Jim K4SQR both similarly suggests making the radials the same 
length as the antenna
ICRA suggests making sure the whip is indeed 102" and not shorter due to a 
design calculation such as a spring load or a quick disconnect.
Jim (KG0KP) notes 102 inch is cut 27MHz works great at 10M and that radials 
at a 22 degree downward slope work well. He suggested I look at the Star 
Duster aerial (description below)
 M-400 STARDUSTER. Re-introducing the classic M-400 Starduster CB radio
 antenna of yesteryear! This antenna is known for it's excellence in
 performance. It is ruggedly made of anticorodal aluminum, brass and nylon
 and is designed for easy assembly. Available in limited quantities.
 * Impedance 52 Ohm
 * Gain 7 Db
 * Max power 800 W
 * S.W.R. 1:1,1 - 1:1,5
 * Length 18'
 The Starduster used a vertical radiator of 102 inches and 3 radials of 107
 inches mounted from a 3 inch diameter base and decending to the lower tips
 with a separation of 75 inches measured between each of them. I just now
 made these measurements. The radial supports were mounted just above the
 half way point of the radials extending to the mounting pole bracket and
 were 17 inches each (separating them about 18 inches from the mounting pipe
 center) yielding the 75 inches of separation at the base. The feed coax 
was
 meant to be fed up through the mounting pole to the center of the vertical
 radiator.