View Single Post
Old 02-27-2008, 03:49 PM
Groucho's Avatar
Groucho Groucho is offline
Rising Sun Ham Guru
Join Date: Aug 2005
Location: Arvada, Colorado
Posts: 1,678
Default Dipole vs. Vertical?

In the 3rd HAM class, we discussed these two antennas, and the formulas of both.

For a Dipole, the formula was:
Length (in feet)= 468 / Frequency in MhZ. That length was then divided in half, and that gave you how long your "legs" of the Dipole should be.

For the Vertical, the formulas was:
Length (in feet)= 234 / Frequency in MhZ. The 234 is just half of 468, indicating what we learned that a Vertical is just half of a Dipole stood on its end.

For a 1/4λ vertical on 2M, the length wil be around 20". Someone (I think it was Scott) asked after we figured this out why he didn't see too many 20" antennas on anyones rig, but my antenna (the homebrew 2M) and Bruce's seemed much longer. I didn't have an answer for it then, but I do now. We have the 2M, 5/8λ vertical on our rigs. For us to understand this, lets figure it out.

Take for example the RS adopted simplex frequency of 146.460MhZ.

To figure out the multiplier in the 5/8λ formula, we take the 1/2λ number of 468 and multiply by 2 (to figure out the 1/1λ number).
468 * 2 = 936.

Divide the 1/1λ by 8 (because the divisor in the 5/8 antenna is 8) , then multiply by 5 (because the multiplier is 5) and you get:
936 / 8 = 117
117 * 5 = 585.

Put 585 into out formula:
Length (in feet)= 585 / Frequency in MhZ
and for 146.460MhZ we get:
585 / 146.460 = 3.994 FEET, = 47.93 INCHES

So for the 5/8λ antenna at 146.460MhZ, the optimum length of the whip should be 47.93 inches. Mine is 48 inches.

Hopefully that helps.
--Choose Wisely--
Blue '99 UZJ100 2" Lift, Replicated Outback stackable drawer system, Engle 63 QT, Custom Front Bumper, Gamiviti Shorty roofrack.
Reply With Quote