Discussion in 'Electronic Basics' started by carl, Jun 24, 2004.

1. ### carlGuest

as i remember from college the speed of rf in a vacuum is about 3x10 to
the 8th meters/second. to get the wavelength of that just divide by
frequency. lets say we have a 300 mhz radar and if i'm thinking right
thats a 1 meter wave, in a vacuum or free space also i'm thinking.
we were chatting at work the other day and velocity factor came up.
if this gentlements figures are right using poly as a dielectric (in
coax) gives us a 60% velocity factor.
when calculating antenna dimensions would one use the "corrected"
wavelength or the actual?

2. ### SBGuest

3x10^8 is the speed of light in a vacuum.
When designing antenna's, we calculated lenths of elements based on a 95-98%
VF.

I'm not sure how much weight people put on these values.
I mean, depending on your length of cable/etc., you end up with capacitances
that must be dealt with anyhow, thus you usually have some sort of matching
network

wavelength = (freq/speed of light) * 98%

3. ### Bob MyersGuest

Antenna dimensions generally come out under what you'd
expect from the free-space wavelength, due to effects
resulting from distributed capacitance and inductance in
the structure. However, this is NOT directly related to the
velocity factor of the coax (or other cable) used to drive the
antenna. (You do use that factor to correct the wavelength
calculations if you're building a wavelength-dependent
structure out of cable, within the feed network - such as a
coax balun.)

Bob M.

thanks

5. ### carlGuest

i was (and probably still am) a bit confused as one of the field
engineers at work used VF and the others not. we were discussing
shortening a piece of coax that was damaged and were wondering where/how
the manuals we were using might have derived their figures as to repair
distances when this one person threw out the VF figures. i was skeptical
and it sounds maybe i was right in my thinking.
thanks for the info
carl  