Amazingly, no one has expressed what units these constants 300 and 984, etc.
represent or the fundamental formula:

wavelength = velocity of propagation/frequency
= velocity (units of length per second)/frequency (cycles
per second or Hertz)
= length per cycle
If the velocity is expressed in meters per second, the wavelength is
expressed in meters, if feet per second then in feet.

In free space, velocity of propagation of radio waves is 300 million metres
per second or 984 million feet per second. When divided by frequency in
millions of cycles per second (MegaHertz), we get the length of one cycle or
wavelength in meters or feet, respectively.

In air, the velocity slows down slightly so a wavelength is slightly
shorter. The coefficient of velocity (or velocity factor) expresses the
fraction the speed is relative to that in free space. In air, it is 99.7%.
In denser media, it is slowed more.

Typically, we don't consider velocity of propagation as the reason for the
approx 95% factor for antenna calculations below 30 MHz but as a convenient
way of accommodating the real-world effects of conductor diameter and
capacitance at the ends on the resonant frequency. It is a rule-of-thumb
factor as a starting point for trimming an antenna for resonance at a
certain frequency - not critical for receiving.

Relative velocity of propagation is important in transmission lines, when
used to set the relative phase at each element of an array of driven antenna
elements, and can be as low as 66% that of free space in coaxial cables
using polyethylene insulation between the inner conductor and the shield.

Thus, at 10 MHz, a wavelength is:
- 30 metres, in free space
- 29.91 metres, in air
- ~ 28.5 metres for a 'full wavelength' resonant antenna (varies with end
effects and conductor diameter)
- 19.79 metres in Belden 8241 (RG-59) coaxial cable.

Regards,

Tom
..
"Michael A. Terrell" wrote in message
...
David wrote:

On Mon, 11 Apr 2005 17:09:32 GMT, "Michael A. Terrell"
wrote:

RHF wrote:

DF,
.
And the 'original' Question Is (Was) :
" formula for calculating the length of a full-wave antenna wire "
.
"How Do I" ? Calculate the Length of Wire I need to build a Wire
Antenna ?
http://groups.yahoo.com/group/Shortw...a/message/2884
.
This webpage does a very good job of providing an answer.
- - - How Do I Find the WaveLength of a Frequency ? - - -
.
GoTo= http://www.radiomods.co.nz/radiomath.html
.
IN THEORY - The Numbers Are :
Meters = 300 Divided by Frequency in MHz
Feet = 984 Divided by Frequency in MHz
Inches = 11,811 Divided by Frequency in MHz.


Not "In theory" but in free space.


IN PRACTICE - {The-Real-World} - The Numbers Are :
Meters = 285 Divided by Frequency in MHz
Feet = 936 Divided by Frequency in MHz
Inches = 11,235 Divided by Frequency in MHz.


This is caused by the propagation delay in the conductor the antenna
is made of. In other words, the wire is measurably slower that free
space. This is speced as the "Propagation Delay" and is stated as a
percentage. Look at the data on Coaxial cable for examples.


Other Questions - Asked-and-Answered :
* How do I find the frequency of a wave length ?
* How do I Calculate the Length of Wire I need to build a Wire
Antenna. ?
[ You must use the following Math to Correctly Cut an Antenna. ]
- One {Full} Wave Length (WL)
- Three-Quarter Wave Length (3/4 WL)
- Five-Eighths Wave Length (5/8 WL)
- One-Half Wave Length (1/2 WL)
- One-Quarter Wave Length (1/4 WL)
- One-Eighth Wave Length (1/8 WL)
.
FULL WAVE LENGTH WIRE (WL) ANTENNA
IN FEET = 936 DIVIDED BY FREQUENCY
.
Plus the Age Old - How Do I Convert :
* Meters-to-Feet ?
* Feet-to-Meters ?
.
.
iane ~ RHF
The wave doesn't travel as fast in a solid conductor and the voltage
reverses sooner (i.e. closer to where it started.)


That's another way to describe propagation delay.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida