Reg Edwards wrote:
There's too much fuss made about resonant length of simple wire
antennas.

(1) Radiating efficiency is hardly affected by being as much as 20 or
30 percent off-resonance in terms of antenna length.

(2) The feedpoint resistance at resonance may indeed provide the
theoretical correct termination for the transmission line for 1-to-1
SWR in the middle of an amateur band. But it is way off resonance at
the band edges with an SWR which causes little if any concern.

(3) The impedance of the transmission line is usually known only
approximately. The reactance of Zo is always ignored. So exact SWR on
the line is never known. SWR varies slowly with small changes in
antenna off-resonance length.

(4) If an antenna tuner can accommodate changes in input impedance
from one band to another it can certainly accommodate the omission of
the correction factor 468/F.

(5) So it hardly matters whether the correction is applied or not. The
minute difference in operating performance will be detected only by an
impossible A/B test. You can put away your pruning shears.

The theoretical resonant length of a 1/2-wave dipole is 150/F metres.
The actual resonant length is very slightly shorter.

In free space, the shortening depends on length-to-diameter ratio of
the antenna conductor. But at low heights above ground the shortening
effect increases.

When the antenna wire is lying on the ground its velocity factor is
approximately 1/2 of the free space value of 1.

So you see the formula 468/F, although good enough for most purposes
if you want to use it, is very approximate.

The question is about the resonant length of a 6m dipole.

Is the dipole 6 metres long or is it resonant in the 6 metre amateur
band? Questioners should make themselves clear.

Assuming it is 50 MHz, resonant dipole length is very nearly 150/F
metres = 3 metres.

But the answer depends on conductor diameter. If frequency is 50 MHz
and the antenna is constructed with 14 gauge wire then the resonant
dipole length is 97 percent of 3 metres.

If the anenna is constructed of 1", 25mm, diameter tubes, then the
resonant length is a little shorter. It is 94 percent of 3 metres.

But actual lengths are non-critical. You can use a tape measure or a
wooden ruler. And don't worry if you can't find your spectacles.
----
Reg.



Hook, line, and sinker!

"John - KD5YI" wrote in message
news:Ct_ng.26884$Xn.3642@trnddc05...

But actual lengths are non-critical. You can use a tape measure or a
wooden ruler. And don't worry if you can't find your spectacles.
----
Reg.



Hook, line, and sinker!

and also the dipole is shorter because you want to cancel out the reactance.
a "real" half-wave would have a resistive and reactive component.

i think if you get into big tubing or pipe, you get too much reactance.

Gravity

The theoretical resonant length of a 1/2-wave dipole is 150/F metres.

i should have thought of that.

c = 3 * 10^8 m/s
F = frequency / 10^6

465/F theoretical (feet)
468/F practical

i think i made a math error.

Gravity