# Monopole antenna length.

#### rmdrmd

Jun 14, 2013
13
I'm after a little help.
Is the correct length for the antenna one quarter of the wavelength?
Is it as simple as that?

The frequency is 27.145 MHz.
From a calculator I see that makes the wavelength to be 11.044 meters.
Does that mean the correct length of the single wire antenna should be 2.761 meters?
Am I on the right track here?

Cheers.

#### davenn

Moderator
Sep 5, 2009
14,077
Is the correct length for the antenna one quarter of the wavelength?
Is it as simple as that?

almost
It's an electrical 1/4 wavelength at the frequency of interest

300 / freq ( in MHz) = wavelength / 4 = 1/4 wavelength x 0.96 (velocity factor) = electrical 1/4 wavelength

The frequency is 27.145 MHz.
From a calculator I see that makes the wavelength to be 11.044 meters.
Does that mean the correct length of the single wire antenna should be 2.761 meters?
Am I on the right track here?

see above and do the math

cheers
Dave

#### Nanren888

Nov 8, 2015
623
With access to equipment some might start a little longerand trim the antenna down slowly while the SWR drops.

Simple it is, but also rather long for most RC toys and applications.
It may be that you can find a more compact design that is also simple.

#### rmdrmd

Jun 14, 2013
13
almost
It's an electrical 1/4 wavelength at the frequency of interest

300 / freq ( in MHz) = wavelength / 4 = 1/4 wavelength x 0.96 (velocity factor) = electrical 1/4 wavelength

see above and do the math

cheers
Dave

Thanks Dave.
Why did you specifically nominate .96 as the velocity factor?

Last edited:

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,720
Because electromagnetic waves travel more slowly on wires than in free air. See here.

#### rmdrmd

Jun 14, 2013
13
Thanks to all of you for your assistance on this.
I now have a simple antenna on my receiver (based on the advice received) and the effective range of the transmitter has gone up from only 10 meters to around 60 meters. More than adequate for the application.
Cheers.

#### davenn

Moderator
Sep 5, 2009
14,077
Thanks Dave.
Why did you specifically nominate .96 as the velocity factor?

Because electromagnetic waves travel more slowly on wires than in free air. See here.

Thanks Harald

@rmdrmd for a bare copper wire the vf ( velocity factor is approx 0.95-0.96
if the wire has a plastic coating the vf can drop substantially down to around 0.55
There are all sorts of numbers in between those extremes depending on the type of cladding

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