Monopole antenna length.

Discussion in 'Radio and Wireless' started by rmdrmd, Jun 23, 2020.

1. rmdrmd

13
0
Jun 14, 2013
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.

2. davennModerator

13,810
1,944
Sep 5, 2009
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

3. Nanren888

475
136
Nov 8, 2015
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.

1,759
677
Nov 8, 2019
5. rmdrmd

13
0
Jun 14, 2013
Thanks Dave.
Why did you specifically nominate .96 as the velocity factor?

Last edited: Jun 24, 2020
6. Harald KappModeratorModerator

11,433
2,622
Nov 17, 2011
Because electromagnetic waves travel more slowly on wires than in free air. See here.

7. rmdrmd

13
0
Jun 14, 2013
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.

8. davennModerator

13,810
1,944
Sep 5, 2009

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