# Antenna question

#### Nathan Hale

Joined Oct 28, 2011
156
Hi guys! Earlier this week I was reading that a radio's antenna should be at least a quarter length of the wave length it is trying to receive.
The scanner we got at home can pick up the 136 Mhz freq. , which i believe has a 2.2 meter wave length. a quarter of that would make it about half a meter.
The scanner's antenna on the other hand is only 12 cms long, when i believe it should be 50 cms. long. How is it able to pick up the 136 mhz frequency even though the antenna is not the size it is supposed to be?

on the same note..... the scanner can also pick up 400 - 480 mhz frequencies. My second question is...... shouldnt the radio have an antenna for each frequency it is asked to pick up? by each freq , i mean an antenna for 400 mhz, another antenna for 401 mhz , 402 mhz etc etc. ? what is the physics that makes an antenna pick up so many different frequencies? me thinks if it is 12 cms long it should only be able to pick up 624567620.83 Hz!!
p.s. (299792458) / ( 12 *4 ) = 624567620.83

Thank You for all the replies.

#### #12

Joined Nov 30, 2010
18,210
You left out a word. An antenna should be at least a quarter wave length for OPTIMUM transmitting or receiving. Any metallic object in a radiated field will be affected to some degree, even a hubcap or a penny. They just don't make optimum receivers.

#### bertus

Joined Apr 5, 2008
20,567

Joined Aug 8, 2011
53
Without getting too deep into antenna theory (I hope).
The general rule of thumb for receiving antennas (in general) has always been that the antenna should be cut for the "lowest" frequency of interest.
This is of course a broad statement any way you look at it.
In your case, you seem to be interested in 136 Mhz so the antenna should be cut for 136 Mhz , your also interested in 400-480 Mhz , if you play with the formula's for antenna cutting for each range you will find that an antenna cut for 136 will have a harmonic in the 400-500 Mhz range also.
One of the more popular scanner antennas is the "DI SCONE" type due to it's wide range of harmonic prosperity's .

#### LDC3

Joined Apr 27, 2013
924
Also on the web page for the "rubber ducky antenna", it states that the antenna could be base loaded. This means that a coil of the proper length is attached between the radio circuit and the antenna. This appears to lengthen the antenna.