102" Whip Questions

[BigMike357]

Using the formula for a quarter wave antenna I get a 103". I see some people not use a
spring and some who do. I also hear 108" is the magic number. Why do some say 108".
I am lost looking for an explanation.
thanks Big Mike

 

[The DB]

The speed of light is 11,802,852,672 inches per second
The frequency for CB channel 20 is 27,200,000 Hz

To get the wavelength take the speed of light and divide that by the wavelength...
11,802,852,672 ÷ 27,200,000 = 433.9284...

Then divide that by 4 for 1/4 wavelength...

433.9284... ÷ 4 = 108.482... inches.

This is where the 108 inches amount comes from.

Although, none of this takes into account various other things such as the fact that our antennas are not made out of an infinitely thin perfect conductor and are not placed over a perfect ground, all of which and more will have their effects.

Now about your 103 inch length... Some calculators try and factor said real world concerns into their calculations, thus giving a lower number than the pure number I calculated above. This is done by removing 5% (although some use other values are sometimes used, 5% is the norm on the ham radio side at least) from the calculation above, so...

108.482 x 0.95 = 103.06 inches... Look familiar?

In the real world this 5% subtraction is only really a rule of thumb. In my experience, tuned CB 1/4 wavelength whips mounted to cars averages closer to 105 inches for me, although, again, this wasn't a specific length and varies from vehicle to vehicle...

Hope this helps.


The DB

 

[BigMike357]

Thank You I was curious and wondering. DE Big Mike

The speed of light is 11,802,852,672 inches per second
The frequency for CB channel 20 is 27,200,000 Hz

To get the wavelength take the speed of light and divide that by the wavelength...
11,802,852,672 ÷ 27,200,000 = 433.9284...

Then divide that by 4 for 1/4 wavelength...

433.9284... ÷ 4 = 108.482... inches.

This is where the 108 inches amount comes from.

Although, none of this takes into account various other things such as the fact that our antennas are not made out of an infinitely thin perfect conductor and are not placed over a perfect ground, all of which and more will have their effects.

Now about your 103 inch length... Some calculators try and factor said real world concerns into their calculations, thus giving a lower number than the pure number I calculated above. This is done by removing 5% (although some use other values are sometimes used, 5% is the norm on the ham radio side at least) from the calculation above, so...

108.482 x 0.95 = 103.06 inches... Look familiar?

In the real world this 5% subtraction is only really a rule of thumb. In my experience, tuned CB 1/4 wavelength whips mounted to cars averages closer to 105 inches for me, although, again, this wasn't a specific length and varies from vehicle to vehicle...

Hope this helps.


The DB

 

[Greg T]

I've installed nearly 100 1/4 wave whips in my time and always, always used the ball and spring mount for 108" NEVER had to modify the length of any of them and the absolute WORST SWR I have even had on an installation was 1.5:1. And that was because the antenna was mounted on a truck directly behind the cab. I also built a quick homebrew base antenna for my shack in the back yard and used a whip and potbelly spring, and used two 105" pieces of copper for a ground pane. 1.2:1 SWR with a 108" vertical. I have also used coax as long as 100', and as short as 8" without any difference.

 

[BigMike357]

The speed of light is 11,802,852,672 inches per second
The frequency for CB channel 20 is 27,200,000 Hz

To get the wavelength take the speed of light and divide that by the wavelength...
11,802,852,672 ÷ 27,200,000 = 433.9284...

Then divide that by 4 for 1/4 wavelength...

433.9284... ÷ 4 = 108.482... inches.

This is where the 108 inches amount comes from.

Although, none of this takes into account various other things such as the fact that our antennas are not made out of an infinitely thin perfect conductor and are not placed over a perfect ground, all of which and more will have their effects.

Now about your 103 inch length... Some calculators try and factor said real world concerns into their calculations, thus giving a lower number than the pure number I calculated above. This is done by removing 5% (although some use other values are sometimes used, 5% is the norm on the ham radio side at least) from the calculation above, so...

108.482 x 0.95 = 103.06 inches... Look familiar?

In the real world this 5% subtraction is only really a rule of thumb. In my experience, tuned CB 1/4 wavelength whips mounted to cars averages closer to 105 inches for me, although, again, this wasn't a specific length and varies from vehicle to vehicle...

Hope this helps.


The DB

The 108" includes the ball mount length also I would assume. Thanks Big Mike