To create an antenna for use with ANY radio, you need to know what frequency or frequencies you wish to operate on.
Radio waves travel at the speed of light. So we need to create the antenna at 1/4 of the wavelength that is calculated at the speed of light.
The formula you need to use to find out the length of wire needed is as follows:
- The speed of light in “Meters” is 300 million meters per second.
- The frequency you wish to operate on in “Mhz“.
- The Conversion from Meters to Inches is 39.37.
Antenna Length = ((300/Freq)/4)X39.37
Your antenna length will now be in inches.
To convert to feet, simply divide the inches by 12.
Lets break down the formula using Channel 19 of the Citizens Band (CB) frequency.
- Channel 19 frequency is “27.185 Mhz”
So this is our calculation.
- 300/27.185 = 11.0384 Meters
- 11.0384 / 4 = 2.7588 Meters
- 2.7588 x 39.37 = 108.6168 Inches
So, have you ever seen the “Whips” on the back of some pickup trucks?
They are 102 inch stainless steel whips. And when you add the 6 inch spring , you have a resonate (108″) antenna for CB radio!
But “CB” frequencies are only good for a couple of miles, so you need to use much LONGER wavelengths to gain more of an angle of attack off the ionosphere to get you across several states or even the entire United States!
I am trying to keep from sounding like you are in school to learn everything about radio transmissions, so I will just tell you.
The “Bands” you are interested in, to get you across multiple states and even the entire United States are: 20 Meters, 40 Meters and 80 Meters. Each band has different characteristics. These bands are regulated by the FCC (Federal Communications Commission) in the United States for amateur radio operation. So you MUST have a license to operate a radio on them! That being said, you can ALWAYS be ready for an emergency!
20 Meters (around 14.275 Mhz and using USB or “Upper Side Band”) will work well during the day time, and typically get you the best range. But at night, it dies out.
40 Meters (around 7.200 Mhz and using LSB “Lower Side Band”) will give you Multiple states of coverage during the hours just before and after Sunrise and Sunset. But will die out after that.
80 Meters (around 3.850 Mhz and using LSB “Lower Side Band”) will give you 1 to 2 state coverage for a few hours before and after Sunrise and Sunset.
Using the formula for 80 meter transmission would look something like this:
- 300 / 3.850 = 77.9220 Meters
- 77.9220 / 4 = 19.4805 Meters (the 1/4 wave)
- 19.4805 x 39.37 = 766.9480 Inches
- 766.9480 / 12 = 63.9123 Feet
So, lets refer to “Figure 1” above. This is a typical “Dipole” and “Inverted Vee” antenna setup.
Element 1 and Element 2 are “Equal” lengths of wire, cut to the length you calculated from your formula. The “Feedline” is typically 50 ohm “Coax,” like “RG-8” or “RG-58“. The center of the “Coax” is going to tie to Element 1 and the “Shield” or “Ground” of the “Coax” is going to tie to Element 2. The “Support(s)” height should be no less than “20” feet high.
You can use something like a “12 gauge” Stranded wire for your Elements.
This method of building antennas is very rudimentary, and you will need to find the “Sweet Spot” for the “least” amount of returned power to the transmitter. This is known as “SWR” or “Standing Wave Ratio“. If the antenna is cut to the wrong length for the target frequency, the “SWR” will be to high, and you can burn the radios transmitter out when you speak.
Most radios have a built in “SWR” meter. You would set to a lower power on “AM” mode, then key up and read the “SWR” reading. Tune the frequency Up or Down to find the lowest “SWR” reading. Then return to your specified operating mode (LSB or USB.)
If the “SWR” reading goes down when tuning up from your target frequency, you need to equally lengthen Elements 1 and 2. This could involve a total replacement of the Elements for the correct length.
If the “SWR” reading goes up when tuning up from your target frequency, you need to equally shorten Elements 1 and 2 in 1/4 inch increments, until the lowest “SWR” is achieved.
Keep going back and forth until the lowest “SWR” is achieved for your target frequency.
You will need to cut 1 antenna set for each frequency band you intend to use.