Amateur Radio Antenna Facts
Here is an initial bit of information about antennas and transmission lines.
I welcome any comments or corrections. Thanks to all who have notified me of inaccuracies. This type of participation makes this site more valuable. Please click here for a link to email your comment, correction, suggestion or question.
SWR
What is VSWR or SWR? VSWR is Voltage Standing Wave Ratio and SWR is Standing Wave Ratio.
Choose Your Topic Below
Formulae:
Cable Length Calculation
Tuning Your Antenna
Dipole Antenna Length
Calculators:
Antenna Characteristics Calculator
Transmission Line Loss Calculator
Cable Loss & Antenna Gain Calculator
Cable Length
It is best to use coax in multiples of .25, .50 or 1 wave length of the center frequency of the antenna you are connecting to. For VHF 2M, the center frequency is about 146MHz. This is derived for the 2M VHF of amateur allocation, 144 MHz – 148 MHz.
To determine the proper length of coax, use the following formula:
length _{ (in Meters)} = (300/CF) * VF
VF = Velocity Factor
CF = Center Frequency
Below is a table of commonly used coax and generally accepted velocity factors.
For exact information regarding your manufacturer's cable, please download the
data sheet from you coax manufacturer's web site.
COAX 
VF 
RG8 
0.66 
LMR400 
0.85 
RG11 
0.75 
RG58 
0.66 
RG213 
0.66 
RG400 
0.695 
For 2 meters, a chunk of coax of 1 wavelength long = 300/146 which is about 2.05 Meters. To get the optimum performance this value should be multiplied with the coax velocity factor. If your coax has a velocity factor of 0.92, then coax of 1 wavelength = 2.05 x 0.92, which equals 1.886 Meters. Now you know how long one wavelength of your coax is, you should cut it in multiples of 1.88 Meters or .25 or.50 of that value.
Example, you need to use approximately 26 Meters of cable.
First, find out how many wavelengths that is.
26 Meters/1.88 = 13.82 wavelengths
13.82 wavelengths is not a whole multiple of 1.88 so use the next highest whole multiple. In this example the next whole multiple is 14 wavelengths or 26.23 Meters. (See my metric conversion page for feet and inch conversion formulae).
Tuning Your Antenna
If SWR on 144 MHz is more than 148 MHz, you will need to lengthen your antenna. Especially for jalopies, this can be done by adding additional spring mounting. For base antennas refer to the length chart that comes with the antenna.
If SWR on 148 MHz is more than 144 MHz, your will need to trim your antenna. Refer to the cutting chart/length adjustment chart that comes with the antenna. Some antennas have a small Allen screw to adjust the length.
If SWR is above 3.0, check for electrical shorts, improper grounding, etc.
Factors that contribute to high SWR

Low quality coax has high loss. Buy coax with close to 100% braid cover.

Coax is too long. Cut your coax as short as reasonably possible.

Bad soldering produces poor contact. Cold joints are the enemy!

Excess coax is coiled. This is actually an aircore BALUN and changes the impedance of the cable.

Antenna length not tuned according to calculations length chart. Insulated wire needs to be longer than the calc shows.

Little or no ground plane. Be sure your environment is suitable.

Location of antenna is not the best. Sometimes you are stuck though... Do the best you can.

When testing SWR on jalopies, do in open area with doors shut.

Antenna mounting not grounded or shorting.
There are other factors too, but these are the most common ones.
Dipole Antenna Length
When computing how long to make each leg of a Dipole antenna, dividing the frequency in MHz into 468 will give the overall half wave length of the antenna. To find the quarter wave, divide that number by two. This will give you the length of each leg of the antenna.
Example — 80 meters : If you are operating on 3.8 MHz, dividing 3.8 into 468 gives 123.16 feet. To find the quarter wave, dividing 123.16 feet by 2 gives 61.58 feet for each leg. Cut the wire two or three feet longer so you will have some to fold back on itself when adjusting the SWR. The part that folds back on itself does not effect anything. Connect the coax at the center point or middle of the antenna, the center lead of the coax to one leg and the braid to the other leg. When adjusting for minimum SWR, adjust each leg the same amount, current fed.
Example — 40 meters : If you are operating on 7.2 MHz, dividing 7.2 into 468 gives 65 feet. To find the quarter wave, dividing 65 feet by 2 gives 32.5 feet for each leg. Cut the wire two or three feet longer so you will have some to fold back on itself when adjusting the SWR. The part that folds back on itself does not effect anything. Connect the coax in the center point or middle of the antenna, one leg to the center lead of the coax and the braid to the other leg. When adjusting for minimum SWR, adjust each leg the same amount, current fed.
Example — 20 meters : If you are operating on 14.5 MHz, dividing 14.5 into 468 gives 32.28 feet. To find the quarter wave, dividing 32.28 by 2 gives 16.14 feet for each leg. Cut the wire two or three feet longer so you will have some to fold back on itself when adjusting the SWR. The part that folds back on itself does not effect anything. Connect the coax in the center point or middle of the antenna, one leg to the center lead of the coax and the braid to the other leg. When adjusting for minimum SWR, adjust each leg the same amount, current fed.
Example — 2 meters : If you are operating on 144.5 MHz, dividing 144.5 into 468 gives 3.24 feet. To find the quarter wave, dividing 3.24 feet by 2 gives 1.62 feet for each leg. When mounting on a vehicle, a quarter wave antenna MUST be mounted on a metal base. A quarter wave antenna can be mounted on a fiberglass or plastic body vehicle with the coax connected to one end of the antenna, center lead to antenna, braid grounded, voltage fed.
Antenna Characteristics Calculator
Calculate the frequency and length of antennas Enter one value only and click COMPUTE for the result.
Clear the form to make new entries
Transmission Line Loss Calculator
Here's a transmission line loss calculator that has values for common coax and balanced line transmission cables. The calculator gives a very close approximation for additional losses due to SWR.


Online Coax Cable Loss / Antenna Gain Calculator
(This produces your result in a popup window)
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