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

RG-8

0.66

LMR-400

0.85

RG-11

0.75

RG-58

0.66

RG-213

0.66

RG-400

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).

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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

 There are other factors too, but these are the most common ones.

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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.

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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


     Frequency in MHz
     Half Wavelength in Feet
Half Wavelength in Inches
     Quarter Wavelength Feet
Quarter Wavelength in Inches

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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. 

Enter Info and click Calc
Line Type:
Line Length: Feet Meters
Frequency:   MHz
Load SWR:   : 1
Power In:   W
Results
Matched Loss:   dB
SWR Loss:   dB
Total Loss:   dB
Power Out:   W

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Online Coax Cable Loss / Antenna Gain Calculator

(This produces your result in a pop-up window)

Enter dB Loss Of Cable Per 100 Ft. At The Desired Operating Frequency

Enter Length of Cable in Feet

Enter Power into Cable in Watts

Enter Gain of Antenna in dBd

  

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