Coaxial Cable Characteristics and Data
It is of
importance when making decisions about getting the right coax for your Ham
Station to understand that there are trade-offs that have to be considered
between transmitter power, antenna gain, coax loss, erp, and your total
Ham Station system performance.
Your bank account may also enter into the equation like most Hams.
The db differences in gains or losses on transmit and receive between the choices available to you are the important issue.
A difference of 3 db either way will not be apparent to the stations that you are communicating with on the other end; around the block or around the world. They won't hear or see (on the S meter) any difference if you run your transmitter at 50 watts instead of its maximum 100 watt output power as an example.
This is a difference of about 3db! The same ratio holds true comparing 500 watts with 1000 watts!
Now take this same analogy and apply it to coax.
Here is an example:
Your present coax is 100 feet long and has a loss of 3db per 100 feet.You change it to the same length, 100 feet, using a more expensive coax that has only about .1db loss per 100 feet.
(Very Expensive Stuff!)
This is about a 3db increase in signal strength to the antenna that you were loosing in the old coax.
The station on the other end usually won't know the difference, and usually, neither will your receiver!
A difference of 3 db or less between two antennas, two types of coax, or two station configurations is usually not sufficient to justify the higher costs of the more expensive, lower loss, coaxial cable unless you are a perfectionist with lots of money.
However, a difference of 6 db may well justify the more expensive approach with the higher quality coax or antenna setup.
Remember, if you can't increase your transmit or receive signal by at least 5 or 6 db or more, if may not be worth the effort and expense.
The tables below should help you decide what if any coax changes you need to make.
Green shade 50 - 52 ohm
Coax dB Loss per 100 Feet using common coax types:
Note: These tables are
typical specifications for comparison only.
LMR-400 Power handling = 2100 watts <30Mhz
Understanding the effects of ERP vs antenna gain.
You will always have some bit of loss in your feedline but this calculator is considering only power and antenna gain.
Example: Coax loss
Using 100 watts output from transmitter and assuming your old coax had 3db loss, with no changes to antenna system except replacing your old coax and also assuming the new coax has 0db loss, the ERP of the antenna system would increase by 3db and would be 199 watts.
This is a 3db gain achieved by the new coax.
Example: Antenna Gain
You use an antenna that has 6dbd gain vs the old one that had 0dbd gain.
What effect does this have on your erp?
By using the chart above, you will see that with 100 watts at the antenna feedpoint, your effective radiated power would be 398 watts!
Remember, you have to
achieve about 6db gain or loss to to make much difference on the air to
the other station on receive.
Using this table, you should see that with every 3db increase or decrease, your effectively doubling the ERP or cutting it in half.
Your feedline will always have some loss so in calculating your total antenna system loss, always subtract the total loss of your feedline from your antenna gain.
Your antenna gain is 6dbd.
Your coax loss is 2 db per 100 feet as stated by the mfg. You use 50 feet.
Your total system gain or loss would be:
Antenna gain 6dbd -
loss of 1db(50 feet) = total gain (or loss) = 6 - 1
= 5 db
You add 100 feet of coax with 3 db loss at your operating frequency to an antenna with 0 dbd gain.
So 0db - 3db = -3db loss in signal strength...you just cut your signal in half.
Velocity Factor of Propagation Through Coaxial Cable
The velocity factor is the speed at which an RF signal travels through a material compared to the speed the same signal travels through a vacuum. The velocity of propagation is inversely proportional to the dielectric constant.
Lowering the constant increases the velocity.
Generally, the higher the velocity factor, the lower the loss through a coaxial cable.
"Typical" Velocity Factor of Coaxial Cable by type
General Rules for Coaxial Cable
D = diameter of insulation under the shield
d = diameter of inner conductor.
Velocity Factor, Velocity of Propagation, Vp
The higher the velocity factor, the lower the loss through the cable.
Raising the D/d has no effect on Vp
Raising the dielectric constant lowers Vp
Raising the D/d ratio lowers capacitance
Raising the dielectric constant raises capacitance
Impedance Raising the D/d radio raises impedance
Raising the dielectric constant lowers impedance
Attenuation or Loss
Raising the D/d ratio lowers attenuation
Raising the dielectric constant raises attenuation
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