1. VHF / UHF Antennas and Coaxial Cable – What is your ERP ???
KARS Net Discussion – Tuesday, 23 August 2011
Presented by:
Del Partridge, W5QQ

2. Introduction How coaxial cable effects overall station performance.
Some different types of coaxial cable and how they effect power output at 2 meters and 70 centimeters.
The importance of knowing what you are using between the antenna and the radio.

3. Goals and Objectives Goal #1 Learn some basics about antenna gain.
Learn a little about the effect that different types of coaxial cable have on overall antenna performance.
Objective:
To become more familiar coaxial cable types and their effect on overall station performance.

4. Vocabulary
dbd: Used as a reference to a theoretical one half wavelength dipole in free space.
dbi: Used as a reference to a theoretical isotropic antenna in free space.
Isotropic antenna: A theoretical antenna that radiates the same level of energy in all directions when power is applied to the antenna. (Most often used antenna reference. Be careful.)
ERP: Effective Radiated Power (ERP) is the term used to describe the calculated power emitted from the antenna.

5. Vocabulary
The most often used antenna reference to describe antenna gain is dbi. This can be misleading.
A theoretical isotropic dipole in free space would have a gain of 2.15db over a theoretical dipole antenna with all other parameters equal. This reference, dbi, would make an antenna appear to have greater gain than an antenna referenced to dbd as a measure of gain.

6. Topic One How coaxial cable effects antenna performance.
In this example we will look at different types of coaxial cable and coaxial cable loss.
We will then examine the Effective Radiated Power (ERP) from a unity gain antenna using different types of coax.
Because coaxial cable loss goes up as frequency goes up we will show examples of the phenomena at both 2 meters and 70 centimeters.

7. Topic One As an example suppose the following:
You are using 100’ of Tandy RG-58 coax.
Your antenna is a quarter wave vertical with unity gain. The antenna SWR is 1:1.
Your radio puts out 50 watts on 2 meters and 35 watts on 70 Centimeters.
Your loss at 146 MHz is: db and your power out is watts.
Your loss at 446 Mhz is: db and your power out is watts.

8. Topic One
Note: For this example a dual band FM transceiver with 50W output on two meters and 35 W on seventy centimeters is assumed. The antenna has unity gain. In this example there are no losses except as shown in the chart above.

9. Topic Two
What would happen to the Effective Radiated power if we connected a dual band J-Pole.
For the sake of discussion we will agree that the J-Pole has a gain of 3dbd at 2 meters and a gain of 6dbd at 70 centimeters.

10. Topic Two .
Note: For example a dual band FM transceiver with 50W output on two meters and 35 W on 70 centimeters is assumed together with and assumed antenna gain on two meters of 3db and an antenna gain on seventy centimeters of 6db. In this example there are no losses except as shown in the chart above.

11. Topic Three
The importance of knowing what you are using between the radio and the antenna.
For short runs use RG 58 or RG 8X. RG-8X has less loss and a foam dielectric. Good and bad.
Up to thirty or forty feet for close in work RG 8X works.
For longer runs look to higher quality coaxial cables.
The most expensive accessory in your station can easily be cheap coaxial cable.

12. Summary
We have discussed two antenna measurement references: dbd and dbi.
We have looked at the effect of different types of coax on ERP.
We have discussed the importance of knowing what you are putting between the radio and the antenna.

13. Where to Get More Information
This site will compute coax loss and ERP for you:
The ARRL Amateur Radio Handbook and the ARRL Antenna Book are invaluable sources of information. Some suppliers on the Internet that I have used:
Where to read about Isotropic antennas:
The effect of SWR on power loss: The usual disclaimers apply. I received no remuneration of any kind from any of these suppliers. I own no part of any of these companies and I offer their names only as sources that I have purchased from before.