Call Search

New to Ham Radio?
My Profile

Friends Remembered
Survey Question

DX Cluster Spots

Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
Author Topic: Effective Radiated Power (ERP)  (Read 7099 times)

Posts: 1

« on: November 11, 2000, 09:54:03 PM »

Could someone PLEASE !! explain in simple terms how to figure ERP as used on the EXTRA exam ?
Any help will be great

                                                                Gary  KD5GIF

Posts: 13

« Reply #1 on: November 12, 2000, 03:52:50 PM »

In order to figure Effective Radiated Power you need to take in all the RF power gain or loss factors from the output of the transmitter to the antenna.  In most cases this will involve three numbers or factors.  

The first is the RF output power of the transmitter.  Usually given in Watts.  

The second factor is the loss factor of the feedline (for example coax, open wire line or possibly twin lead).  These loss factors are usually expressed in decibels (dB).  Total line loss for a specific installation is dependent on the type of line or coax AND the total length of line between the transmitter connector and the antenna terminals AND the frequency of interest.  Manufacturers of line will usually give this factor at several frequencies in a table or graph expressed in decibels per measure of length, typically dB per 100 feet or 100 meters.    

The third factor is the antenna gain in decibels.  The manufacturer of commercially made antennas will usually give this data.  If the antenna is homebrew such as a dipole, these antenna gain factors can be found in many of the Amateur Radio reference books such as the ARRL Antenna Handbook.  Typically antenna gain factors are also expressed in decibels (dB).

ERP is the transmitter output power multiplied by the loss factor of the feed line multiplied by the gain factor of the antenna.  

Lets take a specific example:  Lets say we want to know the ERP of a transmitter, feedline and antenna system on the 30 meter (10 MHz) amateur band.  The transmitter has an output power of 100 Watts on the 30 meter band.  The feed line is 150 feet of RG58U and the antenna is a three element commercially built yagi with a manufacturers specified gain of 6 dB on the 30 meter band.

Looking at the manufacturers sheet on RG58U, it has a loss factor of 2.0 dB per 100 feet.  (I have not looked at the actual table, this is a guess on my part just to show how to use the factor.) So we have to take this and factor it to a length of 150 feet.  So you take the loss factor of 2.0 dB/100 feet times 1.5 (150 feet is 1.5 times as long as 100 feet).  This gives a total feed line loss of 3.0 dB.  If you look at tables of dB vs gain or loss (see the ARRL handbook) a 3.0 dB loss factor means a gain or loss of 2.0.  This is a loss and so we take the recipricol of the factor or 1/2.

So, this means that of the 100 watts of RF leaving the transmitter, only 1/2 or 50 Watts (100 watts X 1/2) will reach the antenna.  The rest is lost in the form of heating in the coax.

Now we have to look at the antenna.  Lets say the manufacturer rated the antenna to have 6.0 dB of gain on 30 meters.  So, if you look at the dB conversion table again, 6 dB of gain is a power gain or loss factor of 4.  In this case this is a gain so we take the gain of 4 times the power at the antenna terminals of 50 watts.  

This then is our ERP.  100 Watts of transmitter power times a loss in the coax of 1/2 times an antenna gain of 4 gives an ERP of 200 Watts.

Hope this helps,  MIKE

Posts: 90

« Reply #2 on: November 12, 2000, 08:38:18 PM »

I thought I replied to this, but must have been dreaming.  The easy way is to just remember that every 3 db CHANGE multiplies power by 2 if it is GAIN, and divides power by 2 if it is LOSS.  You can estimate most problems this way.  I don't think any of the problems on the test ask you to figure lossper 100feet, I think the problem shows the gain/loss.  Some of them do not work out even, but if you use this simple method, and maybe learn the "1 db" you can estimate most of them.  For example, If you have a transmitter with 150 watts out put, that should be at the antenna connector of the transmitter.  ERP wants to know what is the effective out of the antenna.  Just add up all the gains, and subtract all the losses, and the result is the net gain or loss.  If you have a duplexer loss of 6db, a feedline loss of 6 db, and an antenna gain of 9db, Thats a net total of 3db LOSS.  Because this is a change of 3 db, you would divide the power in half--75 watts is the ERP.

Lets say yu have a 100w tranceiver and add an 800watt amplifier? How much gain is that?  You multipled your power 100x2x2x2 to get the 800w total.  That is a change of 3db+3db+3db=9 db gain total.

You can com pretty close on many problems, especially in the field, if you are trying to estimate if a duplexer/antenna is doing its job. A 100ft tower?  Whats the feedline loss per 100 ft?  What kind of antenna is up there?  6-8db gain or so?  What would you expect the duplexer to lose?  You can MEASURE that with your wattmeter.  Same with the transmitter output power.Say you have 35 watts out of the duplexer.  3 db loss in the cable?  6-8 gain in the antenna?  Thats a gain of 3-5 db.  You already measured the power at the duplexer, so that's out of the picture.  Receiver loss is another matter.  Thee ERP here is at least 70 watts.
Pages: [1]   Go Up
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!