Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
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
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Converting dBm to v/m?  (Read 6538 times)
VA2FSQ
Member

Posts: 510




Ignore
« on: January 05, 2012, 07:29:23 PM »

Hello,
Is it possible to convert a signal as displayed on a panadapter to V/m?  In other  words if I have a signal of 20 dBm what would the signal strength be in V/m?
Thanks
Logged

VA2FSQ
K2DC
Member

Posts: 1354


WWW

Ignore
« Reply #1 on: January 05, 2012, 11:40:29 PM »

Tom,

   Congrats on the new license, and welcome to the hobby.  The answer to your question is no, you cannot convert directly from one to the other because you're mixing units of measure.

   dBm is a measure of power, expressed as dB with respect to a milliwatt.  V/m is a measure of field strength, and can only be converted to and from a power density expressed as W/m^2 or dBm/m^2.  That conversion is fairly straightforward.

   Exactly what is it you're trying to do?  If we knew that, we might be able to provide more help.

73,

Don, K2DC


  
Logged
WX7G
Member

Posts: 5977




Ignore
« Reply #2 on: January 06, 2012, 03:52:30 AM »

Yes you can do this conversion. You need to know the "Antenna Factor" or AF or your antenna. A rule-of-thumb is to say a 30 MHz dipole has an AF of 0 dB. A 60 MHz dipole an AF of 6 dB, a 15 MHz dipole an AF of -6 dB.

Dipole AF = 20LOG(F/30)

The AF tells us the loaded (50 ohm) terminal voltage of a dipole immersed in an E-field. The 30 MHz dipole in an E-field of 1 V/m will output 1 volt. The 60 MHz dipole 1/2 volt and the 15 MHz dipole 2 volts.

Now convert your 20 dBm to dbuV. In a 50 ohms system add 107 dB and you have 127 dBuV. Add the AF. If you AF is -19 dB (an 80 meter dipole at 3.5 MHz) the E-field is 127 - 19 = 108 dBuV.

Converting 108 dBuV to volts we get 0.25 volts.
Logged
G3RZP
Member

Posts: 4477




Ignore
« Reply #3 on: January 06, 2012, 08:57:08 AM »

But bear in mind that the uncertainty is quite large. Ground reflections can change things by 6dB or more. For qualification laboratories, the measurement of radiated power or field strength is usually required to be to better than plus or minus 6 dB to a 95% certainty. And that is on a known calibrated test site or in an anechoic chamber (where it's usually much less than 6 dB, but often plus or minus 2 dB)
Logged
Pages: [1]   Go Up
  Print  
 
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!