RadioReference on Facebook   RadioReference on Twitter   RadioReference Blog
 

Go Back   The RadioReference.com Forums > Scanners and Receivers Forums > General Scanning Discussion

General Scanning Discussion For general questions not specific to a model of scanner or general discussion of use of a scanner. Location specific posts should be directed to the regional forums listed below.

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1 (permalink)  
Old 11-05-2012, 6:03 PM
Member
   
Join Date: Oct 2012
Location: Vallejo,CA
Posts: 2
Lightbulb Receiving Antenna vs.Transmitting Antenna

I was wondering how I would check the ohms reading at the coax connection point on a Antenna that I built that I am only using as a Receiving Antenna and not a Transmitting Antenna without a SWR meter? Does the ohms there make a difference?
Reply With Quote
Sponsored links
  #2 (permalink)  
Old 11-05-2012, 6:09 PM
Member
  Premium Subscriber
Premium Subscriber
Amateur Radio Operator
Amateur Radio
 
Join Date: Mar 2006
Location: Washington DC
Posts: 1,935
Default

The instrument used to find the resonant frequency of a receive-only antenna is called a noise bridge. You set it to 50 ohms, for instance, then tune the receiver to a point where the noise changes. Or you set the receiver to your desired channel then adjust the bridge to where the noise changes and then read the impedance from the position of the control on the bridge.

Good noise bridges are precision instruments and are not cheap.
Reply With Quote
  #3 (permalink)  
Old 11-06-2012, 1:08 AM
Member
   
Join Date: Mar 2009
Location: United Kingdom
Posts: 280
Default

Quote:
Originally Posted by big420dog94550 View Post
I was wondering how I would check the ohms reading at the coax connection point on a Antenna that I built that I am only using as a Receiving Antenna and not a Transmitting Antenna without a SWR meter? Does the ohms there make a difference?
Does resistance make a difference or not? Yes it does, but its' worth pointing out that when it comes to undertaking antenna measurements there is a lot more to "Ohms" than we normally associate with this measurement.

To start with we dealing with an measurement taking almost exclusively on a sinusoidal wave, which will vary constantly depending on the point at which its taken from on the antenna element, and the time its taking at i.e. a reading other than 50 Ohms does not mean an antenna is not resonant ...... and then things get more complicated ........!

To add to what W2NJS has contributed, antenna measurement is a science all on its own, which, to undertake requires a whole bunch of instruments. Noise bridges are great pieces of kit to have (and as W2NJS has said, are precision instruments and not cheap), a simple SWR meter is also great to have, and so are things like vector analyzers to undertake things like S-Parameter tests, and these too are big big buck pieces of hardware but are essential to properly measure and characterise antenna performance

Read the following online article.

Impedance of an Antenna

There are hundreds on-line like it if you type in to Google "antenna impedance"
Reply With Quote
  #4 (permalink)  
Old 11-06-2012, 6:31 AM
n4yek's Avatar
Member
  Amateur Radio Operator
Amateur Radio
 
Join Date: Apr 2004
Location: Newport, Tennessee
Posts: 1,959
Default

Here is an example an antenna analyzer:
MFJ Enterprises Inc.

Here is one on ebay:
http://www.ebay.com/itm/ws/eBayISAPI...m=120685683212
__________________
Danny Harp Jr.
HAM Radio Operator: N4YEK
A blood donor saved my life (Matthew 26:28).....Remember, Jesus loves you.
If 'con' is opposite of 'pro', is Congress the opposite of progress?

Last edited by n4yek; 11-06-2012 at 6:34 AM..
Reply With Quote
  #5 (permalink)  
Old 11-06-2012, 12:41 PM
kb2vxa's Avatar
Completely Banned for the Greater Good
  Amateur Radio Operator
Amateur Radio
 
Join Date: Mar 2005
Location: Point Pleasant Beach, N.J.
Posts: 6,127
Default

To clarify, ohms DC is one thing but ohms AC is a horse of a different color. Here we get into VAR or volt-ampere reactive or another way of stating it, impedance and with RF we simply call it impedance. When it comes to antennas a noise bridge is not for the average ham or experimenter being rather complicated to operate and interpret the readings while the MFJ analyzer linked to above is easy and comes with instructions for using it for a variety of applications, not just antennas. Pricy yes but not unreasonably so and well worth the investment for those who do a lot of antenna, tuner, delay line, coaxial matching sections and related work. I don't own one not having sufficient need but borrowed one when I needed it and believe me it's a breeze to work with.

Yeah, I'm back safe and sound after the big blow, a little worse for wear but I'm working on it.
Reply With Quote
Sponsored links
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -5. The time now is 5:19 AM.


Powered by vBulletin® Version 3.8.2
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
All information here is Copyright 2012 by RadioReference.com LLC and Lindsay C. Blanton III.Ad Management by RedTyger
Copyright 2011 by RadioReference.com LLC Privacy Policy  |  Terms and Conditions