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

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: How does one tell if the feedline is radiating?  (Read 6311 times)
KF4YTI
Member

Posts: 3




Ignore
« on: October 04, 2003, 11:46:45 PM »

I have built a J-pole antenna for use on 2 meters, and it appears to work great. I coiled the coax into 4 turns 5 in diameter 4 inches from the feed point, which many have suggested will prevent feedline radiation, and have measured SWR at 1.2/1, which I find acceptable. There's no evidence of TVI, and I find on VHF that high power settings are generally un-necessary, meaning that I usually run about 5 - 10 watts of power.

So, while I would guess my setup is OK, is there a good, established way to determine if the feedline is radiating?

Thanks!
Tim (KF4YTI)
Logged
WB2WIK
Member

Posts: 21807




Ignore
« Reply #1 on: October 06, 2003, 11:49:34 AM »

Lacking complex test equipment, there are still ways...

Evidently, you have an SWR bridge of some sort.  Use it, transmit, and run your hand up and down the coaxial transmission line, gripping it, but loosely enough that you can slide your hand along its outer jacket.

If you run your hand up and down the coax for 20" or more and there is no change in SWR as measured at the transmitter-end of the coax, it is very unlikely the coax is radiating.

WB2WIK/6
Logged
AG4DG
Member

Posts: 539


WWW

Ignore
« Reply #2 on: October 07, 2003, 05:23:19 PM »

Buy a field strength meter.  Cheap ones are sometimes available at hamfests.  (I bought one this past summer for only $2!)  There are also homebrew designs out there.

Now:
1.  Turn on your field strength meter.
2.  Bring your HT next to the field strength meter.  Transmit.  The field strength meter should respond.
3.  Transmit (key-down) with your HF transmitter.  Probe the shack with your field strength meter to see where the radiation is present.  The readings should be highest at the antenna and lessen as you move away.  When you are far away from the antenna, move the field strength meter towards the coax.  Touch the outer (nonmetallic) jacket of the coax with your field strength meter.  If the field strength meter does not respond, then the feedline does not radiate.  On the other hand, if the radiation increases as you approach the feedline even when you are far away from the antenna, then you have a problem with radiating coax.

This is so simple I'm surprised it isn't suggested more often.  Why go to all the trouble of RF grounding at the transceiver when it might not even be necessary?  (Actually, you don't really need an RF transceiver ground period.  Instead, use a balanced antenna or beef up the ground plane of the vertical.)
Logged
KT8K
Member

Posts: 1490




Ignore
« Reply #3 on: October 16, 2003, 03:01:52 PM »

I don't know how sensitive (maybe pretty good) or frequency dependent they are, but some older SWR meters made for CB/ham service have a field strength meter built in.  Look for the extra terminal or telescoping antenna on them.
I agree - use of a field strength meter is rarely suggested, but its an inexpensive and simple tool that works well in these circumstances.
Good luck es 73 de kt8k - Tim
Logged
5R8GQ
Member

Posts: 203




Ignore
« Reply #4 on: May 18, 2008, 04:41:35 PM »

If it works, don't fix it!
Logged
N8CMQ
Member

Posts: 723




Ignore
« Reply #5 on: July 20, 2009, 10:14:01 AM »

I use a diode on the input terminals of my Fluke DVM.
That is all a FSM is, a rectifier and a meter.
You can add an antenna, but it gets too sensitive then!
N8CMQ
Logged

N8CMQ   Jeff Retired...
KJ4EON
Member

Posts: 7




Ignore
« Reply #6 on: August 01, 2009, 06:13:45 AM »

N8CMQ,
    How would I go about making one of those?  Alligator clips on my meter, any # diode?  

       Tnx, Fred
Logged
KM3F
Member

Posts: 858




Ignore
« Reply #7 on: August 01, 2009, 12:14:39 PM »

Putting antennas on VMs; you won't know what your picking up or where it is coming from.
4 turns on a 5" diam is way more inductance than needed for feedline choking for 2m.
On a J pole that is well matched, you don't need to worry about any of this.
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!