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: Any way to measure RF in the shack?  (Read 5983 times)

Posts: 255

« on: November 15, 2013, 09:14:32 AM »

     One of the rules of Continuous Improvement is that you don't know if you improved something if you can't measure it.    I'm still fighting an occasional "Jumping SWR" (from 1.75 to 6 with power cutting back) when using PSK-31 at 35 watts on my ICOM 718 to my new Hex Beam on 10 and 12 Meters.  The Hex Beam has 4 MFJ Ferrites on the COAX right at the connection point per manufacturer recommendation.    Next time I get out the ladder I'll go up on the roof and add some more Ferrites.  My shack is in the basement.

      Getting rid of one Power Strip in the shack reduced the frequency of the problem.   Using a MFJ 949E manual antenna tuner to take the SWR from 1.75 to 1.5 usually stops the problem.   Adding ground wires in the shack usually increases the problem.   I don't think there is anything wrong with the coax or connectors.   Everything was tested individually, temporarily replaced with known good, or eliminated.  I also did a full end to end test with Dummy Load up on the roof.   I can also reproduce the problem on a Hustler Vertical Antenna (totally rebuilt this spring)

       I don't think the problem is the Hex Beam or Vertical Antenna as they work fine on a steady signal (Key down or Tuner button) with SWR being 100% steady.     On PSK, the ICOM-718 SWR meter jumps around "two ticks" (I.e. from 1.75 to 2.0 and back).   I believe the problem is some kind of Ground Loop and or RF in the Shack.   Alternately, the 8 year old ICOM-718 has gotten extra fussy about SWR of 1.75.
      Thus, is there any way to measure RF in the shack?    I have a Rigol 100mhz digital oscilloscope, but an a newbie using it.  I also have a dip meter, field strength meter, plus an assortment of regular digital multimeters, and RF and Audio signal generators.    Any suggestions on how to take some actual measurements?      Random problems are *^($#%@#. Huh   Thanks.


Posts: 719


« Reply #1 on: November 15, 2013, 09:35:23 AM »

Most "RF in the shack" is due to unbalanced currents on your feedline or coupling of your transmitted RF into the wiring in your house.  It can be difficult to measure if you don't the right tools (e.g., a current probe that you can run on batteries, etc).  A pretty effective way to do it is to unplug everything from the radio except the power supply and antenna.  You might check to see if you can reproduce your problem by keying down in CW mode (even just shorting the keying contacts together will work) because that requires an awful lot less wiring.  I wouldn't do anything more up at the antenna if you've followed the vendor's directions.  I have a homebrew broadband hex beam with a similar feedline choke based on a K9YC design.  I have only ever had RF problems when I was beaming toward the house.  It sounds like you're right over the house, so this could be the culprit. 

If your coax is above ground for any horizontal distance away from the antenna, that's a potential coupling problem, too.  You could try putting a few turns on a couple of cores along the feeder somewhere, but I think there's a high probability that your house wiring is picking up your transmitted signal.  Do you come over amplified computer speakers or anything else powered by the AC mains that has a high-gain amplifier in it?  You can probably get away with putting bandaid ferrites on power and audio cables...

Posts: 1849

« Reply #2 on: November 15, 2013, 01:32:31 PM »

Discussion of common mode problems with some hexbeams:

The four ferrites you are presently using probably do not present a sufficiently high impedance to adequately choke off common mode. There are many resources on the net for making simple, cheap and effective current chokes using several turns of coax through a large toroid.

Posts: 3256

« Reply #3 on: November 15, 2013, 05:49:50 PM »

The most sensitive test for RF on the rig is the 'Lip on the big metal microphone test".  Smiley

MFJ sells two current detectors that will detect current on coax (or on whip antennas).  Tom, W8JI has a simple construction article on his website.

Posts: 349

« Reply #4 on: November 15, 2013, 06:44:57 PM »

A field strength meter used as a probe should give you an indication of RF. It won't tell you how much but basically, any is too much.
Dick KH2G

Posts: 1050

« Reply #5 on: November 16, 2013, 06:38:07 AM »

Without a meter you can use a flourescent light tube. Placing the tube near the coax when you key up, it'll illuminate dimly if you have common mode current flowing.

Posts: 2386

« Reply #6 on: November 17, 2013, 03:52:15 PM »

I'm still fighting an occasional "Jumping SWR" (from 1.75 to 6 with power cutting back) when using PSK-31 at 35 watts on my ICOM 718 to my new Hex Beam on 10 and 12 Meters.  

SWR "jumping around" is not due to RF in the shack.  It is an indication of an intermittent connection somewhere in the antenna system. (That includes all the coax and connectors and the antenna tuner.)
« Last Edit: November 17, 2013, 03:54:44 PM by K4SAV » Logged
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!