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Re: [TowerTalk] Balun question shield leakage

Subject: Re: [TowerTalk] Balun question shield leakage
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Fri, 15 Jun 2007 21:44:13 +0100
List-post: <>
jeremy-ca wrote:
>Hi Ian, nice to here from you, its been ages!
Hasn't it just? Good to see you back.

>As far as the ship problem "Not at all" still holds. The network did 
>not use PL259's, they are not even constant impedence. The installer 
>used inferior cable but the proper F connectors for that cable and the 
>ones we checked were all installed correctly.
Well, that's curiouser and curiouser! I guess we've about beaten this to 
death, since we don't know anything about the victim equipment either.

>I was actually looking foreward to the catapult; I had been thru it 4-5 
>times while on active duty and loved it. While traveling around the US 
>on business I also made it a point to enjoy the latest and baddest 
>roller coasters.


Coming back to the effectiveness of baluns, the final decider is the 
amount of unwanted RF current on the outside of the coax shield, 
compared to the wanted current in the antenna element. The only way to 
find that out for sure is to *measure* it, in the system as installed.

I'm a big fan of RF current meters based on simple snap-on ferrite 
beads. Add a few turns of wire, one resistor, a diode detector, and you 
have a real measuring instrument. It's a real eye-opener to be able to 
snap the meter over any cable and *see* the common mode RF current.

By coincidence, this afternoon I threw together an AD8310 wide-range RF 
detector for  my own snap-on meter.  Instead of being blind to small 
signals because of the diode-detector threshold, the meter now has a 
linear dB scale that goes all the way down to about 100uA. Once I have 
beefed up some of the resistors, the same scale will cover all the way 
up to 1A.

Down at the low end, the sensitivity is scary - there's no such thing as 
a totally clean cable, anywhere in this shack! The general level of 
common-mode crud on computer, network and mains cables is on the order 
of a few milliamps. Pretty harmless from an RFI point of view, but it's 
nice to be able to see and measure down to those levels if needed.

(Some reservations about absolute accuracy are in order here. The 
detector sees the combined common-mode currents at all frequencies, from 
VLF up to a few hundred MHz, and the current readings on various cables 
will also depend on the waveform of the signals. This means that a given 
level of measured current can have a widely varying potential to cause 
RFI, depending on the type of signal involved.)

On the RG58 cable connecting the 100W rig to a very well screened 50 ohm 
dummy load, the key-down RF current inside the cable is obviously 1.4A. 
On the outside, the meter says about 10mA. However, that doesn't 
necessarily indicate leakage through the coax, because there is about 
the same level of common-mode RF current on the twin 13.8V power cable. 
Once the RF is out of the shielding envelope, it tends to crawl 
everywhere... with "interesting" implications for crossband duplex and 

Of course this is all anecdotal stuff, but it's so nice to be able to 
*see* what the RF currents are. It cuts out a load of guesswork.

At the higher current levels that are more likely to cause trouble, even 
a simple diode detector will do the job:


73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)

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