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Re: [TowerTalk] RF current in REF + DIR much ?

To: Jim Thomson <>
Subject: Re: [TowerTalk] RF current in REF + DIR much ?
From: jimlux <>
Date: Mon, 03 Jan 2011 07:43:37 -0800
List-post: <">>
Jim Thomson wrote:
> How much RF current typ flows in the REF + DIR  on a 3-el yagi ?     YO  just 
> shows a 'relative'  scale  for the parasitic ele's. 
> IE: just to simplify things,  let's  say we have 10A flowing into the 30 ohm 
> feed-point of the DE  [3 kw] . [monoband yagi]
> How much RF  current do we typ have at the center of the REF  +  DIR  ??   
> [as a percentage of the DE current]    10%, 30% ?? 
> If the  REF + DIR used solid  fiberglass center insulator's and the shorting 
> strap across em was removed, and an RF ammeter
> was strapped across the insulator, what would I expect ?   Is that method  
> valid ?   I was thinking a clamp on RF Ammeter
> could also be used, but typ they are not calibrated..but are only relative.   
> Has anybody ever tried to measure the RF current in the
> center of the parasitic ele's ??      I downloaded 4NEC2 a few days 
> it's  new to me..and at 1st glance,  I don't see any mention
> of exact current flow in the parasitic els.   Now of course, this will all 
> depend on the tuning and spacing of the parasitic ele's....and 
> relative current flow in the parasitic ele's could vary  across the band.  

You can look at the segment currents... in 4nec2, you can click on the 
segment of interest and it will tell you the current in that segment. 
It's most convenient if you have an odd number of segments in the 
element, so one of the segments nicely straddles the center point.

If you're running straight NEC without a front end like 4nec2, you can 
add an excitation source for each element.  Make the voltage for the 
"source" very small (I use 1E-5 volts, versus the 1 volt for the 
"normal" excitation source).  NEC prints the current for all excitation 
sources, so it's easy to compare, then.

   If you're using the single precision version of NEC2 (nobody should 
be doing this, but you never know), then be careful about using this 
technique, because you're down in the precision noise floor.

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