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

Subject: Re: [TowerTalk] RF current in REF + DIR much ?
From: David Robbins <>
Date: Wed, 22 Dec 2010 09:41:24 -0600 (CST)
List-post: <">>
some googling will show some experimental and modeled results for this... but a 
bit of thought will give you some idea of what is going on.  consider for 
example a simple 2 element yagi that is tuned to give a very good f/b ratio.  
when you look at it from behind the fields you see are the sum of the fields 
from the two elements, in order for them to give a good f/b they have to almost 
cancel, and in order to do that with elements that are nearly the same length 
the current magnitudes have to be almost equal and almost out of phase.
a couple of quick references:

Dec 22, 2010 10:21:20 AM, wrote:

Jim, I would opt for the clamp on RF transformer...

Now, having said that, I recognize that anything we do to measure the RF 
current is going to upset balance, impedance, etc. to a greater or lesser 
I suspect the old RF meter ( I have a couple) is going to introduce enough 
resistance in the element to upset the coupling between the elements... These 
were originally intended to be used on open wire line at several hundred ohms 
of Z...

With the clamp on RF transformer you can calibrate your RF voltage relative 
reading by using a signal source and either RF voltmeter or oscilloscope, or 
both, to give you voltage readings that can be mapped out as amperes...

But the issue is what do you expect to learn? I don't see the ratio of 
voltage/currents without being able to measure vector/phase as being 
informative... An example...

Two loaded short elements coupled to form a Yagi/Uda beam will have higher 
currents in both elements than a full size, unloaded Y/U beam for a given 
identical drive power...

A Y/U beam made of a folded dipole (300 ohm twinlead) driven element (or quagi) 
and a shortened, loaded reflector will have significantly different circulating 
voltage/currents between the DE/REF, yet still form a beam pattern...

This is a fascinating question that I don't have many answers for... Maybe some 
of the gurus with electronic engineering background will discuss this... I 
await with great interest...

denny / k8do


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