[TowerTalk] RF current in REF + DIR ..how much ?

[David Robbins]

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:
http://www.jpier.org/PIER/pier100/12.09111501.pdf
http://users.ices.utexas.edu/~cynthia/paper/yagi.pdf


Dec 22, 2010 10:21:20 AM, ad4hk2004 at yahoo.com 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 degree..
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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