Dennis OConnor 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
> 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...
COnsider each of the elements as a separate radiator. The pattern in
the far field is just the sum of the contributions from each of the
elements, adjusted by the phase and magnitude of that element, and the
relative distance to the element.
By adjusting the relative currents, you can create a variety of
patterns. You can control the currents either by driving them
separately (an active phased array) or by using coupling between the
elements (a passive phased array, which a Yagi-Uda is an example of)
The simplest designs set the currents up so that the phase advance
matches the distance between the elements. (imagine a 20m 3 element yagi
with elements spaced 5 m apart, the reflector would be 90 degrees
advanced, the driven in phase, and the director 90 degrees retarded)
But, if you set the phase so it's "faster" than the distance, you get
what's called superdirectivity.. Another way to think about it is that
rather than forming a main lobe, you are arranging the current to
suppress the side and back lobes.
This comes at a cost. In such an antenna, there is more stored energy
in the antenna (aka circulating currents) and so, IR losses are greater.
The design also becomes more picky. With large currents, a small
error in phase or amplitude can have a big effect on the pattern, so the
antennas tend to be narrow band (that is, their gain/pattern performance
degrades as you move away from the design frequency). A side effect is
also that the feed point impedance tends to become lower in general (how
else will you get those high currents) and because of the tight coupling
of many tuned circuits (the elements), the SWR bandwidth will be smaller.
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