antenna-theory.com

[timoth500@yahoo.com]

I came across the slot video by Pete and loved it. But am trying to figure out the impedance of the feed, it's where you show the feed is offset from the center of the slot. I love your explanation but when I do babinet's principle in HFSS I get the impedance duality by feeding the slot in the center of the slot, and not offset. I can actually get a wave impedance equal to that of a dipole offset from the center and in your video you show Babinet's with the dipole driving source offset from the center and not in the center. Am I missing something here? Which is correct, the center? or offset? and how exactly would one define the impedance of the slot's equivalent circuit model

[k3nnw]

My simple understanding is to consider that the driving point impedance at one of the ends of the slot is likely to be very low (lots of current driving into a low impedance "shortish circuit" which is the metal plate). The impedance in the centre of the dslot is likely to be very high, given by one of the formulae on the slot antenna page. this is the dual of a dipole, and duality is what we would expect for a slot cf. dipole of course.

If we want to match into a Zo system of e.g. 50R we know we need to offset the feedpoint away from the "open-ish circuit" centre towards one of the "short-ish circuit" slot ends.

I like to think of <<50R being an impedance which is approaching a short circuit of zero impedance (0+j0 ohms) which is "short(-ish)" and >>50R being "open-ish" ... a few hundred ohms for example. I often imagine 10x being a good rule-of-thumb.

This simple analysis prevails for other types e.g. a half-wave (full sized) patch ... the ends of the patch are high Z, the middle is low Z and somewhere a little bit away from the middle is 50R.

I have seen an interesting paper regarding asymmetric dipoles so that the centre Z can be trimmed" to look like 50+j0 (rather than 77+j something ohms, I forget!).

My final point is that the natural centre Z of a slot is like the natural end Z of a dipole. Neither is useful for driving in a 50+j0 environment and hence the offset.