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Antenna Impedance

We think we know that the impedance at the center of a thin wire antenna in free space is something on the order of 70 ohms at resonance (quarter-wave ears with the end fringing factor thrown in).

We think we know that the VSWR bandwidth increases as that thick wire becomes fat. We think we know that the end fringing factor goes up as the antenna gets fatter.

What I'm suspecting the further I look into it is that the NEGATIVE image of that dipole antenna (a slot cut into an infinite sheet of conductor fed at the center across the slot) is also resonant, but the impedance is going to be something on the order of 350 ohms. Jasik shows a method of off-center feeding that reduces the impedance to something on the order of 100 ohms.

What is NOT mentioned is the aforementioned VSWR "goodness factor" going up with a fatter (wider?) slot.

Is there a better reference to slot antennas than Jasik?

Jim

Reply to
RST Engineering
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Lo and Lee give the simple relationship between an antenna and its "negative" (e.g., slot vs dipole made from flat material the width of the slot) on p. 2-15 of their _Antenna Handbook_: Zslot * Zstrip = 1/4 * Z^2 where Z = the intrinsic wave impedance of the unbounded medium where both antennas are situated (e.g., free space).

Roy Lewallen, W7EL

RST Eng> We think we know that the impedance at the center of a thin wire antenna in

Reply to
Roy Lewallen

Which I calculate to be 266 ohms. However, a couple of other references to Kraus give the answer as more like 500 ohms, so now I'm the guy with two watches who is never sure of what the time is.

Jim

Reply to
RST Engineering

"Antennas" by John Kraus, second edition, McGraw Hill, 1988, (ISBN

0-07-100482-03) has a chapter on slot and similar antennas. It says the impedance is about 500 Ohms, and so to feed with 50 Ohm coax you need to feed well off centre (it suggests about lambda/20). If the slot is vertical, the polarisation is horizontal. There are some journel references.
Reply to
Dave

"Antennas" by Krause and Marhefka (ISBN 0-07-232103-2) has a useful section on slot antennas, They suggest an off-center feedpoint of lamda/20 will provide 50 Ohms. They don't specifically cover your VSWR question, but a couple of trials will provide an easy answer.

Barry Lennox

Reply to
Barry Lennox

same formula, handbook of microwave and optical components vol 1 p.672, eq.11.117

ditto Krause 2nd ed. eq. 13-5-11.

Cheers Terry

Reply to
Terry Given

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