Antenna Impedance

[RST Engineering]

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

 

[Dave]

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

"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.

 

[Roy Lewallen]

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

 

[Barry Lennox]

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?

"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

 

[RST Engineering]

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

 

[Terry Given]

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

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