1. ECE 6341
Spring 2016
Prof. David R. Jackson
ECE Dept.
Notes 12

2. Tube of Surface Currenty x z a 1 2
An infinite hollow tube of surface current, flowing in the z direction: 1 2 x y a
Top view:
From superposition, we know we can solve the problem using
(TMz)

3. The coefficients cn are assumed to be known.
Tube of Current (cont.)
Represent Jsz(f) as a (complex exponential) Fourier series:
The coefficients cn are assumed to be known.

4. Tube of Current (cont.)
r < a
r > a
Apply B.C.’s (at r = a) :

5. Tube of Current (cont.) The first B.C. gives The second B.C. gives
Hence we have, from the first BC,

6. Tube of Current (cont.) From the second BC we have
Substituting for an from the first equation, we have
Next, look at the term

7. Tube of Current (cont.) This may be written as (using x = ka) Hence,
(This follows from the Wronskian identity below.)
Hence,

8. Tube of Current (cont.)
The coefficient is then
Using
we have

9. Tube of Current (cont.) r < a r > a
Generalization: a phased tube of current
r < a
r > a

10. Inductive Post 2a I0 h 2b Coax feed
Parallel-plate waveguide 2a I0 h 2b
The probe current is assumed to be uniform (no z or  variation).
Coax feed
Equivalence principle: remove coax aperture and probe metal I0  
The magnetic-current frill is ignored in calculating the fields.

11. Inductive Post (cont.) Tube problem: I0 For r  a: where
Tangential electric field is zero
(There is no z or  variation of the fields.)
The probe problem is thus equivalent to an infinite tube of current in free space.
For r  a:
where

12. Inductive Post (cont.)
and
Hence

13. Inductive Post (cont.)
We then have so

14. Inductive Post (cont.)
Complex source power radiated by tube of current (height h): or

15. Inductive Post (cont.) or or
where

16. Inductive Post (cont.) Zin Equivalent circuit seen by coax: I0 + V0 -so

17. Inductive Post (cont.) Therefore For ka << 1
We thus have the “probe reactance”:

18. Inductive Post (cont.) or
Xin
Probe inductance:

19. Inductive Post (cont.)
Summary
Probe reactance:
Probe inductance:

20. Inductive Post (cont.) Comments:
The probe inductance increases as the length of the probe increases.
The probe inductance increases as the radius of the probe decreases.
The probe inductance is usually positive since there is a net stored magnetic energy in the vicinity of the probe.
This problem provides a good approximation for the probe reactance of a microstrip antenna (as long as kh << 1).

21. Example
Practical patch antenna case:

22. Microstrip Antenna ModelZ0 Lp
f0 = resonance frequency of RLC circuit
The frequency where R is maximum is different from the frequency where the reactance is zero, due to the probe reactance.

23. Lossy Post
If the post has loss (finite conductivity), then we need to account for the skin effect, which accounts for the magnetic energy stored inside the post. This gives us an internal reactance.

24. Example (Revisited)
Practical patch antenna case:
Assume:

25. Accuracy of Probe Formula
HFSS
H. Xu, D. R. Jackson, and J. T. Williams, “Comparison of Models for the Probe Inductance for a Parallel Plate Waveguide and a Microstrip Patch,” IEEE Trans. Antennas and Propagation, vol. 53, pp , Oct