Active antenna top-cover frequency pulling effects

S. Sancheti V.F. Fusco

Indexing t e r m : Actiue antenna, Load pulling, Radome, Superstrate

Abstract: Active antenna load pulling effects are analysed in the presence of a partially reflecting superstrate (dielectric cover). The analytical and experimental results obtained exhibit distortion in the frequency pulling pattern of the active antenna. The results obtained can be used to design optimum cover placement or to postmanu- facture tune the frequency of the active antenna module.

1 Introduction

High dielectric constant superstrate layers are often used to protect passive printed circuit antennas from physical and environmental damages as well as to improve gain, efficiency and radiation pattern [1, 2). However, for an active antenna, discontinuities presented to the antenna in the form of the cover used for packaging may cause serious degradation of system performance owing to pulling of operating frequency. Several authors have analysed the effects of load pulling on conventional oscil- lator circuit performance [3-51. These effects are of greater significance in the case of an active antenna oscil- lator owing to large reflected signal injection encountered at the device port. Hence the effects of reflection and absorption of the active antenna cover on frequency behaviour must be characterised.

The reflection of transmitted RF energy by a dielectric cover placed over an active antenna may be small enough to be unimportant as far as operating range is concerned, but may be large enough to have other conse- quences. Thus active antennas, which are designed for fixed frequency of operation, may exhibit frequency shifts due to the presence of reflections from a top cover. This phenomenon of frequency shift due to load variation is known as ‘frequency pulling’ and is dependent upon the magnitude and phase of the reflected RF signal. To use a dielectric cover in conjunction with an active antenna circuit with minimal effect on its performance, or to tune its frequency, it is necessary to study the pulling charac- teristics of the system.

Recently, York and Compton [6] have discussed an identical situation in the context of active antenna arrays. The model described in their work, when operated in the presence of a dielectric cover, takes into account the oscillator’s own reflected signal by making the self-

0 IEE, 1994 Paper 1433H (Ell), first received 25th January and in revised form 8th June 1994 The authors are with the High Frequency Electronics Laboratory, Department of Electrical and Electronic Engineering, The Queen’s Uni- versity of Belfast, Ashby Building, Stranmills Road, Belfast BT9 5AH, Northern Ireland, United Kingdom

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interaction term (kii) nonzero. However, their model does not quantify separately the effects of mutual coupling and self interaction. Further, there is no direct analytical evaluation possible for their suggested model. Here we describe the effect of the self-interaction term witH a suit- able model which can be used analytically as well as empirically, to evaluate the shift in the free running frequency of an active antenna when a dielectric cover is placed in its proximity.

2 Theory

It is expected that the pulling effect of an external load on active antenna circuit performance will be much greater when compared to a passive antenna fed from an electri- cally isolated oscillator. The reasons for this may be attributed to higher reflected signal energy at the oscil- lator port owing to practically no isolation between oscil- lator and transmitting antenna. Indeed, in many examples of an active antenna, the radiating element forms part of the oscillator circuit, and therefore cannot be isolated. In this paper, the pulling effect of a partially reflecting dielectric surface on active patch antenna per- formance is theoretically evaluated and the resulting pre- dictions experimentally verified. For the circuit under consideration (Fig. 1) the load consists of a patch antenna

bias

Fig. 1 Actiue patch antenna circuit electrical and physical [SI models

suitably coupled to partially reflecting surface. Using Richard‘s cavity model [7] the passive patch element of the active antenna [SI operated in TM,, mode, is rep- resented as a parallel resonant circuit. The dielectric cover is coupled to the patch element, initially, using a single mode lossless transmission line model having a characteristic impedance (Z,) of 377 ohm. This is termin- ated in an unmatched load to simulate a suitable reflec- tion (Fig. 2). This situation is represented by eqn. 1.

where )p1 is the magnitude of reflection coefficient, q5 is the relative phase shift of the reflected signal with respect

I E E Proc.-Microw. Antennas Propag., Vol. 141, No. 5, October 1994

iI to the transmitted signal at the antenna terminals and Yo is the free space admittance. At resonance, the patch impedance is defined as

= G, + I / jo ,L + j o , C + Y, (2)

I I I _ I I I

YC patch cover

Fig. 2 coupled dielectric cover

Parallel resonant circuit representation for patch antenna with

where Y, = Yo under zero reflection condition from the radome and oo = (LC)-’”. Eqns. 1 and 2 are combined to yield a new pulling expression for frequency change, eqn. 3.

(3)

Q, is the external Q of patch resonator and is defined for a parallel tuned circuit [9] as (w,C/Y,). Eqn. 3, when plotted in the form of frequency offset Af from the nominal frequency fo , shows significant pulling for the large reflection coefficients as compared to the low reflec- tion case, Fig. 3. This figure shows a distorted sinusoidal

0.0 0.8 1.6 2 4 3.2 6.0 L.8 5 6 6.L 7.2 8.0 8.8 distonce, cm

Fig. 3 pure sinusoid as reference I = 2.979 cm - sine fn

.....-- ,, = 0.25 = 0.10

p = 0.50

Frequency pulling under ideal load coupling conditions with

. . . . . . ~ ~~-

pulling pattern with relatively large frequency change as a function of distance d between active antenna and cover. This is unlike the ideally defined sinusoidal pulling pattern suggested in [lo]. The experimental result pre- sented [6] showed some distortion in the frequency variation as a function of antenna spacing. Stephan and Young [ l l ] showed more marked distortion in their report of a mutually injection locked coupled array of two Gunn driven slotline antennas. The distortion remained uninvestigated [6, 111.

IEE Proc.-Microw. Antennas Propag., Vol. 141, No. 5 , October I994

In this paper, distortion is theoretically accounted for by eqn. 3. At distances of greater than five wavelengths eqn. 3 can be approximated to a sinusoidal function. However, this is not of much practical utility for active antenna packaging, as covers are likely to be placed close to antenna due to considerations of size, weight etc. On the other hand, if the cover is placed too close to the antenna, gross load pulling induced frequency changes will occur. This effect can be used to advantage to post- manufacture tune the active antenna module if so required.

Leaderman [12] has shown that reflection coefficient is a function of dielectric constant of the cover sheet, loss tangent and ratio of cover thickness to wavelength. In addition to these classical radome considerations, it is obvious from eqn. 3 that, for the active antenna situation, the frequency pulling of the active antenna Afdue to the proximity of dielectric top cover, is also a function of the relative phase of reflection which in turn is dependent upon the distance between antenna and cover. To account for the reduced reflected signal available at increase distance, I p I has been assumed to be an inverse function of the phase factor k,d. A modified version of eqn. 3 for frequency pulling is given by eqn. 4. Fig. 5 shows the diminishing effect of pulling with increasing distance as predicted by eqn. 4.

where k, the wave number is 2 n / l , and I , is free space wavelength. K is a constant introduced to account for impedance mismatch/coupling loss between the transmis- sion line and the active antenna, which can be evaluated both theoretically and empirically. The theoretical evalu- ation of K would require a correct estimation or mea- surement of radiation resistance of the patch antenna. But this evaluation, especially in our case, is not easy since the patch is also used as a circuit element and is operated slightly away from resonance owing to design requirement [SI associated with impedance matching. It is because of this that an empirical evaluation of K would be preferred. However, to verify the results, an approx- imate theoretical value of radiation resistance, 593.5 ohms, calculated from the equations given [13] can be used. K given by (Rrad - Zo)/(Rrad + Z,) is calculated to be 0.224.

It is known that, for an electrically transparent cover, the distance between the antenna and cover should be an integer multiple n of one half wavelength (10/2) where n = 0, 1, 2, . .., N . Eqn. 4 can easily be evaluated to show that the frequency pulling is at its minimum at intervals of n1,/2. However, it is evident from Fig. 5 that, for an active antenna, the pulled frequency sensitivity, defined here as the change in free running frequency (f,) for a given small perturbation in its cover position from nI,/2, is greater at odd multiples of one half wavelength when compared to even multiples. It is therefore proposed that, to obtain best frequency stabilisation, cover placements in the regions where the pulled frequency sensitivity is high should be avoided.

3 Experimental results

An experimental investigation using the set-up shown in Fig. 4 was carried out. In actual measurement set-up, an expanded thermocoal sheet (E, U 1.05 and thickness = 2.2 cm) was used to hold the top cover in

315

place. The remainder of the instrumentation and cables were placed at the back of the active patch antenna to avoid direct reflections. Scattering from cable and bias

-LO

~ e c x ; ; i c octive ontenno

i

\

d - ’I

positioning arrangement Fig. 4 Experimental set-up for pulling measurement

networks were minimised by use of radar absorbing material. To demonstrate the asymmetrical effect of pulling a rectangular cover sheet (5.0 cm x 6.0 cm) using RTDuroid 6010 material of thickness 0.025 in., relative dielectric contant (E,) of 10.8 and tan 6 of 0.0028 was selected to give a voltage reflection coefticient of 0.5. The resulting measured pulling pattern for normal incidence is plotted in Fig. 5, the free running frequency of the

4 m C

5 0 r ”

active antenna was 10.07 GHz. The value of dielectric cover reflection coefficient was obtained from the equa- tions reported [12] and reproduced here as eqns. 5 and 6.

where rab is the amplitude reflection coefficient at the air dielectric boundary and dd is phase difference between reflection from front and back surfaces of dielectric cover of finite thickness t. Here the external Q used in eqn. 4 was measured to be 32.7 using a conventional injection locking technique [14]. For the active patch antenna under consideration, a value of K = 0.25 was found to be best to describe. the experimental results, which is in very close approximation to the theoretically calculated value of 0.224. The results plotted in Fig. 5 are up to a distance

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of three wavelengths, and clearly exhibit the asymmetry with good overall agreement with theory. Similar results for a second active antenna operating at 10.7 GHz were obtained using an identical design and patch structure, but with a different value of series feedback. This yielded an empirical value for K of 0.26 and theoretical value of 0.223 and almost identical results to those presented in Fig. 5. At distances less than 0.6 cm (d < 0.22,) per- formance is not evaluated, electrically this distance is within the near field region of the active antenna and, in practice, it greatly alters the operating conditions and thereby would invalidate the analysis at small electrical distances which depends on a single mode transmission line model for load coupling.

The case considered in this paper is for a plane dielec- tric cover with normal incidence illumination. When oblique incidence or when curved surfaces are encoun- tered, the pulling effect will in general be less. Hence the results described are for the worst case and, in practice, useful improvements may be obtained by proper material, physical dimension and placement selections.

4 Conclusions

The paper has presented a simple method for the evalu- ation of the load pulling effect induced by the placement of a dielectric cover in close proximity to an active antenna oscillating element. The agreement between theoretical and experimental results is excellent. The results can be used effectively to design either optimum cover placement position or to postmanufacture fre- quency tune the active antenna transmitter.

5 References

1 ALEXOPOULOS, N.G., and JACKSON, D.R.: ‘Fundamental superstrate(cover) effects on printed circuit antennas’, IEEE Trans., 1984, AP-32, (S), pp. 807-815

2 BAHL, IJ., BHARTIA, P., and STUCHLY, S.S.: ‘Design of micro- strip antennas covered with a dielectric layer’, IEEE Trans., 1982,

3 OBREGON. J.. and KHANNA. A.P.S.: ‘Exact derivation of the AP-30, (2). pp. 314-318

nonlinear negative resistance oscillator pulling figure’, IEEE Trans., 1982, M’IT-30, (7), pp. 1109-1111

4 HOBSON. G.S.: ‘Measurement of external 0-factor of microwave oscillator using frequency Dulling or freauencv locking’, Electr. Lett., .. - . . 1973, E, pp. i9i -19i

5 BEHAGI, A.: ‘Derive and measure source pulling figure’, Micro- wauesRF. 1 9 9 2 . ~ ~ . 111-114

6 YORK, R.A., andCOMPTON, R.C.: ‘Measurement and modelling of radiative coupling in oscillator arrays’, IEEE Trans., 1993, MTT-41, (3), pp. 438-444

7 RICHARDS, W.F., LO, Y.T., and HARRISON, D.D.: ‘An improved theory of microstrip antennas and its applications’, IEEE Trans., 1981, AP-29, ( l ) , pp. 3&46

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9 COLLIN, R.E. (Ed.): ‘Foundatons for microwave engineering’ (McGraw-Hill, New York, 1992; 2nd edn.), Chap. 7, pp. 481-487

IO NAGANO, S., and AKAIWA, Y.: ‘Behaviour of Gunn diode oscil- lator with a moving reflector as a self excited mixer and load varia- tion detector’, IEEE Tram., 1971, MTF-19, (IZ), pp. 906-910

11 STEPHAN, K.D., and YOUNG, S.L.: ‘Mode stability of radiation- coupled interinjection-locked oscillators for integrated phased arrays’, IEEE Trans., 1988, MTF-36, ( 9 , pp. 921-924

12 CADY, W.M., KARELITZ, M.B., and TURNER, L.A. (Eds.): ‘Radar scanners and radomes’ (McGraw-Hill, New York, 1948). Chap. 10-12, pp. 259-368

13 BAHL, I.J., and BHARTIA, P. (Eds.): ‘Microstrip antennas’ (Artech House Inc., 1980), Chap. 2, pp. 31-80

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IEE Proc.-Microw. Antennas Propag., Val. 141, No. 5 , October 1994