HK1262809A1 - Antenna structure - Google Patents

Description

Antenna structure

Technical Field

The present invention relates to an antenna arrangement for use in a surgical scope.

Background

Microwave energy and Radio Frequency (RF) energy are known for performing coagulation of deep tissue by contacting the bleeding site with a surgical probe. It is also known that surface bleeding can be controlled in a non-contact manner using Argon Plasma Coagulation (APC), whereby a high energy electric field is applied on the argon jet in order to ionize the gas and strike the plasma. The plasma can then cause coagulation. Vasoconstrictive fluids (vasoconstrictive fluids) used to close open bleeding blood vessels are often used as emergency intervention to control blood flow or stop bleeding prior to administration of coagulants or alternative means for permanently occluding or sealing the bleeding blood vessels.

Disclosure of Invention

At its most general, the present invention provides a helical antenna structure that is connectable to the inner and outer conductors of a coaxial transmission line and that functions both as a radiating antenna or applicator structure and in a mode in which an electric field is generated between its electrodes. In this way, the helical antenna structure can be used both for APC and deep tissue coagulation, and for providing a means for delivering fluids, e.g. therapeutic fluids, such as epinephrine. This can be achieved by using helically arranged electrodes and channels for gas flow. Such devices are also used to deliver epinephrine and RF/microwave energy.

More specifically, the present invention provides a helical antenna structure connectable to a coaxial transmission line having an inner conductor and an outer conductor, the helical antenna structure having: a dielectric support, a first spiral electrode and a second spiral electrode, both located on the dielectric support and electrically isolated from each other, first connection means for connecting the first spiral electrode to the inner conductor of the coaxial transmission line; a second connection means for connecting the second helical electrode to the outer conductor of the coaxial transmission line; wherein at least one of the first helical electrode and the second helical electrode is capable of functioning as a radiating antenna structure for emitting a microwave/RF field outwards; and the first and second helical electrodes are configured to maintain an electric field in a helical region therebetween to generate a displacement current.

In the present specification, "microwave" may be widely used to indicate a frequency range of 400MHz to 100GHz, but preferably indicates a range of 1GHz to 60 GHz. The specific frequencies that have been considered are: 915MHz, 2.45GHz, 3.3GHz, 5.8GHz, 10GHz, 14.5GHz and 24 GHz. In contrast, the present specification uses "radio frequency" or "RF" to indicate a frequency range that is at least three orders of magnitude lower, for example up to 300MHz, preferably 10kHz to 1 MHz.

The helical configuration of the present invention can be used as an effective radiating antenna structure, evidence of which is presented later in this application. This field can then be used for coagulation. The use of the helical electrode ensures that the central region of the antenna structure is not occupied. This means that other structures may pass through the center of the antenna structure to deliver a fluid or gas (e.g., epinephrine or saline). The helical antenna structure is preferably configured for use with endoscopes, laparoscopes and the like and therefore preferably has a maximum outer diameter of no more than 8mm, preferably equal to or less than 5mm, and more preferably equal to or less than 3.5mm, and most preferably no more than 2.5 mm. The dielectric support is preferably substantially cylindrical and may have a rounded distal end. Having a rounded distal end rather than a sharp rounded apex results in a smoother distribution of the emitted microwave/RF energy, giving a more uniform coagulation. In a preferred embodiment, there are only two spiral electrodes on the outer surface of the dielectric support, but may also be three or four spiral electrodes, for example.

The dielectric support may comprise one or more of PEEK, PTFE, ceramic, or other suitable rigid, low loss material.

The first helical electrode and the second helical electrode preferably have the same pitch and may be positioned diametrically opposite each other. In other words: in appearance, the second helical electrode extends parallel to the first helical electrode but with a fixed axial offset such that the coils of the first and second helical electrodes alternate with each other. Most preferably, the first and second spiral electrodes are identical or substantially identical to each other. The first and second spiral electrodes are preferably located on the surface of the dielectric support, or partially embedded therein.

In use, the antenna is inserted first into the distal end with the distal end surface facing the hemorrhage site. Therefore, it is preferred that the maximum heating (as a result of microwave/RF energy delivery) should occur at the distal end and around the outer curved surface of the helical antenna. In this way, effective energy delivery may be achieved by placing the helical antenna first at the distal end or on the side thereof towards the target area. Thus, it is preferred that the microwave/RF energy can be delivered to the distal end of the helical antenna structure by a waveguide structure or transmission line structure. The transmission line structure may be part of the helical antenna structure itself, or alternatively, the helical antenna structure may have a channel or cavity configured to receive a coaxial transmission line structure or other structure capable of conveying microwave/RF energy to the distal end of the helical antenna structure without a significant degree of attenuation. If the microwave/RF energy is delivered only to the proximal end of the helical antenna structure rather than passing the microwave/RF energy to the distal end with any transmission line structure, it is possible that attenuation will occur between the proximal and distal ends as a result of undesirable absorption by tissue contacting the structure. The use of a helical antenna structure as in the present invention means that for example a coaxial transmission line from which the first and second helical electrodes are configured to receive microwave/RF energy can pass through the structure all the way to the distal end of the helical antenna structure.

Alternatively, in a preferred embodiment, the first and second helical electrodes are configured to be connected to the inner and outer conductors of a coaxial transmission line having a hollow inner conductor. Thus, the dielectric support may have a central passage extending through the dielectric support, terminating in an aperture. In this way, structures such as liquid delivery tubes may pass all the way through the helical antenna structure without adversely affecting the radiation properties of the antenna structure. Therefore, it is preferable to expose the center or near-center region of the distal end of the helical antenna structure in the case where it is necessary to deliver a liquid drug (such as epinephrine) to the target region, and thus a liquid delivery tube, needle, or the like can be inserted through the end of the helical antenna. The tube may also be a sealed area (i.e., a space inside a catheter that can contain a microwave cable, a needle actuation wire, and a short length of needle) for fluid flow. Alternatively, the hollow needle may extend from the proximal handle end to the distal end of the apparatus. The hole size of the needle may be 0.4mm or 0.5mm, but the present invention is not limited to the case where the hole size of the needle may be 0.8mm for a laparoscopic device. The needle may be made of stainless steel or the like. A hollow channel or needle channel may also be used to deliver a gas (e.g., argon), and the RF field available at the helical antenna may be used to strike the plasma, while the microwave field may be used to sustain the plasma. In such a configuration, gas would need to be present between the radiators that establish and deliver RF and microwave energy. This may be achieved by providing holes in the dielectric cylinder which allow gas to escape into the region between the electrodes where the electric field is present.

In a preferred embodiment, the helical antenna structure further comprises a third helical electrode located beneath the surface of the dielectric support and preferably embedded within the dielectric support, and preferably located beneath the first helical electrode, and more preferably extending along the same helical path as the first helical electrode but radially inwardly of said first helical electrode. Thus, the first and third helical electrodes also share a longitudinal axis. The first helical electrode may be connected to the inner conductor of the coaxial transmission line at a feed point, and the third helical electrode may be connected to the outer conductor of the coaxial transmission line through the feed point. Then, as the first and third helical electrodes follow the same path, they may act as a continuation of the waveguide structure of the coaxial transmission line and further transmit the signal from the proximal end to the distal end of the helical antenna structure.

The first and third helical electrodes and preferably also the second helical electrode may be in the form of helical strips of conductive material and the transmission line formed by the first and third helical electrodes may therefore be a microstrip line. Preferably, the width of the strip of conductive material forming the first spiral electrode is wider than the width of the strip of conductive material forming the third spiral electrode, and preferably at least twice, and more preferably at least three times, the width of the strip of conductive material forming the third spiral electrode. In this way it can be ensured that there is a significant sufficient overlap between the two spiral electrodes forming the effective microstrip line structure. This is because the current at the edge of the first helical electrode (due to the feed signal) will be low and will not cause significant interaction with any tissue contacting the outer surface of the first helical electrode. The microstrip line structure formed by the first and third spiral electrodes is preferably arranged to have approximately 50_ΩSo as to match the coaxial transmission line at which the feed point is arranged to receive the microwave/RF signal.

At the distal end of the helical antenna structure, the distal ends of the second and third helical electrodes are electrically connected to each other. In this way, microwave/RF energy transmitted by the microstrip line along the length of the antenna structure can excite a corresponding signal that travels back along the helical gap between the first and second helical electrodes towards the proximal end of the helical antenna structure. Preferably, the conductive member connecting the second spiral electrode and the third spiral electrode does not cover the aperture of the central channel.

Rather than having a third helical electrode, in an alternative embodiment, to take advantage of the helical structure, the dielectric support may have a channel extending all or part way through it in a longitudinal or substantially longitudinal direction for receiving a coaxial transmission line supplying microwave/RF energy to the antenna structure. The connection means for connecting the first helical electrode and the inner conductor and/or the second helical electrode and the outer conductor are preferably located towards the distal end of the channel in order to ensure that maximum heating occurs at the distal end of the helical antenna structure, as discussed above.

The inner and outer conductors of the coaxial transmission line may be connected to the first and second helical electrodes, respectively, through holes in the dielectric support. Preferably there are two holes, one arranged to connect the first helical electrode and the inner conductor and the other arranged to connect the second helical electrode and the outer conductor. In use, a coaxial transmission line may be inserted into the channel in the dielectric support and the hole may be filled with solder in order to provide the necessary electrical connection. In this case, the channel may not extend all the way to the end of the helical antenna structure.

Alternatively, in another embodiment, the channel for receiving the coaxial transmission line may extend all the way to the distal end of the dielectric support. The inner and outer conductors of the coaxial transmission line may be connected to the first and second helical electrodes through apertures at the ends of the channel. The shape of the aperture is preferably determined to take advantage of the insulating properties of the dielectric layer separating the inner and outer conductors of the coaxial transmission line. For example, the aperture may be substantially circular and have a radius greater than a radius of the inner conductor and less than a radius of the dielectric layer, and the radius of the radially extending tab is greater than a radius of the dielectric layer. In this way, the outer conductor is exposed only in the area of the tab and remains covered around the remainder of the aperture circumference. The second spiral electrode may then be electrically connected by solder or otherwise to the outer conductor only in the tab section, without any desirable electrical connection to the inner conductor.

In another alternative embodiment, the dielectric support may be two-part. At the distal end, one portion may have a protrusion and the other portion may have a corresponding recess. Portions of the projections may then be plated with a conductive material arranged to provide an electrical connection between the inner and outer conductors of the coaxial transmission line and the first and second helical electrodes.

In another embodiment, the dielectric may preferably include holes or slots between the conductors in the helix to allow gas to be present in the region between the conductors to allow the plasma to be struck using the RF field and sustained using the microwave field.

Drawings

The invention will now be described with reference to the accompanying drawings, in which:

fig. 1A shows an arrangement of an inner helical electrode and a first outer helical electrode according to an embodiment of the invention.

Fig. 1B illustrates an example of a helical antenna including a dielectric support according to an embodiment of the present invention.

Figure 2 shows a helical antenna and liver load arrangement used to run simulations of an embodiment of the present invention.

Fig. 3A to 3D show various results of the simulation shown in fig. 2.

Figure 4 shows another arrangement of a liver load and a helical antenna for running an alternative simulation of an embodiment of the present invention.

Fig. 5A to 5D show various results of the simulation shown in fig. 4.

Fig. 6A and 6B show a perspective view and an end portion, respectively, of another embodiment of the present invention.

Figure 7 shows the arrangement of the helical antenna and blood load as shown in figures 6A and 6B, which was used to run simulations of this embodiment.

Fig. 8 to 9B show various results of the simulation shown in fig. 10.

Fig. 10-12 illustrate alternative configurations of how a coaxial transmission line may be connected to a first helical electrode and a second helical electrode.

Detailed Description

Fig. 1A is a view showing the proximal end of a helical antenna 100, which can form a first electrode and a second electrode, and the conductive structure of the present invention. In the figure, the direction from the proximal end 100a to the distal end 100b of the helical antenna is parallel to the z-axis, as shown in the lower right hand corner of the figure.

A first outer helical electrode 102 and an inner helical electrode 104 are shown in fig. 6. The inner helical electrode 104 has the same pitch as the first outer helical electrode 102 and has a smaller diameter such that it extends directly below the first outer helical electrode 102 and parallel to said first outer helical electrode 102. The proximal ends of the two helical electrodes 102, 104 are fed with microwave/RF energy from a coaxial transmission line at a feed point 108 (as shown by the line and cone). The first outer helical electrode 102 and the inner helical electrode 104 together form a first electrode having a diameter of 50_ΩAn impedance spiral microstrip transmission line (in the presence of an alumina dielectric, see description of the figures below).

Fig. 1B shows a view of a probe tip 111 having a helical antenna 100 supported thereon. Probe tip 111 is composed of a cylindrical dielectric material 112, which in this case is alumina, having a cylindrical hole therethrough, forming a central channel 115 extending in the z-direction as shown from the proximal end to the distal end. The central channel terminates at its distal end 115b in an orifice 116. The orifice is unobstructed so that a liquid channel (not shown) or other tool can pass through the probe tip 111 for use on a target area (also not shown).

In addition to the first outer helical electrode 102 and the inner helical electrode 104, a second outer helical electrode 106 is also supported on the dielectric material 112. The second outer helical electrode 106 is diametrically opposed to the first outer helical electrode 102, but has exactly the same pitch. In FIG. 1B, the first outer helical electrode 102 and the second outer helical electrode 106 and the inner helical electrode 104 have a pitch of 3.3 mm. Since the inner helical electrode 104 is embedded within the dielectric material 112, extending directly below the first outer helical electrode 102, only the distal end surface of the inner helical electrode 104B is visible in fig. 1B. At the distal end of the dielectric material 112, the distal end of the second outer helical electrode 106 and the distal end of the inner helical electrode 104 are connected by a connecting member 117. Connecting member 117 is a disc-shaped piece of conductive material (e.g., copper) having a hole 119 in the center to coincide with aperture 116 so that aperture 116 remains clear.

In operation, microwave/RF energy is fed into the proximal end of the helical microstrip transmission line formed by the first outer helical electrode 102 and the inner helical electrode 104. When the microwave/RF energy reaches the distal end, the microwave/RF signal is excited between the first outer helical electrode and the second outer helical electrode and propagates back toward the distal end of the probe tip 111 along a helical path through the gap 110 between the first outer helical electrode 102 and the second outer helical electrode 106. When the probe tip 111 is connected to a coaxial transmission line having a gas channel (not shown) positioned therearound (e.g., positioned in a jacket spaced apart from the coaxial transmission line), the first and second outer helical electrodes 102, 106 and the gap 110 therebetween are located in the flow path of the gas exiting the gas channel. When an electric field exists between the first outer helical electrode 102 and the second outer helical electrode 106 as a result of the microwave/RF signal propagating along them, the electric field acts to ionize the gas and generate a plasma.

Fig. 2 shows a model for simulating the effect of the helical antenna 100 as shown in fig. 1A and 1B when placed end-on against the liver load 120. The dielectric material 112 in the model is an alumina ceramic, i.e., a strong non-porous dielectric with good dielectric breakdown properties. The dielectric constant is 9.4 and its loss tangent is 0.0004 at 5.8GHz, which represents a very low loss material at the microwave frequencies employed. The copper spiral wire (i.e., spiral antenna 100) was modeled on the outside of a 3.3mm diameter alumina cylinder, which was 7.5mm in length. The pitch of the helix is 3.3mm and the width of the copper measured in a direction parallel to the axis of the cylinder is 0.9 mm. The copper strip in the model shown is 0.1mm thick, but in practice may be as thin as 0.003 mm. The second copper spiral is modeled diametrically opposite (i.e., rotated 180) from the first copper spiral. This resulted in two inner wound copper spiral wires with a 0.75mm gap between them (in a direction parallel to the axis of the cylinder).

The internal diameter of the alumina cylinder (i.e., the diameter of the probe tip channel) was 2.5 mm. A 2.3mm diameter internal alumina cylinder was modeled with a 0.6mm diameter hole in the center on its inside and a 0.5mm diameter steel needle on its inside. An inner copper spiral wire, which is 0.35mm wide in the axial direction and also has a pitch of 3.3mm, was modeled on the inner alumina cylinder. The inner copper spiral is positioned directly below the center of the width of one of the outer copper spirals.

The distal end of the inner copper spiral wire is connected to the distal end of the copper spiral wire which is not directly below the inner copper spiral wire.

The helical antenna consisting of three copper spirals is fed at its proximal end with 50 between the inner spiral and the first copper spiral_ΩA feed and terminates between the proximal ends of the two outer spirals. A liver load is created and used to determine the power absorption around the tool, which gives an indication of the desired coagulation pattern that can be achieved by using the tool in this way. In the simulation shown, the distal end of the probe tip was inserted 2mm into the liver load.

Fig. 3A-3C show plan views of the power absorption of the liver load member around the distal end of the probe tip in three different orientations (two in longitudinal cross-section with the probe tip and one in axial cross-section), as shown in fig. 8. Taken together, these plan views show that between 60% and 70% of the microwave power is absorbed into the liver load. Figure 9D shows the results of a return loss simulation at different depths of penetration of the probe tip into the liver load. At 5.8GHz, it can be seen that the return loss increases from 4dB to 5dB as the insertion portion increases from 0 (line A) to 2.5mm (line F).

Fig. 4 shows an alternative simulated setup where the probe tip is inserted 1mm sideways into the very same liver load as fig. 2. Fig. 5A-5C show plan views of the power absorption of the liver load around the probe tip when placed side-on. These plan views show that the helical antenna is capable of generating a substantially uniform microwave field around the probe tip. Figure 5D shows the results of a return loss simulation at different depths of penetration of the probe tip into the liver load. At 5.8GHz, it can be seen that the return loss increases from 4dB to 7dB as the (lateral) insertion portion increases from 0 (line G) to 1.5mm (line K).

The results of placing the helical antenna 100 from the side and from the end show that the helical antenna 100 can operate effectively as a microwave transmitting antenna structure, in addition to being able to strike and sustain plasma in the helical gap between the first outer helical electrode and the second outer helical electrode.

Fig. 6A and 6B illustrate an alternative embodiment of a helical antenna 200 according to the present invention. There are several similarities between the helical antenna 200 of fig. 6A and the helical antenna 100 of fig. 1B, for example. In the case where the features are identical, they will not be described in detail.

The helical antenna 200 includes a dielectric material 212 (which is PEEK in this case), and may be divided into a cylindrical portion 212a and a hemispherical portion 212b that are integrally formed with each other. The outer diameter of the helical antenna structure 200 is 3.3mm in this embodiment. A channel 215 extends through the center of the two portions 212a, 212b of the dielectric material 212 to receive a coaxial transmission line 220. The first spiral electrode 202 and the second spiral electrode 206 are connected to the inner conductor and the outer conductor of the coaxial transmission line 220 by a metallization extending into an aperture (not shown). For protection purposes, an insulating plug 213 is placed over the connection. This arrangement is shown in more detail in fig. 11A and 11B and discussed below. The dielectric material 212 also has off-axis needle channels 221 extending therethrough for situations where it is also necessary to dispense liquid to the target area. Two outer spiral electrodes 202, 206 are located on the surface of the dielectric material 212. In use, the coaxial transmission line 220 is inserted through the channel of the helical antenna structure 200. Fig. 10-12 show different examples of the geometry of the dielectric material 212, each showing a different arrangement by which a coaxial transmission line can be connected to each of the helical electrodes 202, 206.

In fig. 10 to 12, the electrodes 202, 206 are not shown. To connect coaxial transmission lines using the dielectric body 300 of fig. 10A and 10B, a coaxial transmission line 320 is embedded along the central channel. The coaxial transmission line 320 must be stripped to expose the outer conductor 320a, the dielectric layer 320b, and the inner conductor 320c in sequence as shown in the drawing. The dielectric body 300 shown in fig. 10A and 10B has two holes 322a, 322B drilled therethrough. When the coaxial transmission line 320 is inserted, one of the holes 322a intersects the exposed inner conductor 320c and the other hole 322b intersects the exposed outer conductor 320 a. The holes may then be filled with solder to establish an electrical connection and secure the coaxial transmission line 320 in place.

In fig. 11A and 11B, the coaxial transmission line 420 extends all the way to the distal end of the dielectric body 400. In this embodiment, the outer conductor 420a of the coaxial transmission line is stripped back to expose the dielectric layer 420 b. The dielectric layer 420b and inner conductor 420c then continue to the end of the dielectric body 400 and are exposed at the holes 424 as best shown in fig. 11A. Tabs 426 are located at the edges of holes 424. When the coaxial transmission line is in place, the end surface of the outer conductor 420a is exposed by the tab 426. Importantly, the outer conductor 420a is electrically isolated from the inner conductor 420c by a barrier layer formed by the intervening dielectric layer 420 b. As shown in fig. 11B, the inner conductor 420c of the coaxial transmission line 420 may be recessed. The tab 426 may be filled with solder and solder connected to one of the spiral electrodes 202, and the recess may be filled with solder (which does not contact the solder in the tab 426) and solder connected to the other of the spiral electrodes 206. Although not shown, as described above, a metal plating may be used to connect the conductor of the coaxial cable to the helical electrode, and the recess defined by the inner surface of the hole and the end surface of the coaxial transmission line 420 may be filled with an insulating plug.

Another alternative is shown in fig. 12A and 12B. In this case, the formed dielectric material 500 is formed in two pieces 500a, 500b, which pieces 500a, 500b are joined together to form the complete helical antenna structure. The first workpiece 500a has a protrusion 528, which protrusion 528 corresponds to a depression 530 on the second workpiece 500 b. The second workpiece 500b also has a central channel 515 for receiving a coaxial cable 520. When held in place, the base of the recess 530 covers around and exposes only half of the upper surface of the coaxial transmission line 520. The base of the recess 530 has a notch 532 to receive the inner conductor 520c of the coaxial transmission line 520. The surfaces indicated by the arrows in fig. 12B may then be plated with a conductive material that extends to the hemispherical surface 512B of the dielectric material 512 so as to connect the inner conductor 520c and the outer conductor 520a to their respective spiral electrodes 202, 206.

Referring back to fig. 6A and 6B, the operation of the apparatus will be described. The operation is similar to that of the embodiment of the invention shown in fig. 1A and 1B. The main difference between the two embodiments is that in the present embodiment, the coaxial transmission line (e.g., 320) is directly connected to the first helical electrode 202 and the second helical electrode 206, whereas in the previous embodiment, the microwave/RF energy is delivered to the distal end of the helical antenna structure 200 through the microstrip transmission line formed by the helical electrodes 102, 104.

In the helical antenna structure 200 shown in fig. 6A and 6B, the coaxial transmission line 220 is connected to the helical electrodes 202, 206 as described above and delivers microwave/RF energy to the helical electrodes 202, 206. Because of the potential difference between first spiral electrode 202 and second spiral electrode 206, an electric field exists in spiral gap 210 between first spiral electrode 202 and second spiral electrode 206. If the field is high enough and the gap is placed in the gas flow path, this can cause the plasma to strike in the spiral gap 210. This means that the helical antenna structure can be employed in the APC mode. Furthermore, due to its geometry, the helical antenna structure can also be used as a radiating antenna for radiating microwave/RF energy outwards for deep tissue coagulation. A needle may also be inserted through the off-axis needle channel 221.

In a similar embodiment shown in fig. 6C, the outer diameter of the dielectric material 212 ' is only 2.4mm, and the channel 215 ' and the needle channel 221 ' are positioned off-axis. The dielectric material 212' with this geometry is equally suitable for connection to a coaxial transmission line using the same internal arrangement as shown in fig. 10 to 12.

Fig. 7 shows a test arrangement for testing the performance of the helical antenna 200 shown in fig. 6A and 6B when used as a microwave radiator. The simulation setup is similar to the setup shown in fig. 2. However, instead of a liver load, a blood load 240 is used. Again, energy is fed to the antenna structure through a coaxial transmission line 212.

Fig. 8 shows a graph of return loss similar to fig. 3D and 5D. It should be noted that the precise form of the graph may vary depending on the position of the device relative to the blood load, e.g., on the side thereof. It can be seen that at 5.8GHz, the return loss is-12.08 dB. Fig. 9A and 9B illustrate the power loss density within a blood tissue sample immediately in front of an antenna. The plan view shows that the power loss density is uniform, which means that an antenna (such as the antenna of the present invention) will likely produce uniform heat/solidification.

Claims (19)

1. A helical antenna structure for connection to a coaxial transmission line, the coaxial transmission line having an inner conductor and an outer conductor, and the helical antenna structure having:

a dielectric support;

a first spiral electrode and a second spiral electrode, both of which are located on the dielectric support and are electrically isolated from each other;

first connecting means for connecting the first helical electrode to the outer conductor of a coaxial transmission line;

a second connection means for connecting the second helical electrode to the outer conductor of a coaxial transmission line;

wherein:

at least one of the first and second helical electrodes is capable of functioning as a radiating antenna structure for emitting a microwave/RF field outwardly; and is

The first and second helical electrodes are configured to maintain an electric field in a helical region therebetween.

2. The helical antenna structure of claim 1, wherein said first helical electrode and said second helical electrode have the same pitch.

3. The helical antenna structure of claim 2, wherein said first helical electrode is positioned diametrically opposite said second helical electrode.

4. A helical antenna structure according to any one of the preceding claims, wherein the dielectric support is cylindrical or substantially cylindrical.

5. The helical antenna structure of claim 4, wherein the dielectric support has a rounded distal end or a hemispherical portion at its distal end.

6. The helical antenna structure of any one of the preceding claims, further comprising a waveguide or transmission line structure for conveying microwave/RF energy from a proximal end to a distal end of the helical antenna structure.

7. A helical antenna structure according to claim 6, wherein the waveguide or transmission line structure is in the form of a third helical electrode located beneath the surface of the dielectric support or embedded within the dielectric support.

8. A helical antenna structure according to claim 7, wherein the third helical electrode follows the same helical path as the first helical electrode and is located radially inwardly of the first helical electrode.

9. A helical antenna structure according to claim 8, wherein the first and third helical electrodes are made from a strip of conductive material such that the first and third helical electrodes form a microstrip line.

10. A helical antenna structure according to claim 9, wherein the first helical electrode is at least three times wider than the third helical electrode.

11. The helical antenna structure according to any one of claims 7 to 10, wherein a distal end of the second helical electrode is electrically connected to a distal end of the third helical electrode by a conductive member.

12. A helical antenna structure according to any one of claims 1 to 5, wherein the dielectric support comprises a channel or cavity for receiving the coaxial transmission line such that a distal end of the coaxial transmission line is located at or near the distal end of the dielectric support.

13. The helical antenna structure according to claim 12, wherein the distal end of the coaxial transmission line is embedded within the dielectric support, and the first and second connection means are located in holes through the dielectric support arranged to connect the first helical electrode to the inner conductor and the second helical electrode to the outer conductor respectively.

14. The helical antenna structure of claim 12, wherein:

a distal end surface of the coaxial transmission line is located at the distal end of the dielectric support and is exposed by an aperture,

the first connection means connects the first helical electrode to the inner conductor through the aperture, and

the second connection means connects the second helical electrode to the outer conductor through the aperture.

15. The helical antenna structure of claim 14, wherein only a tab portion of the aperture exposes a portion of the outer conductor.

16. A helical antenna structure according to any preceding claim, wherein the dielectric support has a channel extending therethrough from the proximal end to the distal end, the channel terminating in an aperture.

17. A helical antenna structure according to claim 16, further comprising a retractable needle slidably mounted in the channel.

18. A helical antenna according to claim 16 or 17, wherein the dielectric support comprises a plurality of holes between the channel and an outer surface of the dielectric support, the holes being arranged to allow gas to flow between the electrodes.

19. The helical antenna according to claim 16 or 17, wherein the channel is arranged to convey a liquid, such as epinephrine.