The Cat Whisker is a broadband antenna of bidirectional radiation pattern covering continuously 14 - 30 MHz frequency range. This feature has been achieved by combining several wire loops.
I presented this antenna in Świat Radio 2017/01 (Polish ham radio magazine) and QST 2018/03. It was awarded the Second Prize in the QST 2017 Antenna Design Competition in the category: 80 Through 10 Meters.
You can find its description also in The ARRL Antenna Book 24th Ed.
The overall antenna dimensions are: 7.4 x 4.5 m. Its turning radius equals 3.7 m. The drawing below presents all dimensions of the Cat's Whiskers in meters.
As you can see in the drawing, there are four wires going to the left-hand side and four wires going to the right-hand side from the antenna feed point. Their ends are joined to a rectangular wire frame. The four wires connected to the frame corners are run along the fiberglass spreaders. The remaining wires hang in the air. The wires are connected in the spots shown in the drawing as small dots. I used soldering in the prototype antenna, but screw terminal blocks could be used instead.
The upper spreaders are tilted 47° upwards and lower ones are tilted downwards by 8° w.r.t. horizon. The upper ones are ca. 5.4 m long, and the lower ones are ca. 3.7 m long. They are made of fiberglass fishing poles 8 and 6 meter long respectively. Two thinnest sections were removed from every fishing pole. The remaining sections were glued together in the extended position and cut to length. The spreaders were slit into aluminum tubes and mounted on a steel plate that is held in turn by metal brackets.
In the prototype antenna, a copper strand wire of 1 mm diameter was used.
The antenna dimensions were adjusted in the computer model to minimize variation of its impedance. The Cat Whiskers impedance changes from 150 through 650 ohms in the 14-30 MHz range. The calculated SWR in the antenna feed point for the 300 ohm source is quite low and changes from 1.8:1 to 2.6:1 in the 14-30 MHz range. The best match to 50 ohm coax may be achieved with a 6:1 balun.
When a coax is connected to the balun, the SWR is reduced due to non-zero cable loss. For example, if you use 30 m long RG-58 coax, then the SWR measured at the transmitter coax end will be no higher than 1.85:1 in the whole frequency range. It means that many transmitters not equipped with antenna tuners can drive the Cat Whiskers with no fear of damaging their final stages.
The prototype antenna shown in the photograph was fed with 40 m long RF-7 cable, which is less lossy than the RG-58. That's why the SWR reduction was not as high as for the RG-58. The graph below shows a real SWR measured with an antenna analyzer. The SWR changes from 1.1:1 to 2.2:1 in the 14-30 MHz range.
Similar SWR performance has the well-known T2FD antenna. However, it must be stressed that the Cat Whiskers do not dissipate RF energy in a resistor as the T2FD does. As can be shown, depending on frequency, the T2FD can lose in its resistor even 50% of the supplied power. Therefore, the T2FD has significantly smaller gain than a half-wave dipole. And what about the gain of the Cat Whiskers? In the 14-30 MHz range, it varies from 2.5 to 3.8 dBi in free space. A reminder: the dipole has gain of 2.15 dBi in free space.
However, to compare the Cat Whiskers to a dipole in properly, one must take into account the additional loss in the coax feeder due to higher SWR than a dipole has. The graph below shows the gain of the Cat Whiskers compared to a half-wave dipole calculated in the 4nec2 and MMANA-GAL simulators and reduced by the amount of signal loss in the feeder calculated in the TLDetails program (a freeware created by AC6LA). The 30 m long RG-58 coax was assumed as the feeder in those calculations. As you can see, the MMANA-GAL results are slightly more optimistic than 4nec2. We may say that the Cat Whiskers are on par with a dipole in 20 m band (~0 dBd), and are better than a dipole in the higher bands (up to 1.5 dBd @ 29 MHz). Should the coax be shorter or less lossy than RG-58, the supremacy of the Cat Whiskers will be even more visible. Please note that it was assumed in these calculations that the transmitter is equipped with an antenna tuner that matches its output impedance to the impedance of the antenna-balun-coax system.
After simulating the antenna in free space, ground effect was taken into account in the model. The antenna was modeled at various heights. The simulations showed that antenna impedance and its SWR depend very little on the installation height. What was even more interesting, small changes in the antenna dimensions (in the order of 5-10 cm) had almost no effect on the antenna performance. It means that the antenna can be built without utmost precision and yet it will work perfectly in the 14 - 30 MHz range.
And now a few words about the construction of this antenna.
The antenna central mount was made from a 400 x 400 x 4 mm steel plate. The drawings below shows basic dimensions. In the pictures below, you can see an additional tube perpendicular to the antenna plate. It was not used in the Cat Whiskers prototype. However, you can use it to add a short boom and tie the ends of the antenna spreaders to the ends of the boom with non-conducting ropes. Then, the spreaders will not wobble during strong wind periods.
The total weight of the prototype antenna was 8 kg. A light Yaesu G-450C rotator (3.5 kg) has been added to turn it around.
The 6:1 balun is based on three toroidal transformers. The first one (TR1) transforms 50 ohms to 75 ohms, and the remaining two (TR2 and TR3) form a 4:1 Guannela balun transforming 75 ohms to 300 ohms. All transformers were wound on the FT140-61 cores.
TR1 has 10 turns made with 5 enameled copper wires of 0.5 mm diameter. TR2 and TR3 have 10 turns made with pairs of insulated copper wires. Those wires have thick high voltage insulation. Thanks to that, they are more separated from one another than normal insulated wires. Ideally, the TR2 and TR3 should be wound with a symmetrical transmission line of 150 ohm characteristic impedance. Thick isolation helps to bring the impedance of a pair of wires to something close to that value. If desired, the Cat Whiskers can be fed with 75 ohm feeder. In such case, omit TR1 in the balun.
When compared with a nearby multiband GP installed on a 5 m high mast, the Cat Whiskers shone. On reception, they always produce stronger signal, sometimes even 2-3 S stronger than the GP. This was more than the theory can explain, so evidently the multi-band GP performed worse than advertised. Moreover, because the Cat's Whiskers radiates mainly in horizontal polarization, noise background was significantly lower and QRN less noticeable. The very first contacts showed that the Cat Whisker can be successfully used as a DX antenna. Of course, it is important to hoist it high enough. Similarly, as for the dipole, it should be at least 0.5 wavelength above the ground (10 m for the 14 MHz band). If you install it at a lower height, peak radiation will occur at higher elevation angle, what does not help in DX hunting.
The Cat Whiskers will forgive you some inaccuracies in dimensions. Just build it, hoist it and operate it without any trimming.
Click here to download this antenna models.