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Bobtails and Half Squares These antennas offer big gun DX performance on the low bands, yet they are surprisingly simple to build. Try one- you will like it! |
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Q.
When is a half square not really a half square?A1.
When you direct feed the tail with coax at ground level against
radials.A2.
When you invert it and corner feed it directly with coax.
CQ Magazine for January, 2000 had comments about the bobtail and half square with numerous errors that have probably caused some confusion about these antennas. Figure 2 on page 94 shows the antenna very much out of proportion. A half square is 1/4 wave high and 1/2 wave long, or 2 times as long as it is high, not 5 times as shown. The article states that half squares do not need a ground plane to operate properly, yet it shows the antenna working against one. The feed method shown is not correct for a half square. The chosen method probably forces the two tails to be current maxima- The antenna is therefore a conventional ground plane with a full wavelength folded radiator. The low SWR reported on the coax is strong evidence of this. Were the antenna truly an inverted ground plane, the tail impedances would be very high, and so would the SWR on the coax caused by direct connection to a roughly 20:1 to 80:1 impedance mismatch. This high ratio probably could not manifest in practice, but the SWR would probably behave strangely and the coax would have a strong tendency to become a radiator. The article states that a half square will show about 3 to 4dB gain over a dipole but neglects to point out that this is with respect to a vertical dipole. Horizontal dipoles have reflection gain, verticals do not. A half square may not show any gain at all over a horizontal- it depends on many factors such as local ground quality, the antenna site, the height of any horizontal comparison dipole, and propagation conditions at the moment. The article erroneously claims that LC tuning networks are not appropriate for feeding half squares and will not work well. Alert readers may be aware that the former Antennas West company sold half square antennas for years which were direct coax-fed up in a corner. You can't have it both ways, so which of these designs is right? Antennas West has correctly implemented the half square by putting the current maxima a quarter wave up in the air where they belong. Corner feed will not produce the cleanest possible radiation pattern, and will limit the antenna to single-band use, but it will at least make it function as an inverted ground plane as intended. On 10 meters there might not be much difference between a half square and the CQ article design, but there would be on the low bands, where the half square is a truly top notch DX performer without any radials. Verticals with their current maxima at ground level need a lot of radials to work efficiently- the half square with its current maxima a quarter wave up in the air needs little more than a small counterpoise at the fed end to work as well or better. The CQ article stresses the use of ferrite beads or choke coils at the feedpoint for decoupling- this is not surprising, as it is not uncommon to see antenna/feedline interaction with direct coax-fed, current-driven verticals. I have successfully used LC tanks to feed half squares and bobtails on 160, 80, 40, and 20 meters, and they always worked very well for me. LC voltage fed bobtails and half squares have been shown in the ARRL Handbook, the ARRL Antenna Book, ON4UN's Low Band Antenna Book, and other major books and ham magazines for over 50 years now. It is often the only feed method shown. Certainly if there were any major problems, it would have fallen out of favor long ago and been withdrawn from publication. It was the method of choice for W6BCX when he published the first article about the Bobtail in 1948. The CQ article's claim that LC tanks will not work well with these antennas is absolutely not true. Whether voltage feed is necessary in every case is a separate issue and is open to debate, but the effectiveness of LC feed has been well established over the years. It is regrettable that the editiors at CQ did not catch these errors and allowed misinformation to be published. The impedance to ground at the tail end of a half square is very high, in the thousands of ohms. This would obviously not be a good match for direct feed with 50 or 75 ohm coax. The correct feed method at this point on the antenna is 'voltage feed', which is best done using an LC tank circuit to match the coax impedance to the antenna impedance. (Another feed method is open wire line with a tuner, but coax is far better and easier at HF, in my opinion) The SWR on coax feeding a properly designed resonant LC network will be be low. With an efficient tank circuit, overall losses will be low as well. As a bonus, the network is frequency selective and will attenuate harmonic radiation and decrease receiver overload from strong signals on other bands, especially if link coupling is used. When transmitting, the ends of the tails are points of high RF voltage, and users must be careful to insulate them well to prevent RF burns to people or animals touching the antenna! L.B. Cebik, W4RNL, has an interesting collection of vertical 2 meter arrays with coax corner feed described on his website. He calls them half squares, although, strictly speaking, they are really inverted half squares or inverted, inverted ground plane arrays. I would call them ground plane arrays, myself. It would also be less confusing if they were referred to as "inverted half squares". This is a minor point- it does not detract from what is no doubt one of the best antenna websites on the internet today. At VHF, there is probably no advantage to having the current maxima at the top of the antenna, inverted ground plane style, as there is at HF. If you can visualize two ground planes, side-by-side, then remove all but one radial from each vertical, and then join these two radials at their tips, putting a short across one feedpoint and coax across the other, his antenna results. I hear they work very well on 2 meters W4RNL asks an intriguing question at the end of the article: would an array of 3-element (bobtail) vertical arrays be practical for 2 meters, and how would it be fed? I must leave that for real antenna experts to work out, but I would suggest that voltage feed of the center tail of the array(s) might be the best answer with bobtails. This might best be accomplished with open wire feed at the antenna, followed by impedance conversion to coax using a stub and balun, and might involve either parasitic or all-driven operation. It might be easiest to invert the array once again and run it inverted ground plane style. :o) Recently, W4RNL stated on his website that he will soon be publishing such a design in Communications Quarterly. I am looking foreward to seeing how he works out the details. |
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