Site hosted by Angelfire.com: Build your free website today!



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!


Tuning Network Design
Here are some calculated inductance values for bobtail and half square tuning networks. Choose a capacitance of about 1 pF per meter of wavelength for good SWR bandwidth. The exact value used is not critical. Also shown are solutions for the 80 meter CW band and phone DX window using a fixed inductance and varying the capacitance. Any combination of variable and fixed capacitors that can handle the RF currents and voltages involved may be used. RG-8 or RG-213 coax can be used to make excellent, inexpensive high voltage RF capacitors. Efforts to optimize the inductor and capacitor Q's will be rewarded with lower network power losses, however.
Freq. MHz C pF L uH
1.85 160 46.3
3.50 80 25.8
3.50 86.9 23.8
3.65 80 23.8
3.80 73.7 23.8
3.80 80 21.9
7.00 40 12.9
10.1 30 8.28
14.0 20 6.46
Coaxial Capacitors These are easily made from RG-8 or RG-213 which have 30pF capacitance per foot. Simply divide the needed capacitance by 30 to get the number of feet needed. For example, 80pF divided by 30 equals 2.67 feet, or 2 feet, 8 inches. Allow a little extra length for tuning and connections. The coax is gradually shortened until the desired capacitance is obtained. Leave some extra center insulation extending beyond the braid at both ends to assure that the best possible high voltage breakdown rating is maintained during high humidity conditions. The trick is to trim back a little of the outer jacket at the high voltage (center conductor) end and then cut off the braid flush with the jacket. Each inch you remove is worth 2.5pF- this is a slow tuning rate of about 54 KHz per inch at 3.5 MHz using the inductance value from the table. The rate is about 208 KHz per inch at 7 MHz, however, so it pays to make smaller adjustments on 40 meters. The coax can be coiled or bent to take up less space. Minimum coil diameter of the homemade capacitor must be ten times the coax diameter, or about 5 inches. A little more is better. Only polyethylene dielectric coax should be used, never foam! Although you can make direct connections using the coax center conductor and braid, soldering some smaller diameter wire leads to them makes things a little easier, especially at the braid end. #14 awg is a good choice. Remember to wire the coax braid to ground to avoid the possibility of a hot braid arcing. Keep in mind that the center conductor at the braid end is hot and must be clear of other objects.


Note that coaxial capacitors may introduce more network power losses than air variables, vacuum variables, micas, or ceramics. The best choice for most hams would probably be the familiar 'bread slicer' type air- variable capacitors- but coax capacitors definitely will do the job and can get you on the air in a hurry when needed.


Inductors These are single-layer, solenoidal coils similar to those found in the output circuits of amplifiers. It is best to use at least the minimum conductor sizes shown in the table, but in practice, you can get away with going slightly smaller. Conservative design minimizes power losses, however
Power W 20/30/40m 80/160m
500 #12 awg #14 awg
1000 #8 awg #10 awg
1500 1/4" Cu
Tubing
3/16" Cu
Tubing
Inductors can be made from commercial coil stock or hand-wound, or surplus coils can be used. Large ceramic coil forms can be found at many hamfests, cleaned up, and used for winding coils. Military surplus coils on ceramic forms are easy to find. Some are suitable and can be used as-is, or new windings can be put on the forms. Note that many of the coils offered at hamfests do not have enough turns-per-inch to obtain the inductance needed. Since this kind of ceramic form is usually notched to set the wire spacing, those particular forms are useless for bobtails. It is a good idea to know what diameter, length, wire size, and turns-per-inch will work on your band while shopping at the hamfests. This will prevent wasting time and money. A copy of the inductance formula and a small scientific calculator would be very useful items to bring along. I have used the formula so much I have it memorized.

For these frequency ranges, at moderate power levels, most wire coils are going to be in the 2-4 inch diameter range, and about 3-6 inches long. As an example, one coil I wound for 80 meters was made from 26 turns of #12 awg copper wire on a 2.5 inch diameter ceramic form. The winding was 3.7 inches long. Calculated inductance was 22.7uH. To make the network easy to tune when a coaxial capacitor is used, the inductor can be wound for a little more inductance than needed and a 'top tap' can be used to find the resonance.

You will find a direct reading LC meter such as the AADE LC Meter II (or similar) very handy for working with these networks- or if you are only instrumented to measure L, or only C, then in addition, a Grid Dip Meter is very helpful to determine where your resonance frequency is.


One caution for home builders- some materials are too lossy to be used for coil forms. Avoid PVC (polyvinyl chloride) tubing and similar plastics used for plumbing. Good materials for forms include ceramic, fiberglass (like G-10 used on printed circuit boards,) and many clear 'engineering' plastics such as Plexiglass® acrylic and Lexan® polycarbonate.

A note to those who may be squeamish about building an LC tank circuit: It would certainly be possible to dispense with the LC tank and direct feed the antenna 'tail' at ground level using open wire line and a balanced antenna tuner, if you prefer. This approach has its own problems and challenges, but does do away with the requirement for the LC tank for voltage feed. You would need to assure that the open wire line is spaced up sufficiently from the ground (say 4-6 inches for 450 ohm window line), and you should put some twist on the line over the full run, to help maintain balance. You also need to make sure you deal with any potential tripping hazard from the balanced line near ground level. It would probably be best to gradually slope it up until it's at least 8 to 10 feet high. Coax is so much easier in this regard- simply lay it across the ground, or perhaps bury it just slightly. See the article by K3BC on the Bibliography page for an example of a balanced feed arrangement.

I would be concerned about possible antenna/feedline interaction when using balanced line. I certainly encourage the reader to go ahead and get their feet wet with building LC networks, if they have not yet had the experience. They are the basis for all HF tuning and matching networks a ham might build. With coax feed on the low bands, SWR is generally low so that losses in longer runs are not much of a concern. As a bonus, if you are able to get hold of some cheap or free CATV hardline, as I have, you can have some incredibly long runs (at low SWR) without incurring much of a power loss at all. Open wire is only really needed if you are going to be running under high SWR conditions- then it is the hands down winner. For Bobtails, open wire feeders would need to be run away at a right angle from the array. For the Half Square, it could also be run away from the end being fed, probably for about a half wave or more before changing direction, in all cases.

Regardless of whether you use coax with an LC tank or you use open wire feeders, your 'return' should consist of an appropriately sized ground screen- ground rods are optional and not really necessary or helpful. Radials at this point are definitely not desirable, and might cause significant problems if improperly applied.

Revised and updated 02 Aug 2012

EMAIL Send Comments to K3KY Bobtails Bibliography PREV

HOME K3KY's DX Toolbar   Construction/Safety Issues NEXT

Web page created by K3KY. All original content including text and photos is Copyright © 1999-2012 by David E. Sinclair. Use elsewhere without permission is expressly prohibited.