|Simple 10M Loop||| Print ||
|Simple 10M Loop||| Print ||
|Written by George Fletcher|
A Simple 10-Meter Band Loop Antenna
By George Fletcher, AD5CQ
This report documents the efforts by the author to design and build a small loop antenna for the 10-meter band. The specific design frequency was chosen to be 28.4 MHz which coincides with the middle of the band reserved for Technician class privileges. The overall design is simple and the construction does not require any special components or processes beyond basic soldering and mechanical assembly.
Most literature on the design of small loop antennas classify the loop as "small" if the length os the loop is less than .1 of a wavelength. Loop antenna design of less than .1 wavelength can be made to operate but with a sacrifice in overall efficiency. This design uses a loop length that is greater than .1 wavelength at the design frequency so as to achieve greater efficiency in the antenna system performance.
The loop conductor is made from 77 inches of ¼-inch copper tubing like the tubing that is sometimes used to connect water lines to a refrigerator. The exact length of the loop is not critical because more or less length can be compensated for with the tuning capacitor. My experience has shown that keeping the length of the loop between .1 and .25 times the wavelength is a good design parameter. This loop (77 inches) is .18 times the wavelength (calculated as 28.4 MHz). Bending the tubing into a loop is not too difficult but not very easy either. It takes some time with small adjustments made each time so as to not put a crease in the loop. The loop itself does not have to be precisely circular. The final shape of the loop will have an effect on the overall radiation pattern. Making the loop circular just looks better.
The base (Figure 1) is made from a piece of fence picket. Wood fence pickets are about 5.5 inches wide. That is wide enough to mount the antenna components and still have a little room for mounting brackets if you need to mount the antenna on a mast. I used #6 wood screws for mounting parts of the antenna. The loop is attached to the wood base using two copper pipe clamps that have been reshaped to fit the ¼-inch tubing. These clamps are thin and can be easily flattened and reshaped using a heavy set of pliers. Try to use copper pipe clamps instead of aluminum clamps to avoid galvanic corrosion that can occur between copper and aluminum metals. Copper clamps will also facilitate soldering of connecting wires. Solder the clamps to the ends of the tubing then screw the clamps to the wood base using the holes that are already drilled into the ends of the clamps. Connecting wires can be easily soldered to these clamps rather than trying to solder them directly to the ¼-inch tubing.
The tuning capacitor is mounted in the center of the antenna. The electrical value of this capacitor was measured as approximately 4.66 pF. Each half of the split stator capacitor was measured to be approximately 11.27 pF. Placement of the capacitor itself is not critical except that lead connection lengths can be critical and cause large changes in the resonant frequency. So mounting the capacitor in the middle with short leads makes sense. Speaking of lead lengths almost all of the wires used in the antenna system need to be kept as short as possible. If you breadboard your design and then later make the final connections by shortening the wires it will normally change the input impedance significantly so make your breadboard as close to the final design as possible.
“L” brackets are used for mounting most of the components. The brackets are especially convenient for the antenna connector. They are easy to use for mounting the variable capacitors to the wooden base.
This particular antenna uses a split-stator capacitor (Figure 2) as the main tuning component. The split-stator design is not critical. A single gang capacitor could be used if it enables the system to be resonate at the desired frequency. The split-stator design enables the signal currents to pass through the capacitor without having to cross mechanical joints. Avoiding mechanical joints is critically important in this type of antenna because the high currents will cause significant voltage drops in the system and rob the available signal energy. Sometimes mechanical joints are unavoidable. Turning the shaft of the tuning capacitor will cause a large change in resonant frequency. Use a large knob or some other method of moving the capacitor shaft so that it moves only a small amount at a time. The connecting wires between the loop ends and the tuning capacitor were 5 and 6.5 inches. Altering these lengths will have a significant effect on the resonate frequency of the antenna system. The left connecting wire could have been shortened by about a couple of inches but the author chose the leave the length in place since the antenna was performing well with this extra length.
This antenna uses a gamma type matching system. The gamma capacitor (Figure 3) is mounted at the front left of the antenna base. The gamma rod is made of 3/32-inch brass rod which is bolted with a screw into the side of the capacitor mounting bracket thus making connection with one side of the capacitor. Adjusting the gamma capacitor is not as sharp as the main tuning capacitor so a smaller knob can be used. Larger knobs are useful for keeping your body from detuning the system as you make adjustments. The gamma capacitor used in this design was measured to be approximately 71.77pF.
The gamma match rod (Figure 4) can be placed on either side of the antenna. Note that the size of the gamma rod diameter, distance from the antenna and location of the shorting bar all work together to achieve a 50 ohm match. This antenna design used a distance of approximately 2 inches between the gamma rod and the loop. The curve of the gamma rod does not have to match the curve of the antenna but doing so makes the antenna system look nicer. The shorting bar (Figure 5) is made of two spring loaded clips that have their ends soldered together to make the assembly as rigid as possible. The small clip attaches to the gamma rod and the large clip attaches to the antenna. The location of the shorting bar makes a significant difference in the real value of the input impedance. Adjust the position of the shorting bar so that the impedance at the input connector is a close to 50 ohms as possible. Then adjust the gamma capacitor to null out the remaining reactance. You will likely have to adjust both the shorting bar position and the gamma capacitor several times to achieve a perfect match (SWR 1.0/1). Some adjustment of the main tuning capacitor may also be required. All of the elements of the antenna system, length of loop, main tuning capacitor, gamma capacitor, location of shorting bar, distance between the gamma rod and the antenna loop, height of ground and surrounding objects all have an impact on the impedance at the input connector. Final tuning adjustments should be made as close the final operating position as possible since the antenna is very sensitive to its environment which can cause the antenna to become detuned.
A perfect match at the input connector (Figure 6) is not always possible. If your tuning adjustments are only able to achieve, for example, an SWR of 1.2/1 then that will be OK. In that case the length of the coax line between the antenna and the transmitter will have some effect on the SWR at the transmitter end but that should be easily corrected by using a transmatch (commonly called “antenna tuner”).
Wires used in the construction should be of heavy gauge since the currents will be significant. A large gage wire will also provide a larger surface area for the RF currents which is desirable.
The final loop (Figure 7) should be approximately two feet in diameter. The copper loop conductor was measured to be approximately 1.7 uH. The loop conductor plus the two connecting wires was measured to be approximately 1.93 uH. Using the antenna outside will require weather proofing the during capacitors and electrical connections. If the loop is to be used indoors then be mindful of RF exposure limits. Move the antenna further away from the operating position or operate with reduced power if RF exposure limits are being exceeded.
Use of an antenna analyzer (MFJ-259) will enable you to see both the real and imaginary impedance values along with the overall SWR and observe how they change when adjustements to the antenna are made. Antenna asjustements can be made with an ordinary SWR meter but use of a radio transmitter will be required as a signal source. Using an antenna analyzer will likley reduce the number of adjustments that need to be made because the analyzer provides more information about the antenna response than a simple SWR meter.
Use of the antenna in the orientation show above in the photographs will provide both horizontal and vertical polarized RF waves. In an ideal environment most of the signal from the antenna will be in the plane of the antenna. Stations that are broadside to the antenna will not be heard as well as stations tha tare in the direction of the plane of the antenna. When the antenna is used at a distance less than ½ wavelength from the ground the overall signal pattern will start to resenble a fishbowl with a signal pattern that is approximately the same all around the antenna. The height above ground will also affect the input impedance at the antenna connector so final adjustments should be made in the final operating position.
|Last Updated on Saturday, 27 November 2010 09:20|