Constructing a LowFER Antenna
by Robert Bicking, W9RB
Getting on the air with a LowFER station is more complicated than getting on the ham bands since commercial equipment isn't available and construction is involved. The "RB" LowFER antenna is 31 feet tall with a 24 foot diameter top hat. At 186 kHz, the antenna primarily looks capacitive with a small resistive component consisting of the radiation resistance of the antenna in series with the loading coil resistance and the ground resistance. These three resistances are the factors under control of the experimenter and determine how well the antenna works. The radiation resistance of any Part 15 antenna is a small fraction of an ohm, since the 15 meter length limitation means that the antenna will be less than 1 % of the wavelength, as opposed to 25 % which is the generally accepted rule for AM broadcasting and 160 M ham operation. By reducing the rf resistance of the loading coil and ground and increasing the radiation resistance, the radiated power (I2RR) is maximized. The fact that NDB's in the 200-500 kHz band running 100 w or less can be heard consistently over 1000 mile plus distances demonstrates that the LowFER band is capable of good DX, but power and antenna limitations make propagation conditions and background noise very important. The top hat is important as it not only makes the antenna electrically longer but changes the current distribution to put the maximum current at the top instead of the bottom and reduces the size of the series inductor needed to resonate the antenna, further reducing losses. A good ground system is necessary, with as many radials as you can tolerate installing since ground resistance is typically the largest resistance in the system.
The antenna described herein is mainly based on info past Lowdown articles and is not too difficult or expensive to build. Practical construction details are included and references are given for those interested in more background.
The antenna is a 31 foot vertical with a 24 foot diameter top hat, using guying to support it,
insulated at the bottom, and fed with a series inductor. A 30 foot push-up mast is available at a
very reasonable cost ($ 49) from Surplus Sales of Nebraska. They also have a ceramic base
insulator, although since it is only 1 inch thick, I would use several with a bolt to hold them
together and to a galvanized steel plate about 6 by 12 inches. The steel plate keeps the antenna
from sinking into the ground. Alternately, solid fiberglass rods of various diameters are available
from Max-Gain Systems and a 12 inch length would make an excellent base insulator, again using
the steel plate for support. Phillystran guy material is very long lasting, has excellent electrical
properties, doesn't need any insulators and is available from Texas Towers, among other places.
Cable clamps and thimbles are used to terminate it with a 1.5 foot screw-in ground anchor as the
support. The hardware is available from a store such as Farm and Fleet. For a 30 foot tower,
placing three anchor points 16 feet from the tower in an equilateral triangle configuration should
be adequate. One important point when using Phillystran is to cover the ends with plastic caps. I
used a bit of clear silicone sealant to make sure that the caps stayed on, preventing moisture
A top hat is necessary to get a desirable current distribution and as high a radiation resistance as possible. Additionally, the top hat adds a lot of capacitance to the antenna, reducing the inductance of the loading coil needed to resonate it and reducing the losses associated with the loading coil. The larger the top hat, the better, until it exceeds the height of the mast (references 1 and 2). The generally accepted rule for Part 15 antennas is the antenna plus the radius of the top hat should not exceed 15 meters. A 24 foot diameter top hat is a reasonable compromise and one which should withstand windstorms. It is much easier to get the mast set up with the bottom set of guys to hold it in place and then mount the top hat than to try to do it in reverse order. A separate top hat assembly facilitates this.
A solid aluminum plug about 12 inches long can be machined to fit in the mast to hold the top hat tubes. The plug should be machined two inches from the bottom to fit in the mast. Then, drill a 0.75 inch hole one inch down from the top of the plug. Then drill a second hole at 90 degrees from the first hole, spaced 1.5 inches on center. Then, drill another hole at 45 degrees from the second one, spaced 1.5 inches on center. The final hole is again spaced 1.5 inches on center and 90 degrees from the third one. This will result in a top hat with 8 equally spaced arms. Aluminum tubing with good mechanical properties is available from Texas Towers in sizes which fit nicely into each other. Each top hat tube consists of an 8 foot length of 0.75 inch tube, a 1 foot length of 0.625 inch tube located in the center as a stiffener, two 4.5 foot lengths of 0.625 inch tubing and two 4.5 foot lengths of 0.50 inch tube, with the two smaller diameters each inserted 6 inches into the adjacent tube. The tubes should be held together with # 6 stainless sheet metal screws. Another stainless sheet metal screw can be used to hold the top hat tubes in the plug and a # 10 stainless screw to hold the top hat plug in the top of the mast.
Three skirt wires will nearly double the capacitance of the top hat (reference 3), without
adding much weight or wind drag. AWG 17 electric fence wire is low cost, plated,
solderable and quite strong. It also can be used to make a good Beverage antenna for
receiving. The skirt wires can be attached using the same screws that hold the sections of
top hat tubes together using solder lugs. This will result in one skirt wire at a 4 foot radius,
one at an 8 foot radius and a final one near the tips of the smallest tubes at a 12 foot radius.
The skirt wires do not have to be taut and in fact, a little slack might prevent breakage.
Erecting the Antenna:
The mast is placed on the base insulator and the bottom set of guy wires are adjusted so that the mast is vertical. Note: all the guy wires should be attached to the antenna so all you have to do is anchor them at the ground as the antenna is pushed up. The top mast section may now be pushed up and locked in place. Push up the middle section and lock it in place. Next, adjust the guy wires so that all sections are roughly vertical. Don't tighten the guys too much as it will make lowering it to attach the top hat difficult. Now the mast sections can be lowered and the top hat assembly attached and the mast sections raised. Attach a 6 inch piece of ground braid between the top section and the next section using stainless steel hose clamps to ensure a good electrical connection. Repeat this with the next section as it is raised. Finally, the guys can be adjusted so the antenna is vertical. Turnbuckles facilitate later adjustment due to guy stretching, etc.
I have 32, ranging in length from 40 feet to 150 feet as dictated by my property, with 8 terminated by 5 foot ground rods. The first 4 ground rods made a big difference in the ground resistance but the last 4 only improved it a few percent. I also have two 10 foot ground rods at the antenna base. Menards has heavily plated copper ground rods at a low price. Don't bother with the ones with just a flash coat of copper, they will be rusty and ineffective in a short time. A power edger is an easy way to cut grooves several inches deep for buried radials. If you don't own one, they are available at most rental shops.
Once the grooves are cut, a tool such as shown in the picture which consists of an 8 inch
diameter 1/4 inch plywood disc with a groove cut into the periphery and mounted on a 2 x
2 inch stick makes pushing the wires into the ground fairly easy. My wife spooled out the
wire and I pushed it in. I used AWG 14 insulated wire since it is low cost. My yard is
heavily wooded and in the woods, I didn't bury the wire but instead, used J-hooks as
shown on the radial burying tool to hold them down until nature buried them. These are
available from any building supply store and their intended purpose is to attach chain link
fence to its supports.
Once that the antenna is up and the ground system is in, a loading coil is needed to resonate the antenna capacitance. The first step is to measure the capacitance of the antenna. This can be done with a RF analyzer at 1 MHZ or so if you don't have a low frequency impedance bridge, since capacitance of an air dielectric capacitor doesn't change appreciably with frequency, at least up to a few MHZ. Then, you can calculate the inductance needed from L = 1/ [4(Pi)2 f 2C]. As a check, 280 pF at 186 kHz needs 2.61 mHy to resonate. As explained in reference 4, lower losses result from using a rotatable inductor (variometer) inside the main inductor instead of tweaking the system into resonance with a small air variable capacitor connected from the antenna to ground. However, a small air variable capacitor of, say, 100 pF is useful to determine how much inductance will be needed in the variometer. Connect your main coil to the antenna and attempt to resonate it using the external capacitor connected across the antenna to ground. If you can resonate it, then measure the capacitance. The additional inductance needed is given by L = Lmain x C/Cant. The variometer should be wound in the same direction as the main coil and they are connected in series. The mutual inductance of the variometer is given by M = k[L1L2]1/2. If k, the coupling factor, is 0.5, L1 = 2.5 mHy and L2 = 0.1 mHy, M = 0.25 mHy, so the maximum inductance of the variometer is L1 + L2 + 2M =3.1 mHy.
Bill Bowers provided some very useful info in reference 5 on building coils. He showed that the optimum diameter to length ratio is 2.5 and that Litz wire with many strands is the ultimate. My coil has a diameter to length ratio of 1 and is wound on a white PVC coupling which is 7.25 inches in diameter. The PVC was slotted using a saber saw to make it more like an air core per reference 5.
The bottom 15 turns are solid AWG 14 copper with a bare spot in the insulation every turn (left end of coil) so I could experiment with coupling to the transmitter. The remainder is wound using 175 strand unserved AWG 40 Litz wire from MWS Wire Industries. The Litz wire is wound with no spacing based on the results reported in reference 6. It should be noted that the Litz wire is about 7 times more expensive than ordinary copper wire. I used Strip-Eze to remove the insulation from the Litz wire for soldering. If you cut slots in the form, round their edges if you are using Litz wire to avoid cutting any strands of the fine wire as it is wound on the form. The Variometer coil was wound on a 5.25 inch diameter PVC coupling, again slotted. The Variometer was held in place inside the main coil using nylon 1/4 x 20 screws and nuts, tightened so that the coil will stay in place but could be moved to adjust it. It should be located so that the tops of the two coils are even. The Variometer coil is visible tilted at the right hand side of the above picture.
I mounted the coil in a plastic toolbox which keeps water out and used an electric fence
insulator to feed the high side of the coil to the antenna.
1. Optimizing the Part 15 Antenna, Mike Mideke, Lowdown, April 1987.
2. The Texas Beacon: Part 3, Bill Cantrell, Lowdown, Oct. 1997.
3. Antenna Top Hat Capacity, Bill Bowers, Lowdown, March 1995.
4. LowFER Transmitter Antenna Tuning Analysis, Bill Cantrell, Lowdown, March 1999.
5. Low Frequency Coil Q, Bill Bowers, Lowdown, February 1996.
6. Coils, Coils, Coils, Bill Bowers, Lowdown, July 1996.
1. Surplus Sales of Nebraska, 800 244 4567, www.surplussales.com
2. Texas Towers, 800 272 3467, www.TexasTowers.com
3. MWS Wire Industries, 818 991 8553, www.mwswire.com
4. Max-Gain Systems, 770 973 6251, www.mgs4u.com
P. S.-- If you want to build a Beverage antenna for receiving with your left-over fence wire,
Lectrokit makes a kit consisting of a matching transformer, terminating resistors and small
PC boards to mount the components on at $ 12 each or 3 for $ 29. 401 W Bogart Rd.,
Sandusky, Oh. 44870. I use nail-on fence insulators and nail them to the trees for my
Beverages. TV coax works fine and is cheap. For Beverage antennas less than a wavelength
in length, a height of 13 feet gives the best signal.
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