Limited-space antennas

Sometimes at night, when things are well and all is right in the universe, I dream. I dream of owning (or at least renting from a radio enthusiast land lord) about 500 hectares of flat land good for farming. Not that I intend to do any farming (unless 'antenna farm' counts), but good farmland is moist and composed of enough conductive minerals to make a very good radio frequency (RF) ground for antennas! Dreams are fun to indulge, but on awakening one finds the sometimes harsh nightmare of antenna construction on the limited estates that most of us can afford. And I have lived in some tight spots. In my novice days, living in my parents' home, the lot was respectable by American suburban standards, but was criss-crossed with alternating-current (AC) power lines to both our house and the houses of two neighbors. That problem sharply limited the area open to safe antenna erection. The student boarding house where I lived in college had a sympathetic landlady (who in any event was too poor of sight to notice an antenna short of a beam and tower) was on a shallow, narrow city lot with few possibilities below 20m (and even that band strained the antenna constructor's imagination). After being married, I lived in an apartment and then a small house of my own for a number of years - all of which were limited spaces. Even with my present home, which seems a mansion compared to our first house, my ability to put up full size antennas is a testament to my wife's tolerant attitude. So what to do? Fortunately, there are some things that can be done to accommodate the limited space situation. Nearly all of these schemes use some form of compensation antenna, and as a result don’t work as well as the full-sized equivalent. The TANSTAAFL principle reigns supreme: 'there ain't no such thing as a free lunch.' Whenever one attempts to reduce the size of an antenna for any given frequency, the result is that something else falls down a bit: gain, directivity, impedance, efficiency, bandwidth, or all of the above.

While these factors may lead one to a depressed sense of gloom and doom, that is not the best attitude. The correct way to view the situation is not comparing the limited space antenna to a full size antenna, or the best in class, but rather to whether or not it allows you to operate at all. After all, a 10m long, end-fed inductor loaded Marconi draped out of an upper-story window may not work as well as a 75m dipole, 25m in the air, but it works well enough to make contacts.

SOME OBVIOUS SOLUTIONS

Not all limited space antenna choices are bad. For example, one could restrict operations to the upper bands, i.e. those above 21MHz and into the very high-frequency (VHF) region. I know one fellow who was a 300+ country DXCC award holder who operated only on the 15m ham band because of space limitations (he lived on a small 1 12 hectare lot). He was able to erect a 15m two-element quad beam that worked quite well. Another fellow earned his DXCC as a newly licensed amateur (when operating skills are presumably undeveloped as yet) with a 50W kit-built transmitter and a 10-15-20m trap vertical mounted on the roof of a four-story apartment building. It can be done.

The vertical antenna is a decent solution for many people. A full-sized 40m (7-7.3MHz) vertical is only 10m high, and that is right at the legal limit or 10.7m in my county for an installation that does not need mechanical inspection by the local government. Adding a switchable loading coil at the base of that antenna would also allow 75/80m operation, especially if an antenna tuning unit is provided.

Modern trap verticals are also a good bet. Several commercially made verticals allow operation over the 40m band through 10m and are only 6.5m tall when mounted on the ground. A good set of radials makes this antenna reasonably efficient. The only drawback is that the omnidirectional pattern makes interference rather constant. The half-wavelength bottom-fed ham-band verticals currently on the market are quite decent DX antennas.

Another obvious solution is to use any of several inductor loaded antennas such as the loops and shortened dipoles shown elsewhere in this guide.

Some of these antennas can be quite useful. Also, I once operated with a pair of commercial coil-loaded mobile antennas mounted back to back to form a kind of dipole. Did it work well? No! But it worked well enough for me to rack up a lot of DX using an old Heath HW-101 transceiver.

I have also used commercial mobile antennas as fixed verticals. In one case, right after we moved into our first home, I mounted a Hustler mobile antenna for 20m on the window sill of the upstairs bedroom that served as my office (until the kids started coming along). A pair of radials out of the window, sloping to a fence, completed the 'ground' side of the antenna. It worked rather well, actually. Later on, I added 15 and 40m coils using one of those attachments that allow three coils to coexist on the same base mast.

It worked very well at 15m and 20m, and passably well at 40m.

SOME OTHER SOLUTIONS

FIG. 1 shows Marconi ( FIG. 1A) and Hertzian ( FIG. 1B) antennas for use in limited space situations. In the Marconi version, the antenna wire is mounted to lay flat on the roof, and then (if possible) to an attachment point on a support such as a mast or convenient tree. The Hertzian version is a dipole, preferably a half wavelength (if you want to use coaxial feed), positioned such that the feedpoint is in the middle of the roof line.

FIG. 1

The idea in building these antennas is to put as much wire in the same direction as possible, but don’t worry too much if the goal is not met.

The real goal is to get on the air or use your receiver, depending on your interests.

Another solution is the loop antenna shown in FIG. 2. The best loop is one that is a full wavelength on the lowest frequency of operation, but any convenient loop can be made to work at least part of the way. This is especially true if tuned parallel feeders and a decent antenna tuning unit (ATU) are used.

All of these antennas can be used either on the outside of the roof, or inside the attic. In neither case, especially the latter, would you want to try it on a roof that has a copper or aluminum base. That approach is not used on residential properties much anymore, but if you have an older dwelling the roof may well be copper covered.

Mounting the wire can be a bit of a problem, especially when the antenna is on the outside of the roof. What seems to be the best way is to use nail-in or screw-in stand-off insulators attached to the wooden roof. This works well only when you can seal it against rain water seeping in. Water will wick along the threads, and rot the wood - leading to leaks and expensive repairs. A better approach would be to design a wooden fixture attached to the soffits or overhang in a way that prevents extensive damage. Inside the attic, however, one can easily use standoffs, but they must be attached to a roof rafter ( FIG. 3), rather than the roof covering. A screw thread that penetrates to the outside surface will allow as much water damage (or more) than a thread going the other way.

FIG. 2

Marconi and Hertzian antennas may not be as good a choice as the loop because they tend to have high voltages at the ends. Use of even moderate RF power levels could cause an arcing situation that represents a fire hazard. As a result, even with the loop, if the antenna is used with a transmitter the power level should be limited.

These ideas work especially well for people with restrictive local ordinances that make decent antennas unworkable, or nasty neighbors, or restrictive covenants in their deed (and a militant homeowners association board that enforces them!). Some situations allow an antenna such as that shown in FIG. 1A or FIG. 1B as long as it’s not visible from the street.

Zig-Zag Dipoles

FIG. 3

The antenna purist would probably roast me over a slow fire like a marsh mellow for suggesting some approaches to the limited space problem. FIG. 4 is especially suited to drawing the ire of the pure. But it also works passably well for people with limited space. This antenna uses a zig-zag path for the dipole elements. Try to put as much wire in one direction as possible, and wherever possible make the pattern symmetrical. Although the angles shown in FIG. 4 are acute, any angle can be used. Of course, the closer the pattern is to the regular straight dipole installation, the better it will work.

The antenna shown in FIG. 4 is seen from the side, i.e. a ground view.

The zigging and zagging can be in any direction in three-dimensional space, however. Some combinations won’t work well, but others will work well enough to get you on the air.

You will find that the pattern suffers when this is done, but again the goal is to get you on the air - not to produce a perfect antenna. Also, expect to use a 'line flattening' antenna tuner to wash away the sins of the voltage standing wave ratio (VSWR).

FIG. 4

Linearly Loaded 'Tee' Antenna

The traditional method for loading an antenna that is too short is to put a coil in series with one or all elements. In the 'tee' antenna in FIG. 5, however, a different approach is used: linearly loaded elements. This antenna has been popular with low-band operators and receiver users who lack the tremendous space required for proper antennas at those frequencies. The overall length of the element (A) is usually around j/3, and the spacing between the three segments (B) is about 25 cm (a little more or a little less can be accommodated by the design). The vertical portion of the antenna (C) is a quarter-wavelength single wire, and is fed at the bottom by parallel tuned transmission line.

The critical lengths are

A = 50 F_MHz meters

B = 25 cm C = 75 F_MHz meters

The Right-Angle Marconi

FIG. 6 shows a simple quarter-wavelength Marconi antenna that can be used with 52 or 75 ohm coaxial cable. The antenna consists of two wire sections, each j/8 long, erected at right angles to each other. The right angle Marconi (sometimes laughingly called 'half-Hertzian') produces both a vertically and horizontally polarized signal. This method of construction reduces the linear property needed for the antenna by half, so is more accessible to a larger number of people.

Shortened Hertzian Radiators

The zig-zag dipole shown earlier is only one possibility for reducing the size of a Hertzian antenna. The antenna of FIG. 7 is a species of zig-zag doublet, but is a little less random in its construction than the previous antenna of this sort. The overall length is j/2, but the two j/4 elements are bent into right-angle sections of j/12 (vertical) and j/6 (horizontal).

The antenna is fed with open-wire parallel line operated as tuned feeders.

FIG. 5

FIG. 6

FIG. 7

The antenna in FIG. 8 looks like a folded dipole, and on one hand that is exactly what it is. The antenna length is cut for j/4 at the lowest frequency of operation. At the second harmonic of this low band the antenna acts like a folded dipole, while at the lower frequency it’s a species of 'tee' antenna. It’s fed with open-wire tuned feeders. If you want to operate it only one band (the lower band), and wish to use coaxial cable, then a quarter-wavelength matching stub can be provided, and the antenna driven with a 4:1 BALUN transformer.

The doublet antenna in FIG. 9 is a bit longer than 5j/8, and is fed by coaxial cable through a shorted matching stub and a 4:1 BALUN transformer. The overall length (B) of the radiator element is:

B = 102 F_MHz meters

FIG. 8

FIG. 9

The matching stub consists of a length of parallel open-wire transmission line, divided into two segments either side of the feedpoint. The dimensions of the two segments are:

C = 32 F_MHz meters

D = 8:8 F_MHz meters

The tap point on the stub represented by these lengths will provide a good match to 300 ohm twin-lead, or to 75 ohm coaxial cable if a 4:1 BALUN transformer is provided.

The extended folded dipole shown in FIG. 10 uses 300 ohm twin-lead for the folded dipole section (A) and single conductor wire for the vertical extensions (B). The length overall of section A is found from

A = 123 F_MHz meters

While the vertical sections are

B = 9:2 F_MHz meters

The antenna of FIG. 10 can be fed with 300 ohm twin-lead, but this requires the antenna tuning unit to have a balanced output. Alternatively, you can feed it with 75 ohm coaxial cable if a 4:1 BALUN transformer is provided at the interface between the coaxial cable and the antenna feed point.

FIG. 10

Limited Space Radial Layout

Radials are quarter-wavelength (usually) pieces of wire connected to the ground side of a transmission line at the antenna. These radials can be on the surface, under the surface, or elevated, depending on the particular antenna. The radials are used to form a counterpoise ground, i.e. an artificial ground plane. It’s seen by the antenna as essentially the same as the ground.

By the way, the preferred 'ground level' configuration is buried, rather than on the surface, because it prevents injury to people passing by, either from RF burns or stumbling over the fool wire. But what do you do in the limited space situation? A representative solution is shown in FIG. 11. The radials shown in textbooks are straight, and that is the preferred configuration. But if you don’t have the space, then you need to use some variant of the two paths shown in FIG. 11. The radial can either go around the perimeter of the property, or zig-zag back and forth in either a triangle or rectangle pattern (the latter is shown here). I have even tacked radials to the baseboard of a student boarding house room (not recommended over a few watts of QRP power levels).

FIG. 11

Imagination, a bit of engineering, some science, and a whole lot of luck make radio operations from limited space situations possible - or at least a lot easier.

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