Some Techniques for Building a QRP Field Vertical

L. B. Cebik, W4RNL (SK)



The requirements for a QRP field vertical--usually base-loaded--are few but critical. It must be sturdy and easy to handle in the field. The field may be anything from a hotel room to an actual meadow. The parts must be easy to hold but hard to lose. The entire structure should breakdown into a set of pieces that fit within a small bag or case.

Over the years, I have seen a wide variety of parts used for such antennas, some better than others. These notes simply add to the collection of techniques available to the QRP field operator, as he or she designs a personal field vertical.

Fig. 1. Basic elements of a vertical with base loading.

Fig. 1 shows the basic elements of a field vertical intended for base loading. We must have a main radiator. The height of this radiator is caught between two demands. On the short end, it should be usable within the 8' high ceiling of a hotel or motel room. On the long end, it should be as tall as we can manage for the sake of efficiency. The shorter the radiator, the greater the size of the loading coil and, hence, the lower the radiated power for a given amount of power fed to the system.

The antenna requires a support system that will keep it vertical and stable (even in a modest breeze). The larger the support spread, the sturdier the mount. However, once more we encounter conflicting needs. The support system should break down or collapse into manageably short pieces for transport and not be unreasonably heavy. The support system may also form the hub of the radials that we use to complete what is essentially a shortened vertical monopole. The better the radial system, the better the performance.

Finally, we need a way to feed the antenna. For this type of antenna, feeding also includes loading and matching. A shortened vertical has a low resonant impedance: the shorter the antenna, the lower the resonant impedance and the higher the inductive reactance required to offset a high capacitive reactance. We need a small assembly-- consistent in size with the pieces of the system--and a means of connecting it to the antenna and the radials.

With these essentials in mind, let's look at a few of my personal preferences for the pieces of the system.

The Main Radiator. Fig. 2 shows how I prefer to make up the main radiator for a field vertical. T6063-832 aluminum tubing is light and readily available from sources such as Texas Towers (http://www.texastowers.com). It comes in 6' lengths to avoid UPS excess shipping charges, but we shall use shorter sections than this length. The figure shows the largest diameter as 0.75". This specification depends on two major factors. First is the question of weight. The larger the tubing, the higher the weight. For a home ground- mounted vertical, I might start with 1.25" diameter tubing, but this antenna is for the field.

Fig. 2. A radiator made up of nested aluminum sections.

I use certified 6063-T832 aluminum with a wall thickness of about 0.05" for a reason: the interior will be smooth, allowing me to nest the tubing easily. Hardware depot tubing sometimes has a seam that roughens the interior. (At the end of these notes, I shall make some maintenance suggestions, including keeping the tubing interiors clean.) All of the sections shown in the figure collapse into a single unit that is still light, not to mention compact.

The section lengths are all equal, and we have a choice for that length. Ignoring the top rod, 4 sections of 2' tubing will make a 7' 3" radiator, which fits nicely under a room ceiling, if we overlap the section by 3" for strength. 5 sections of 3' pieces, including the top rod, make a 14' vertical, which is significantly more efficient as a ground-plane antenna. The choice depends on your intended operations and the length of your carry-bag.

Fig. 3. Key hardware items for the portable vertical.

For field use--but not for permanent installations--we may simplify the junctions. The figure shows small holes about 7/64" to 1/8". These holes are for hitch pins, shown in Fig. 3. The figure also shows the source: page 2971 of the McMasters-Carr on-line catalog (http://www.mcmasters.com). Hitch pin clips, also called hairpin cotter pins, come in a variety of sizes and materials. Stainless steel is best, but cheaper plated pins are adequate if never subjected to rain. The main radiator uses a pin size intended to hold 1/2" to 3/4" diameter rods or other similar fixtures. They also have large round ends for easy gripping. However, I hot-glue ribbons through the round ends so that the pins cannot hide in the grass.

Now we can return to the optional top rod of the main radiator. A pin of the size specified is a bit large for the 3/8" tube to 1/4" rod junction, adding a different hitch pin size to our hardware. As well, the weight per unit length of the rod is higher than that of the 3/8" tube. The difference will not make the rod unusable, but only a bit troublesome relative to our otherwise simple structure.

Drill the hitch pin holes through both pieces to be joined in one operation to assure alignment. Be sure to deburr the holes so that the drilled aluminum tubes still nest smoothly for storage. Since it is likely that the holes will be only close to perfectly centered, make a pencil line from one section to the next for easy field alignment. Renew the line after every few uses of the antenna. The hitch pin clips are easy to use and mechanically sound for this application. However, the antenna relies on the metal-to-metal contact between tube sections for electrical continuity in the antenna. The practice is satisfactory for short-term field use, but not for long-term permanent home installation. The condition of using this system is guaranteeing clean aluminum for the junctions.

The Support Stand. A 4-legged stand provides good stability for the field vertical under indoor or gentle weather conditions. The stand that I built from scrap PVC appears in Fig. 4. It consists of 5 separate parts. 4 of the parts are 2' long legs of Schedule 40 1/2" nominal diameter PVC (Part A in the figure). Each leg has an in-line junction cemented on the outer end (Part B). This junction allows you to insert additional lengths of tubing for longer legs and added stability. As well, you can drill the junction to accept aluminum tent pegs to hold the legs firmly to the soil in the field. Alternatively, you can bend stiff steel wire into Us that fit over the legs near the outer ends: press these Us into the soil.

Fig. 4. Details of the 5-piece PVC support stand

The hub of the legs is a 4-way junction of the same PVC material (Part C). However, I modified the hub by drilling a 5/8" hole through the center. Into this hole, I cemented an 18" length of 1/2" nominal CPVC tubing, which has a 5/8" outside diameter (Part D). Because CPVC has a thinner wall than Schedule 40 material, I inserted a 3/8" wood dowel and then filled the interior of the tube with fiberglass resin used in auto repairs (Bondo). This addition stiffens the vertical section against stresses created by the main radiator.

At the base of the CPVC is a 1.5" length of 1" diameter aluminum tubing (Part E). On opposing sides of the short tube, I installed 1/2" long #10 stainless steel bolts, with the heads inside the tube. A little filing flatted the round heads so that the tub fit over the CPVC. The figure shows one of the bolts aligned with one leg of the stand, but that is a function of my limited drawing skills. The bolt actually is aligned between the legs on opposite sides of the tube. To center the tube and provide an insulated separation between the 1" tube and the main radiator, I added a very short section of 3/4" nominal diameters CPVC (Part F). The 3/4" CPVC fits inside the 1" aluminum tube and over the 1/2" CPVC. I cemented the CPVC separator in place, thus locking the short base aluminum tube in place as well. The main radiator simply slips over the CPVC and slides down to the separator.

The small base aluminum tube serves two purposes. With a wing nut on each bolt, the tube forms the hub of any radials used with the antenna, as well as for any special ground rod connection. I recommend a full set of radials for each band. You can construct these from flat 4-wire TV rotator cable, cutting each strand to a quarter wavelength for 10, 15, 20, and 40 meters. The exact length is less important than the presence of as many radials as possible, with a field minimum of 4 recommended for adequate efficiency. Connect the 4 strands together at the inner end and use a ring connector under the wing nut and over the hub bolt. Above the hub bolts, we may connect the match/load/feed system base, with its top end going to the main radiator.

The Match/Load/Connector System. Fig. 3 showed not only a sketch of the hitch pin clip, but also the outline of a handy tool clip. The clip is also available from McMaster-Carr (page 2726) and comes in numerous sizes in both plated and stainless types. Once more, stainless steel is best for durability, but plated clips will last well if not subjected to rain. The size shown (1/2" to 3/4") will work, even with the 1" base tube, but a larger size is also dandy for a base coil and feed system.

Fig. 5. Mounting plate for the loading and matching components.

Fig. 5 shows the outlines of a plate that holds the loading and matching components. I recommend 3/16" thick (or thicker) Plexiglas or polycarbonate. The part of the clips with a mounting hole (which uses #6 hardware) comes arched. As you tighten the hardware, the section flattens against the plate, forcing the spring section to close very tightly. When fully tightened, the arch exerts considerable force on the plate and can deform 1/8" thick material over the span of a few hours. The stiffness of the tool clips makes excellent electrical contact with the tubing.

The plate shows a sketch of a simple loading system using two coils. The high-band coil covers 10-20 meters with an 8-10-foot vertical. An added plug-in 10 micro-Henry coil allows loading on 40 meters. A lead from the coax center pin to coil taps does the tuning for the simplest version of this base-loaded vertical.

However, finding a tap that gives a low SWR is often difficult with higher levels of loading. The resonant impedance gets lower and lower, and by the time we find a tap near 50 Ohms, the reactance is considerable. To overcome this problem, you may add a capacitor across the coax terminals. A receiving variable with about 1200 pF total capacity should work with all but the shortest verticals on 40 meters. (Higher bands are less problematical.) The physical size of the capacitor depends on the desired power handling capabilities. For an old-fashioned multi-section receiver capacitor, you can offset the tool clips toward the coax connector side of the plate. There will be about 1/2" of space between the plate and the main radiator, due to the shape of the tool clips.

Adding the capacitor converts the simple base loading system into an L-network for matching low load resistances to 50 Ohms while compensating for the capacitive reactance of the short vertical. Jumpers can use banana plugs and chassis jacks to simplify the overall set-up. How much of the assembly consists of plug-in components and how much you permanently mount to the plate depends on your operating needs and the sizes that you choose for the main radiator sections.

I have not given any specific values on a band-by band basis for some components, especially for the loading coils and the L-network capacitor. My reasons are many. First, the required values will vary with your selection of the main vertical length. Second, they will vary with the height of the antenna base above the real ground. Third, they will also vary with the number and length of the radials that you use--and somewhat with variations in how you arrange them. Two 10 microHenry coils--one for the upper bands and tap-able, the other a fixed coil added in series for 30 meters and below--should handle most cases, along with a 1200 pF shunt capacitor on the coax side of the coils, if you choose the L-network option. (You may parallel a 100 pF capacitor with the big one to provide a fine-tuning control.) Nevertheless, there are too many variables involved in the personalized versions of this system to guarantee a good match in every circumstance.

Fig. 6. Other ways to use the tool clips.

The tool clips are handy in a variety of circumstances that range from field antennas to prototype experimental designs. Fig. 6 illustrates a couple of situations where they work well. The simple plate and its coax connector are useful for feeding a dipole center, illustrated by the left side of the vertical sketch in the figure. By using a fiberglass rod inside two sections of tubing--fixed in place with hitch pins--the center connector simply snaps into place. For mid-element loading coils in field or trial use, the plate and tool clip system, along with a fiberglass separator, allows one to develop the exact loading coil or trap needed for a given design, all without having to remove and replace screws in the element.

Maintenance. The system just described requires no tools in the field. The tool clip hardware is shop-tightened and requires no further field work. Everything simply snaps into place, whether a hitch pin or a tool clip. This feature allows for quicker assembly and disassembly, leaving more operating time. The entire collection of parts (nested main radiator, 5-part support, and matching/loading/connector plate) fits into a bag as short as 24" long. (Extras, such as additional field braces, SWR meters, etc., are the user's responsibility.)

However, the system will only work well if you give it the required pre- and post-use maintenance.

1. Before use: Test assemble the antenna to ensure that all parts fit together as prescribed. Perform any adjustment necessary (such as burr removal). If the aluminum has become cloudy during storage due to oxidation, clean the surfaces with a plastic scouring pad. (Do not use steel wool or course sandpaper.) Check all coils for electrical and structural soundness. Count parts to ensure that everything is available.

2. After Use: Clean all parts of the antenna thoroughly. Remove any dirt from the stand, using any good cleaner or detergent. Check for use damage to the vertical section of the stand to ensure a smooth fit with the lower aluminum tubing at the next use. Clean the outer surface of the aluminum tubes with a mild cleanser to remove field dirt and stains. Clean the inner surfaces of the tubes with a stiff long-handled bottlebrush. Recheck the coils for any damage--and repair immediately. Re-wrap components together to restore a factory-like presentation. Count components to ensure nothing is missing. Replace any missing component immediately.

With these measures, the antenna should provide many years of satisfactory service. However, I fully suspect that long before the antenna wears out, you will be experimenting with other improved field antennas.

The ubiquitous base-loaded vertical is far from the most efficient antenna that you can use. However, it is cheap, compact, and reliable, even if the gain goes down with the frequency and the amount of loading required. The system shown here uses a longer main radiator than most whip-based systems for a modicum of higher efficiency. The L-network can provide a somewhat better match for the rig. Even with these improvements, the system remains light for easy field transport. It requires no tools for assembly and disassembly. Allowing for leftover aluminum tubing and hardware bought in excess quantities (all useful for other projects), the net cost is about $25 to $35, depending on the sources used and the size of your junk box. (If you purchase components for the matching/loading system, the cost may go up accordingly.)

Even if you do not build your own field vertical, perhaps the use of hitch pin clips and tool clips will give you some ideas for other projects.

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