Support Bolt

Fig. i. Details of overall construction.

Fig. i. Details of overall construction.

I held the foil on the wood with a generous dose of glue. Position the four blocks around the inner tubing and use dabs of epoxy to fasten them to the pipe. This is only really necessary on the lower blocks because they will be too far down inside the outer pipe to be reached, The blocks at the upper end of the pipe can be tapped into place at the top of the pipe.

As a means of supporting the pipe at the correct length, two small aluminum bolts are used to hold the inner pipe in position. These are deliberately placed at the lower end of the inner pipe to avoid any mechanical weakness at the junction.

We cannot really depend upon this for good electrical connection, although it will likeiv be quite sufficient. However, Fig. 3 shows the bridging connection at thejunc-tion of (he pipes. This is made quite simply by using strips of grounding braid and stainless steel iiose clamps.

Fastening the completed radiator in position is the next siep. The simplest way is by using two large muffler clamps, well primed with several coats of rust inhibitor. These are ideal for gripping the large tubing firmly and yet not denting it. The clamps are placed around tire tubing and mounted onto the two angle aluminum cross pieces on the frame.

At this point, decide upon the distance you want from the ground level to the bottom of the radiator, and then slide the radiator through the clamps until it is in the required position. The distance will depend somewhat on personal preference or availability of space for radials. If you intend to bury the radials, the base can be very close to the ground, but in my case I have the radials above ground and sloping downward at a 10-degree angle. This gives a little better match for RG-8/U cable, and also allows the radials to be rolled up easily when summer lawn mowing becomes necessary.

Before attaching radials, the antenna can be raised into position and guyed. By pegging the base to a short pipe or stake driven into the ground, the antenna can be raised by one man. However, have a helper on hand to tie down the guy wires or disaster may overtake you! Two sets of guys are used. The lower guys are attached to the main frame, and can be your choice of materials. I used heavy aluminum clothesline wire covered with heavy plastic — it's easy to work with and not prone to kinking. A strain insulator in each guy breaks up any possible resonance in the wire.

Mark the positions for the three lower guys, in an equal triangle and about 10 ft out from the base. The stakes should be at least 4 ft long and driven into the earth to almost their full length. As a suggestion, drill the ends of the stakes and insert a steel U-bolt, which will offer much easier attaching of the guy wires. A steel turnbuckle between this U-bolt and the guy wire will offer precise adjustment of tension on each guy and save a lot of bothersome hit-and-miss tightening later. I have also extended the guy wires from the turn buckle back to the base of the frame. This prevents any tendency of the base to move sideways under high winds.

The upper set of guys are made from an insulating line (nylon, polypropylene, etc.), but try to use a line with minimum stretch and also one which is impervious to rapid changes in temperature or wetness. Be especially certain it will not rot. If available in your area, it's hard to beat the woven line used for deep-sea fishing nets. This is virtually indestructible and good for years of use.

Fasten the upper set of guys directly to the radiator at the top of the 3 in. pipe. These guys may be fastened to the same stakes as the lower set, but a much stronger arrangement can be had by using another set of stakes some 5 — 10 ft farther out from the base.

When the antenna is raised and guyed, the radial system can be installed. How many? Simple. The most you can get. More radials mean more efficiency and lower angle of radiation. Of course it's very hard to measure the slight improvement that one more radial makes, but 10 more radials inner pipe spocer slocks (see text)

outer pipe

;lower end of pipe onlv) Fig. 2. Details of pipe junctions for tight fit.

Fig. 3. Method of electrically bridging the junction of the radiator sections.

may get your attention..Thfey can be made from any wire you can get.

The length of the radials should be about 10% longer than the radiator. The ends of the antenna can be all cleaned, soldered together, and then clamped or soldered to a short piece of copper braid which in turn is soldered to the shield of the coax fcedline, either directly or via a connector. Although the ideal pattern for the radials would be to space them equally in a circlc around the radiator, they can be bent or crammed into whatever odd space is available without seriously lowering the efficicncy of the antenna.

Standard 5Oil cable is used to feed the radiator, and direct feed to the base of the element will result in a reasonable swr in the neighborhood of 1 :1.5 or a little lower. However, a closer match (and an swr nearer unity) can be achieved by use of a coaxial matching section.

The antenna will perform very well, I believe my best DX (in rarity, not miles) is FBSXX, but the log contains many QSOs with VK, ZL, 9J2, 5H3, 524 and most European countries. The wind loading is slight, and with reasonable attention to painting the frame occasionally and checking the guys, the antenna should give excellent satisfaction for many years.

0 0

Post a comment