Refurbishing Old Trap Antennas
Hygain, Mosley, Cushcraft and others use traps to get the antenna to perform on several bands. In principal, trap antennas are still a very good compromise antenna for a compact multi-band design. But in time, the traps deteriorate and problems arise. The owner usually buys another antenna and sticks the old antenna underneath the house in a disassembled state where it deteriorates further. At the extreme end you end up with a pile of traps, with no labels or part numbers and with the plastic end caps cracked and quite useless. Worse, you have no idea what frequency that traps actually resonate on and whether they should reside on the director, reflector or driven element.
The manufacturers rarely design a trap to resonate where you think that they will resonate. A 15M trap that you may think should resonate on 21,200 Mhz will often be designed to resonate around 20.500 Mhz. Why? If the trap resonated in the middle of the band, then extreme voltages would exist at high power levels which will warm the trap and cause arc-overs to damage the trap. When the trap design resonates below or above the antenna design then those voltages are not a problem. You can research the Hygain trap frequencies on the internet. Typically 13.6 Mhz, 20.6 Mhz and 27.4 Mhz are common. Think about it. The length of the elements determine the resonance of the antenna, not the traps! A common trap (if you will pardon the pun).
Traps are assembled in the factory and can be disassembled. That’s a given. HyGain put a little dimple in the aluminium outside cover to hold them in place. The dimple may be drilled out and the trap coil will slide out of the outside cover. A little more theory here: The outside cover is actually the capacitor for the trap. The air spacing to the coil is the dielectric, or insulator. After the trap cover is removed from the coil you will see the ends of the aluminium wire joined to the tubes with a set screw. It is almost a certainty that the galvanized steel screw is corroded and the connections will be compromised electrically.
Now is the time to do something about it. Clean the wire loops, clean the tube surface that they touch and fit new stainless self tapping screw with star washers which will bite into the metals and make good contact. Before the coil is slid back in the shroud capacitor, take a stiff brush and clean the trap and end tubes in the kitchen sink with a good detergent. You will have purchased a set of new end caps from the manufacturer and now the traps may be re-assembled good as new.
There is a drain hole in each shroud. Make sure they are facing down when the antenna is up on the tower. A trap full of water will cause major embarrassment to the owner and is a source of great amusement to the next door neighbour when it is on fire! If you decide to use a balun, be aware that the standard HyGain BN-86 balun is not a good choice if you run 1 KW. They fail. They are fine at 500W. They should be mounted BELOW the boom, not above. And make sure you enlarge the drain holes when you check the internal soldering connections.
I have already discussed resonance above, but how do you determine it’s resonant frequency? The shroud, being a capacitor of under 50 Pf in value is very susceptible to surrounding objects which will detune the trap. The best way of measuring a trap frequency is to tie a loop of thin nylon fishing line around each end tube of the trap and hang it from the ceiling so it is in free space about stomach level ready for measuring. Then use your grid dip meter with its coil end on to the end of the trap tube about 10 or 15mm away. You may find that only one end will give you a resonant dip. take several measurements and get a precise average.
Don’t hold the GDO. Stack some boxes or a chair or whatever to get it in that position so you don’t have to hold it. You simply have to adjust the GDO looking for the dip. And it will be a challenge to find the dip because the “Q” of the trap assemble ywill be around 300 and the dip will be minuscule and very sharp and very easy to miss. Eventually you will find it. Write the frequency on the shroud with a pencil and move on to the next one. You will eventually will have 6 traps in pairs. Some traps will have different end tubes on them and you will know where they go. Download the manual and study it. Eventually it will make sense and you will be able to sort out what the trap is you have in your hand that has no label.
Where tubes fit into each other and fit into traps, the aluminium must be cleaned and shine for a good electrical connection. Elbow grease and steel wool will do the job. The insides of the tubing may be cleaned with a round file or a rifle bore cleaner. Old tubing clamps should be disguarded and replaced with new stainless steel clamps. Finally, a very fine coating of UTILUX jointing compound (Most electrical wholesalers have it in stock) or the like should be applied to the tubing surfaces before clamping.
An antenna refurbished and repaired in this way will give many more years of service. I have refurbished several HyGain TH3s, TH4s and TH6s this way and they worked as good as new.
73, Lee ZL2AL