Hy-Gain Trap Repair

Traps on Hy-Gain HF yagis are not really designed to be serviced by users. Of course we're hams so we do it anyway. Despite a lot of disparagement of traps including those of Hy-Gain yagis they can be effective performers. With the wide availability of inexpensive Hy-Gain yagis on the used market many hams will find themselves with a good tri-band yagi that has been in the air some time and may need some repair. The traps are usually the only daunting aspect of the job.

I purchased my TH6 and Ham-M rotator from a local ham back in 1985. At that time it had been on his tower for at least 10 years. In those early days the hardware wasn't stainless steel so much of that had to be cut off and replaced. The dreadful BN86 balun that comes with the antenna was soon discarded in favour of a coax choke. A couple of the traps needed service and I puzzled out their disassembly and repair -- I couldn't benefit from an internet search back then.

When done it performed flawlessly until 1992 when I dismantled the tower and antennas and exited amateur radio for over 20 years. I kept the antenna, and indeed I sold very little of my equipment in the intervening years. I restarted in the hobby in 2013 in a small way, gradually increasing the size of the antenna systems. The driven element was put to use as a trap dipole for a short time.

In 2017 the antenna was refurbished and raised to the top of my new 150' tower. A couple of mechanically suspect traps were given cursory attention since it was the onset of winter and time was of the essence. Trouble reared its head the next winter. I had planned to have it up there for no more than 6 or 8 months but changed plans meant leaving it up there another year. Hope is a poor strategy for ensuring antenna reliability.

Again I failed to repair the suspect traps when I had it on the ground this fall. It went intermittent during CQ WW SSB this past weekend. This time I would have to take action. It was a simple matter to isolate the problem. With an antenna analyzer attached I wiggled the elements within reach of the tower. The SWR bounced around when one half of the driven element was shaken. The trap itself wiggled which made the failure point obvious. It was a simple matter to pull the half element out and lower it to the ground.

Trap structure

Hy-Gain yagi traps are not designed to be opened and serviced. Of course countless hams, myself included, do it all the time. The first time a trap is opened it is difficult to separate the parts without causing damage if you are unfamiliar with how they're built. A mistake usually makes it unusable. It is worthwhile to learn how to do it properly if you insist on doing repairs yourself.

The first thing to know is what's inside: the components and how they're put together. All the Hy-Gain traps for the tri-band yagis are the same no matter the model, be it an ancient TH6, a relatively recent TH11, a TH3, an Explorer 14 and so on. Indeed there are only 4 traps for the tri-band yagis:

• 10 meters: there is a different version for parasitic elements on some yagis
• 15 meters, for directors and reflectors
• 15 meters, for the driven element

They are mostly distinguishable without the part numbers printed on the outer body, which is a good thing since the labels fade with age. The 10 meter traps are shorter than the 15 meter traps, the 10 meter and 15 meter parasitic element trap coils are wound with aluminum wire, and the 15 meter driven element trap coils are wound with copper wire.

Compare body lengths and peek in the weep holes and you have all the data you need to identify the trap. That said, I haven't directly compared the two versions of the 10 meter trap. Hy-Gain sells new traps at a reasonable price, and even most of the component parts. That's an alternative to repair. Go to their web site and search for "trap".

I pulled a broken 10 meter trap from my antenna junk box that I scavenged from another ham's antenna nightmare. The coil form and tab connecting the shell (capacitor) to the right tube are broken. I don't know the story of how it broke. Laying out the pieces as I've done gives an idea of what's inside. These traps are really very simple devices and quite robust when not abused in service, disassembly or reassembly.

The three sheet metal screw that secure the coil end and the shell tab are frequently criticized. While not a superb choice the screws are tough and have an integrated lock washer, and when the traps are properly assembled the screws are under very little stress. I judge the design a reasonable compromise between cost and robustness. The trouble comes when the traps are reassembled after service.

The plastic spacers are critical to the strength of the trap. Install them incorrectly and fatigue failures will occur. Their purpose is to keep the inner tube and coil rigid with respect to the outer tube. Since the weakest component is the coil (wire and form) these must not flex under wind and ice loads. When the spacers are properly positioned flex is minimized. It takes two spacers on either side of the coil to prevent the inner tube from flexing.

The inner end of the 10 meter traps (on the right, with the tab) flex the most since the spacers are close together. Over the years the tab can fatigue and break. This is usually only a problem on the driven element since the larger and heavier 15 meter trap on the outer end of the element increases stress on the 10 meter trap.

The spacers are positioned correctly when manufactured. Dimples (small depressions) in the shell hold the spacers in place. There are cavities in the spacers that align with the weep holes so that the spacers don't dam water flow.

To disassemble the trap remove the screw on the shell tab and push the inner tube left with respect to shell as oriented in the picture above. Some force may be needed to push the two left spacers over the dimples. Unfortunately the spacers may suffer some damage but there's no good alternative to getting them out. The spacers on the right will most likely stay where they are during disassembly and reassembly.

To reassemble the trap you push the inner assembly in from the left of the shell. If the rightmost spacers moved reposition them first. The one on the outside comes close to the tab. The other should be pushed from the left to its approximate position. Use the dimples as a guide but don't obsess over getting it perfectly aligned. But do try to have a weep hole aligned with the outer tube weep holes.

Even it isn't in far enough the trap coil screw push it further when it is inserted. Avoid this if possible since is a small risk of damage to the spacer and coil. If the spacer is too far outboard use a blunt metal rod to lightly tap it toward the coil through the outermost spacers weep holes, going around from hole to hole so that it doesn't jam. The most common reason for the spacer to be too far outboard is pushing the inner assembly too far to the right. Move slowly and watch for the tab screw hole to appear. Screw the tab to the inner tube.

Push a spacer in from the left until it is at or near its dimple on the left side of the coil. Hold the outer tube in a non-marring vice while for best control and least risk of damage to the tab. A 1" PVC pipe can be used to slide over the inner tube and apply even pressure to the spacer. Don't push it in too fare (right up against the coil) or you'll have start over again.

Finally insert the leftmost spacer and you're done. Hold each end of the inner assembly check for flex. There should be little to none. Remember to slip on the trap covers. I wrap the covers with Scotch 33+ for extended UV protection. Replacements are cheap if you need them.

Repair

For my repair job I disassembled the 10 meter and 15 meter traps. Although only the 10 meter trap was flexing I chose to do both since I had them in hand.

No wire breaks or loose hardware were evident. This can be a problem when the traps are mechanically unstable because the coil wires or forms will flex and eventually break. There was no damage of this kind just some debris and corrosion. An old insect nest was removed.

I undid the screws at either end of the coils and lightly sanded the aluminum surfaces to ensure good electrical contact. The inner end of the 10 meter trap tube was slightly distorted by the flexing so I pulled it from the coil form and gently applied pressure to the tube with a vice to reduce the freedom of motion it had developed. Perhaps a needless repair but I prefer that the form fit snugly.

As expected the spacers were out of position and that is what caused the flexing. The spacers were correctly positioned (as described above) during reassembly. The half element was then put back together and the dimensions checked. The entire job took less than 2 hours despite proceeding carefully. I wish that I'd taken the same care the last time I did this job!

Back in the air

For late October the weather was warm so I rushed to put the driven element back together before the weather turned. The high wind that day didn't dissuade me since the half element is easy to lift. I stuffed it and the insulator back into the boom clamp and attached the clamp and wires going to the balun and hairpin (beta match shorted transmission line stub). Within minutes I was ready to test my repair.

The element no longer wobbled when shaken. I connected the antenna analyzer to the balun and the SWR was what it ought to be. So far so good. To my dismay the SWR again misbehaved when I wiggled the driven element in exactly the same fashion as before the repair. That was not good. Although the poorly assembled trap need fixing the problem was evidently elsewhere.

Oops!

Further troubleshooting showed there were two problems not one. The first was found by wiggling every wire and connection on the balun, driven element and hairpin stub. The wire from the stub to the clamp on the suspect half element was intermittent.

It seems that after 45 or 50 years the stranded wire had few intact strands remaining. Nothing lasts forever. I now realize that this was the same intermittent problem that developed earlier when the antenna was on top of the 150' tower.

Hy-Gain yagis of that vintage did not use stainless hardware. The screws and nuts are pretty well welded onto the aluminum tubes of the stubs. Since I was not prepared to tram the antenna to the ground to destructively remove the hardware I used a hose clamp to attach the new wire to the tube.

The wire quality is not the best but it should hold through the winter. I put a few layers of tape on the wire insulation where it might touch the boom clamp for added protection from high voltage and stray capacitance.

Finally the antenna survived the wiggle test. The SWR tests perfectly on all three bands.

The next problem was further down the tower. Where the short length of RG213 from the antenna to connects to the long length of Heliax the UHF to N adaptor was visibly bent underneath the layer of weatherproofing. Although the SWR was fine in the shack this could not be left as is due to risk of imminent failure.

The Heliax terminated with a male N and the RG213 had a male UHF. Adaptors with an N female on one end and a UHF female on the other end are rare. The 10 meter length of RG213 I grabbed from my junk box had a heavily taped N female on it that looked perfect for what I needed to hook up the newly side mounted TH6.

When I cut away the layers of plastic and rubber I discovered the problem. This old bit of coax was older than I realized. Inside was an adapter I home brewed back in the 80s for LDF4 runs, all with N male terminations.

Oh, to be young and foolish again! Unwrapping this contraption brought back memories. The shells of the N female and PL259 reducer were merely pressed together. It had no mechanical strength at all. I'm surprised it lasted as long as it did. Only the tape holding the halves together allowed for electrical continuity.

The abomination was replaced with an N female barrel connector and a more common N male to UHF female adapter. I have both in abundance since I routinely connect Heliax main runs to RG213 and LMR400 terminations. I taped up the joints and this time I really did had the problem resolved.

Rush jobs coming up

The weather is finally turning colder, windier and wetter. Time to finish the many antenna jobs I have ongoing is running out. Just this week I found time to (finally!) complete my 80 meter vertical yagi. An article on that should be out next week. The 20 meter and 15 meter stacks are slowly progressing, with two yagis tested and working. I am rebuilding the boom of the upper 20 meter yagi so that it is stronger and lighter. Permanent gamma matches to replace the testing/tuning gamma matches are under construction. Other hardware to side mount and mast mount the yagis has recently arrived or will be fabricated shortly.

The time taken to repair the TH6 delays every other job making this problem very unwelcome. The antenna was really proving its worth during CQ WW SSB before it quit working. As the number of towers and antennas grows the probability of a problem arising increases. Problems come in all types. For example, this week I discovered a small pit dug into the bundle of Heliax and other cables running from the 150' tower. Some animal, probably a skunk, dug up a wasp nest that was down there. I inspected the cables -- thankfully there was no damage -- and refilled the hole.

Maintenance is never ending in a growing antenna farm. I am spending some time on the air, entering contests and working DXpeditions. I count myself fortunate to have the time to do all of this and for the friends willing to help even when the weather is unpleasant. They seem to enjoy being a part of this journey.

However it is better to do things right the first time and avoid unwanted repair jobs.