Wednesday, March 25, 2015

Buying a Used Yagi: Hams Ripping Off Hams

I described my research into choosing a tri-band yagi in a series of articles last year (2014). The antenna I eventually purchased was not the first one I looked at. Originally I preferred to locate a TH3 since that seemed ideal to my needs and tower capacity. The Explorer 14 I bought has the same boom length and more efficiency (4 less traps) but more wind load.

While shopping for a TH3 I encountered something unfortunate. There was a transaction involving an antenna where, in my opinion, one ham ripped off another. That story is the subject of this article. No calls or names are mentioned, and indeed I don't know the identities of certain parties. While stories like this are atypical in the ham community, fraud and frayed relationships sometimes just seem to happen when money is involved.

Inspection

I found a TH3 on the local used market and contacted the seller. He explained he didn't know much about antennas, especially yagis, and would willing to negotiate the already reasonable asking price. I drove over to have a look. Problems were already evident from the ground, which is not a good sign. For example, one element tip dipped downward from the outermost trap.

I climbed the tower for a closer look. More problems appeared. He wanted the antenna down and since he was a decent fellow I gladly performed that small task for him. It was already becoming clear to both of us that I would not buy the antenna.

With the antenna on the ground I began to disassemble it. It quickly turned into something like a forensic analysis at a crime scene. What I found shocked both of us. He soon realized he'd been had by the ham who sold it to him.

How we got here

Someone he connected with in a local club offered to sell him a yagi when he was looking for something small and inexpensive to work some DX on the higher HF bands. That individual was friendly and willing to help put the antenna on his tower. A deal was negotiated.

The seller delivered the antenna and assembled it. In retrospect the buyer realized he should have been suspicious since the antenna looked odd and the seller gave him no opportunity to consult the manual that he'd downloaded in advance. He quite sensibly wanted to make measurements and just to have a closer inspection of his first HF yagi. The seller explained he was pressed for time and wanted to be quick with the job. So up the antenna went and coax attached.

Signals were heard and the transmitter seemed happy. Money changed hands. But it was soon clear that not all was as it should be. The SWR was quite high in many places. When contacted the seller told him that was nothing unusual and just to use the rig's ATU to match.

He had no basis for comparison and so could not check gain and I don't believe he checked the F/B. He only knew that when he turned the antenna the desired signal peaked and he was usually able to get through. Of course even a dipole shows directivity, and a yagi is like a dipole in its basic azimuth behaviour: broadside gain and a null off the sides.

This is a real story and a real antenna so I am able to provide physical evidence. You'll soon see why I was outraged by what had transpired.

Busted trap

The cause of the dipping element tip was discovered when the mass of tape holding the tube to the 15 meter trap was removed and the trap opened.


Pretty, eh? The plastic coil form is severed and the tab connecting the trap shell (capacitor) to the element was sheared off. The stand-off toroidal insulators are heavily scored. A violent impact would account for this degree of damage.

Trap covers

All the trap end covers are taped. The few I uncovered were severely damaged by UV radiation. This is a case of the shutting the barn door after the horse has bolted. Tape (the necessity of which is debatable) should have gone on earlier to reduce sun exposure.


The decay of one cover was so bad that the tape was actually holding the cover together. In another case removal of the tape revealed an ill-fitting plastic plumbing cap rather than a Hy-Gain part. Hy-Gain sells replacement kits which are reasonably priced. There really is no excuse for what I found.

Parts substitution

One tube joint could not be taken apart. There wasn't even a clamp at the joint. A closer look (see upper right of above photo) showed no compression slots in the larger tube and that the tube was not square cut. This is a press fit that was likely seized by corrosion or an oversize wall in the larger tube.

I can only presume the original tube was damaged beyond repair and replaced by another tube that was not properly selected or prepared.

Hy-Gain multi-band yagis have few trap varieties. This is good design and good business. All the 10 meter traps are identical. The 15 meter traps are different for the driven and parasitic elements. This allows kilowatt power without heating the driven element traps to destruction. In a conventional yagi the driven element current is higher than the parasitic elements. These traps are wound with copper wire to reduce resistance and therefore I²R loss. All the other traps are wound with cheaper aluminum wire.

The difference can only be discerned by the part number printed on the affixed label (too faded to read in the present case) or by peering through the drip holes to inspect the wire colour. The wire in the trap wound with copper should be brownish instead of silvery gray.

Element-to-boom clamps


I found two cases of substitution of proper Hy-Gain parts. Have a look at the adjacent picture.

On the left is a clamp made from half of the correct part and half that homemade from sheet aluminum.

First, the alloy is unknown and therefore of uncertain strength. Second, it is improperly shaped. The tubes inserted in this clamp had their ends severely crushed.

Why someone go through so much misspent effort when a replacement part is readily available and inexpensive is mystifying. I can only imagine that it was someone with more time on their hands than good sense.

On the right is the clamp for the driven element. This clamp must be larger to accommodate the plastic inserts which insulate the driven element from the boom. This is needed for the beta match feed. Except I found that the wrong clamp was used. To get a compression fit to the boom the clamp had to be tightened so much that the plastic inserts were crushed and split (not visible in the photo). Further, the anti-rotation set screws were replaced with longer hardware to bridge the gap.

Boom

The TH3 boom is a little over 4 meters (14') long. It comes in 2 identical halves that are joined at the centre with a clamp.


The ends of the two halves (off the left side of the photo) are aligned. It is readily apparent their lengths are unequal. The longer is the correct length for this antenna. The other must have come from another antenna since it shows no signs of having been cut. My guess is that it is from a TH3jr (12' boom) since the length is correct for that antenna.It would also explain the lack of copper-wound 15 meter traps in the driven element. That is, this antenna may be a mix of parts from multiple antennas.

Not only is the resulting antenna physically unbalanced the tuning of the elements will be sub-optimal for the altered inter-element spacing. The result will be poorer performance and SWR behaviour. Even if the difference is small it makes no sense to put up a 3-element yagi that and not get the performance you paid for.

Notice the deep indentations in the longer boom half. It's worse than it appears in the photo. Similar damage is present at the position of every boom-to-element clamp. The set screws (2 per clamp, to help prevent element rotation) were overtightened, in a few cases piercing the boom wall. There are more dimples than set screws which tells me that this antenna was improperly assembled at least twice, with the boom partially rotated the second (or third) time.

Although this is not a large antenna there is no excuse for this abuse, which will reduce survivability in severe weather.

Hardware

On a somewhat positive note most of the hardware -- hose clamps, bolts, washers and nuts -- were stainless steel. However the sizes were often incorrect. This is the best I can do to say something positive.

Prescription

There is great fellowship among radio amateurs. We help each other out with antenna-raising parties, sharing expertise and software, advice and training, and even simply offering pointers to help out the novices among us. Elmers -- those hams who mentor others before and after they first join our ranks -- are rightly venerated.

Yet hams are only human. It is a mistake to imagine that we are all cut from better cloth. Just as in the general population we have our misfits, anti-social miscreants and worse. The number in the latter group is small but can have devastating impact when they prey among the novices in our hobby or those who are overly trusting of fellow hams.

We like to believe we are good judges of character. It is not so easy, though it can become easier as we grow older and wiser, often by the "benefit" of bad experiences. Getting ripped off not only cost time and money it can also cause acute embarrassment. I have seen disputes come close to fisticuffs in flea markets. I have looked sellers in the eye and asked hard questions about an item they were selling, and then watched them squirm. Even the dishonest have consciences and you may see it on their faces.

Too often we keep those incidents quiet, fearing that other will think us foolish. That can be a mistake, one that the criminal class counts on for the continuation of their careers. Perhaps my story can, in a small way, shed some light where it's often absent.

In any transaction where you lack expertise about a product or don't know what question to ask, bring a knowledgable friend along. It won't hurt and it can help avert a bad experience. When we lean on each other we are all stronger. Pass along the same favour when you have a chance. Just don't go overboard and become suspicious of everyone; happily the bad apples are the exception not the rule.

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