Monthly Archives: November 2014

Universal Balun

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Baluns may be would in all sorts of configurations to change impedances and change input/output line feeds. One of the two most popular baluns are the 50 Ohm Un-balanced to 200 Ohms balanced configurations. The other is the 50 Ohms un-balanced to 50 ohms balanced configuration. The first balun 50 Ohms to 200 ohms are often used to feed Of Centre Fed dipoles and G5RV and antennas while the second balun 50 ohms to 50 ohms simply provides a match and keeps RF off the feedline. Both configurations require the same initial toroid and windings to cover to HF bands from 3 t0 30 Mhz with little actual loss.

One toroid wound the same but connected differently for two different applications.

One toroid wound the same but connected differently for two different applications.

The photo shows how the windings are wound on the toroid which must be robust enough to handle 1KW and be wound with fibreglass tape to prevent the copper windings digging in on the edges of the toroid and shorting a few windings or arcing into the ferrite toroid under higher power transfer.

The finished toroid Balun ready to to be sealed in it's plastic box and connected into the dipole antenna

The finished toroid Balun ready to to be sealed in it’s plastic box and connected into the dipole antenna. This particular version used 16 Turns of #14 wire evenly spaced around the  T-43 ferrite core. The core was 55mm in diameter and 20mm thick. Amidon in the USA and many other suppliers around the world sell them.

 

 

The finished balun should be put into a suitable plastic box sealed from weather and rain with suitable connections to each leg of the dipole and and with an SO-239 socket for the coaxial feedline.

Baluns are not difficult to wind and are very much less expensive than buying a commercially made unit.

73, Lee ZL2AL

Miracle Antennas

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Over the years every ham has read the advertisements about the various incantations of various types of vertical antennas. “Pick me, pick me, pick me” they all shout. And of course they are backed up with the usual testimonials. Published gain figures are dubious as they are often compared with antennas we have never heard of at locations that don’t exist. Of course we can always search eHam reviews to get a better idea of what any given antenna is like. Even after you have done your research and made your purchase, your results will vary as your soil will be different, your height above ground, whether or not you have used radials and the list goes on. The other variable that cannot be underestimated is the skill of the op. A really good op using a poor antenna will usually outperform an unskilled op using a great antenna. Indeed – mileage does vary.

One local trader on our main NZ trading site is currently listing OCF (Off Centre fed) Windom antennas and making outrageous claims in his advertising pitch to get you to buy his antenna. The vendor suggests that we look up K4(Callsign withheld). I have just researched the DXCC lists and he appears NOWHERE on any recognized DXCC or CQ listings in any mode. If the antenna works as well as he says it does, he would have the plaque on the wall. Surprise, surprise. “Work 200 countries in a weekend” sounds good when you read the advertising copy. It would be foolish to believe that the antenna would do it for you.

At the end of the day, the OCF antenna actually does work OK but uses a bit of jiggery pokery to the lengths of each side and a few well placed ferrite cores to make you think that it is more than a dipole. It isn’t a dipole as we all know! It is a dipole of suspect parentage.

Advertising is advertising. Claims are claims. Our trader is using the same techniques that the purveyors of women’s anti-wrinkle cream use. It is the same language that Proctor & Gamble have used since the last century to sell toothpaste, which after all is simply cream and a bit of abrasive that makes you feel good when you blind the women you are chatting up in the pub. And visa versa.

Over selling radio antennas is the not the sole domain of our trader. Gotham Antennas in the USA sold untold 23′ lengths of aluminium in the 1950s. Man, you were nowhere until you had a Gotham vertical. “Work the world with a Gotham vertical” was the slogan And did it work; just ask the man who owns one. (It did, after a fashion if your put attached radials and mounted it above the garage roof and used an antenna coupler. Sorry, you don’t get that info in the ad!!)

Original Gotham Advertisment in the 1950s

Original Gotham Advertisment in the 1950s

The USA was not the only country that they had the ultra compact answer to the 4 el full size yagi. The Partridge Manufacturing Company in England marketed a 6′ tube wound with wire indoors in the corner of the shack that they said would outperform anything. And of course the testimonials where there on the page to backup their claims. And of course, thousands of hams around the world bought the “Joystick” but there wasn’t much joy after you paid over your money. Just a pain in your bank account.

A home brew version of the "Joystick"

A home brew version of the “Joystick”

An original "Joystick"

An original “Joystick”

So the cry is “lynch the trader, hang em high and bring out the Fair Trade Act”. Being drawn and quartered at least would satisfy our thirst for blood.

Is this going to stop the vendor from printing his advertising crap? I doubt it. What will stop this sort of thing is every club that has newcomers to the hobby making sure that the vendor’s products and claims are well known at club level. Clubs and locals have contacts with most new amateurs and can do that. It comes back to what I mentioned a few months ago about mentoring the newcomers, getting close to them, advising them that antennas can be made that will work well for a fraction of the price.

What I am saying here is to look at the adds, do your research and the ask around in your amateur community for opinions and what they use. It wont be long before you will realize that you don’t get something for nothing in this antenna business. Gain is hard won by plenty of resonant wire or alloy up very high and rotatable.

Complaining about rogue traders in the ham section of eBay or Trade-me and burying your head in the sand with newcomers is not smart which is why these traders survive. They may not get a lot of repeat sales, but he survives. I won’t discuss pricing, as that is a whole new can of worms that I have no intention of opening except to say that every trader must make and is entitled to a profit to stay in business either on turnover or markup. Oil companies do it on turnover and jewellers do it on markup.

At the end of the day… it is and always has been Caveat Emptor. As experienced capable amateurs, we can mentor newcomers and cut down on this sort of trading abuse.

DX IS!

73, Lee ZL2AL

Refurbishing Trap Antennas

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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.

Trap resonance
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).

Trap maintenance
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.

Trap Resonance
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.

Antenna re-assembly
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