Ever have one of those “little things go wrong” that turns into a massive time-sink?  One such “quick fix” bit me on the behind this week and as soon as this morning’s column is done, I’ll be back after it.

Ham Antenna School

I won’t try to explain the ARRL’s Antenna Handbook – because honestly, most 4-year electrical engineering grads will still be challenged by some of its contents.  But I can give you a top-down, recipe approach to my Off-Center-Fed Dipole (OCFD) antenna which is giving me fits.  Basics first:

• An OCFD is a half-wave antenna.  Meaning one-half of a radio frequency’s wavelength at the lowest operating frequency.  Which for me is 3.5 MHz – low end of the 80-meter ham band.
• The length of this antenna (overall) is about 468’/3.5 which pencils out to 133.7 feet long.  Since this is a decimal length, we convert to inches by multiplying the 0.7-part times 12 and arrive at 8.4 inches.  So, 133 feet plus 8.4 inches overall.
• Antennas have a “characteristic impedance” when installed.  You of course know what resistance is in Direct Current circuits.  Impedance of an antenna is the A.C. (alternating current) equivalent of resistance as installed.
• Now, a simple dipole, fed exactly in the middle, will – at some elevation over ground – have an impedance of somewhere between 40 and 80 ohms.  Picture a “normal Gaussian curve” centered at around 50-ohms.
• The energy transfer is maximal when the antenna “feed line” (like coaxial cable) matches the load (the antenna) perfectly.
• In a dipole antenna, typically the center feed point will be around 50-ohms.  Which is why coaxial cable is so popular.  Because its characteristic impedance *(of an infinitely long feedline) is also 50 ohms.
• When there is a “mismatch” it causes “standing waves” of energy to arise on the antenna system.  This is quantified by the (non-dimensioned) number called a “standing wave ratio.”  Thus, if the antenna impedance as installed is 275 Ohms and yet we insist on feeding it with coaxial cable, then this (275/50) number results in a “Standing Wave Ratio” (called SWR) of 5.5 to one.
• Generally, ham radio fanatics will tinker with their antenna systems in order to arrive at a maximum SWR of no more than 2:1 on any operating frequency.  (Trust you are following this so far?)

Special Case of OCFD Antennas

Thus far, we have been talking about a “normal” center-fed half-wave dipole, meaning a quarter wavelength on either side of the center.  Call it 67-feet to ballpark this in your head.

There is one problem with the center-fed dipole:  It does not work on even harmonics worth a dam.  Because its impedance goes skyward.

An 80-meter center-fed dipole may present that ideal 50-ohms on the 80-meter band at 3.5 MHz, but when we go up to the 40-meter band at 7 MHz, the impedance rises to as much as a few thousand ohms.  IF – just to pull a number here – the impedance on 40-meters is 1,568-ohms, then the SWR is now (1568/50) which is spoken of as a 31:1 SWR.  In other words, useless.

Harmonics are always a “times two” calculation.

On the third harmonic frequency (3.5 X 2) is your second harmonic and doubling this (7 X 2) we get to 14 MHz which – around here, anyway – is our all-time favorite ham band.  Daytime global distances are easy-peasy when the sunspots are right.

Ham radio types realized that there’s nothing holy and sacred about feeding an antenna in the precise middle.  “What, if we fed it at a different position, wouldn’t we get a relatively common impedance on many ham bands, not just every-other harmonic?”

Indeed, it is so.

The two popular offset5s from the “center” are one-third of overall length and 20 percent of overall length.  The one-third offset feed point is a “traditional OCFD” setup.  They generally run about 45 feet on one side of the feed point and 90-feet on the other.

All great – except the 50-ohm feed line coax might now look at 200 ohms on 80 meters, 300 ohms on 40-meters, and 175-ohms on 20-meters.  The measured results will be determined by ground conductivity and height above ground.  Higher *(over 35 feet, or so) and the antenna impedance will change, but on the higher bands (like 20 meters) the take-off angle toward the horizon will decrease which is great for working DX (short for far away stations).

On the OCFD, a matching device is used to bring the (roughly) equally bad SWRs down to something coaxial cable will play with an be happy.  The easiest solution is called a Balun (meaning a balanced output to an unbalanced (like coaxial cable) input.  These are available in a fair range up to 49:1 but for OCFDs a 4:1 to 9:1 is most common.

So, There I Was

A north-south OCFD and my super-antenna OCFD variant east-west.

I decided to use one of my classic Icom 761’s on 20-meters on the north-south OCFD.  As a matter of good operating practice, I checked the SWR.  It was reporting back 8.5 to one!  Unusable, in spite of the 761’s onboard antenna tuner to “flatten out” mismatches.  Won’t handle one that large well.

The first trouble-shooting test was to lower the antenna, unscrew the coax from the balun.  It its place, a 50-ohm “dummy load” (non-inductive resistor) was used.  Which revealed the likely culprit was NOT the coaxial cable connections because the terminated feed line reported an SWR of 1.05:1.

Oh-oh…something else had bitten the dust.

The second stop was to immediately suspect the 4:1 balun. Sure enough, it was full of water.  Not only that, but it looked like it may have been zapped last month by an incredibly close lightning hit that was so close there was no discernable delay between its flash and the sound report from air splitting!

I couldn’t tell if the balun had been sprayed with a coating, or whether that was from lightning damage.  It had to go.

The next stop was to model the antenna location and decide what would be the best balun to pull out of back-up stores.  A quick visit to the antenna modeling software (which you can still download free from Ray Lewallen’s site here) suggested that a 2.5 : 1 ratio would work well.  Best of all it would have some nice gain over a center-fed antenna on the 20-meter band:

To keep the story short:

An hour, or two, was spent getting wires ready, stripped, and set to replace.  The new balun was drilled in a couple of places so water could drain out of it.  When people put up outdoor weatherproof boxes, they overlook condensation – which is where the water likely came from in the suspect balun.

A lot more sweat – it was 90 and humid as hell from the rain the past couple of days – and it was back up in the air.

And the SWR?  Well, NOW it’s 9.55 : 1. 

Which leaves only one other (really off the wall) suspect:  The antenna wire itself.

The Modeling program says these ought to be:

As you can see in the yellow area, the 90-45 dimensions are not what we come up with.  So having wire (and swaging gear) we’ll do what the software proposes.

If you’re a gearhead/car dude, this is about like shaving off half a second in the quarter-mile, but similar thinking:  Half a second here, half a second there…next thing you know you’re running against AA fuelers, right?

Which is where we will go today as soon as there’s light.

Key Takeaways

The main thing to grok is the “troubleshooting process.”

In my time in electronics (over 60-years now) the process goes something like this:

• Step 1:  Go to the area of suspected damage first. Look closely.
• Step 2: Go back the “signal path” and start from the good end, next.
• Unless there’s no power in which case you start at the power source.

In my case, discovering a Balun full of water was a good find, no doubt.  But with good coax, could it be the double run of thin wire has broken somewhere?  It’s a hard-drawn pair of thin stranded wire and is suspect.  So that will be checked next.

Same as when troubleshooting a radio receiver:  After confirming the power supply is correct, you go back to the speaker, inject a signal and make sure the audio stage(s) work.  Then back to the detector stage, then back up the intermediate frequency amplifier string, then the mixer… although you can skip a lot of steps by just injecting some audio at the detector…

Point being, you narrow the search progressively until you finally nail the culprit – which is why ham radio is so much fun.  Part ancient telegrapher, part detective, part RF design engineer, part community-minded hobbyist, and so forth.

Winter Shop Tips

A few metal roof joint cracks have evolved from the summer sun-blast this year.  So, first sign of cooler weather, time to get up the roof and sweep things off.  Leaves and twigs can form dams – especially around vents – which can lead to nightmares when the heavy rains show up.

Since the odds of some really bad economic times are rising, we had a bid this week on a roof replacement.  Better to do those kind of projects now, rather than wade into an uncertain future only to find a year or three in you have a roofing project come up with there may not be materials or people to get things done and kept up to snuff,

Picked up a new winter jacket – some really nice ones are out there and cheap now.

Time to repair and refill the BBQ for the fall cooking season.

Fall/winter garden should sprout over Labor Day weekend, if you haven’t gotten that one done yet.

Not too early to work on outdoor stormy weather pet shelters, either.

For the Road

Quick way to check a balun?  Put what you think it should be (like 200-ohms on a 4:1 balun) across the balun output.  Then measure the input with your SWR tools.  Should be close to 1:1 SWR if your 4:1 has 200 ohms on the (antenna) load side and is doing the job.  If not, you still have balun problems.  Which we’ll get to just as soon as the Sun gets its lazy-ass out of bed!

Wait!  Is this a sign from God/Universe that it’s OK to buy a time domain reflectometer? (TDR)  Just $167 on Amazon now…$600+ for Fluke…

Write when you get rich (or work all continents in a single day),

George@Ure.net (ac7x)