In just a few short months, winter will be upon us. With it, ham radio comes into its own. Nightime propagation in winter can be spectacular on the lower ham bands; 40, 75/80, and 160-meters. These correspond to the 7-7.3 Mhz, 3.5-4 MHz, and 1.8-2 MHz bands.
During summer months with longer hours of daylight in the Northern Hemisphere, even at the lows of Solar Cycle 24 (going into the upswing of Cycle 25 in a year or so), the maximum useable frequency (MUF) somewhat dependably rises to 15 MHz during the daytime. At frequencies above the MUF, signals are radiated into space, while more than 5-10 MHz below the MUF, there’s not atmospheric (F-2 layer ionospheric) “bounce” – what the CB’er’s called “skip.”
When the Solar Cycle is at it’s peak (as it was around 2011-2013) the MUF was even higher, sometimes reaching above the six-meter (50-54 MHz) band. At 14 MHz during this bonanza time, I worked many stations in Europe with a 3-watt, home-brew QRP (low-power) CW rig. Since the Cycle is bottoming now, I’m back to high power; linear amplifiers and a renewed quest for the perfect antenna.
Is There a “Perfect” Antenna?
Like boats, sports cars (and relationships, lol), it depends how much time and money you are willing to invest.
A couple of truths about “Cycle-Low” antennas:
- The Low Band Antennas eat all kinds of real estate.
This is because the most useful common size & type is a “half-wave dipole” antenna. The simple formula for antenna length is (roughly) 468/frequency in MHz. So, to illustrate, if we design a ½-wavelength antennda for the middle of the 75/80 meter band – 3.75 MHz, it means the antenna should be approximately 124.8 feet in length.
Because a di (meaning two) pole antenna consists of two quarter-waves (in this case of 62.4-feet each) the first complication is the antenna feedline must be attached at the mid-point. 99-times out of 100, there’s a house in the way.
The second problem? The Perfect antenna would also be a quarter-wavelength above the ground over its entire length.
About here, we run into other stumbling blocks, not the least of which is finding 60 foot- high support poles. Especially in a city, especially where there are neighbors, and views, and Homeowner Associations, and other “obstacles to the hobby” like that. Insurance companies aren’t keen on things that can fall on some else’s property.
Still, the proven dependability of the inverted-vee antenna – essentially with a single pole in the middle sloping down to about 10-feet (or less) on each end, is a workable compromise. With this kind of antenna, unlike the usual dipole radiation pattern where the maximum power is radiated at right-angles to the wire, the inverted-vee configuration’s maximum horizontal lobe remains at right-angles, but the vertical component of the signal become end-firing.
Inverted Vee antennas are a great choice for US hams. You simply orient the antenna wire so it points at the “other corner” of the country; perpendicular to the wire.
A California station might run their wires north-northwest to south-southeast. This would position the horizontal lobe into the Northeast corridor. The vertical lobe would end-fire, giving a great “look” into Russia and east into China/India and Japan via the north (magnetic) pole. KL7’s (Alaska) ought to be good, too.
How Can a Dipole Be Improved?
The simplest answer? Higher support poles. Again, there comes a practical limit. Even those of us lucky-enough to have towers with higher band beams on them, still have the problem of the wire ends.
My tower is 52-feet up to the wire antenna pulley sits (and 8-feet further up to the beam). How to position the “two ends?”
As luck would have it, a few years back I picked up a used 43-foot MFJ aluminum vertical. This will be placed in service from the top of the carport on the south of the shop/office/ham shack building and that will result in about 55-feet of altitude 61 feet from the tower.
The other end is being “shopped” right now. Several good options are available. One that gets great reviews is the 28-foot Jakite thick pole (desinged for flying kites and wind-socks) available from www.jackite.com. $70-bucks and a great deal.
Competing with that option is a higher 50-foot pole (and $239 dollar) pole from Max-Gain Systems. This option is a bit more money but if you’re willing to put in the recommended guying, that’s certainly an attractive option because there’s no substitute for altitude with antennas! See www.mgs4u.com/fiberglass-push-up-masts/
Horse-fence, Dipole, OCF, or What?
Boy, we just opened a can of worms here! The short summary (based only only 55-years in the hobby, lol) ranks them in this order:
- Simple. They only work well on one band, though, so consider a “fan-dipole” or a “trap dipole” for multi-frequency operation. Traps have some minimal losses – but are easiest to install and tune. Fan dipoles might be a tad-better far away, but you could be playing with as many as 10-12 end points for five and six-band coverage.
- Horse-Fence dipoles get rave reviews on the ham radio sites like eham.net. Unlike a single-conductor, the horse-fence antenna uses stainless 1-1/2 to 2-inch wide electric fencing designed for farm use. The claim is that because of their spacing between multiple conductors,, they are slightly “hotter” than a conventional antenna. Engineers say “No…” but user’s love them based on reviews.
- The OCF is an Off-Center-Fed In other words, if the basic half wavelength is 135 feet, for example, the antenna is fed at about 33-percent of the antenna length. Call it 45-feet in from one end, leaving the longer end to be stretched-out 90-feet. Reviews tend to track manufacturer and how good the installation is. These work best when all parts are at the same level, and less well when installed as variants of the inverted-vee.
This Year’s Antenna Projects
Somewhere in my background, I “wrote the book” for one of the major antenna tuner manufacturers. And the late president of the company “took me to school” on HF antennas. Let me paraphrase what I learned:
“George, as an HF antenna radiates, there will be voltage and current nodes on the antenna. If you want to have a maximally-efficient antenna, you need to use a conductor that is as large as practical. This way, when the voltage is down – and current high – you will pick up efficiency.”
OK, sounds fine. While it is nominally true that an antenna made of #14 copper will “handle” a couple of kilowatts (PEP), it only takes a moment to see the truth of what my mentor was conveying.
We can determine current in an antenna by using the simple formula:
Power equals current squared times impedance. Also stated as:
We’ll avoid using z (impedance) and stick to r for clarity. There’s no test here.
Now let’s fire up one of my stations…here’s a TS-590S and SB-220 linear that we can dial-back to exactly 1,000 watts CW output. This becomes “P.”
Since we know the antenna presents a 1:1 SWR at resonance (or close enough for ‘government work’), we can calculate antenna current will be using 50 ohms?
Power divided by impedance equals the square of current, so the actual current is the square root of whatever i2 is.
So…. 1000/50 = 20 0 (i2) So the square root of this is? 4.7 AMPs.
Except not quite…
When we run full legal limit out, the current rises to 6.7 AMPs.
But we also know that many antennas, low to the ground at these lower frequencies will really have impedances that might be as low as 25-Ohms.
At these impedances, the current might be 14-amps at many places on the wire. And as my mentor explained, the “snap” (current reversal) is where the radio signal really “fires from…”
Perhaps, that’s why the “horse-fence antennas” work OK? I ran to the American Wire gauge tables. Horse-fence antenna makers claim as many as 18 conductors but it’s often cabe be smaller than 30 -gauge wire.
They key factor is wire diameter times number of conductors. That’s because the actual radiation takes place very near the surface due to “skin effect.” If comparing antennas, we’d be best-served comparing wire diameter not thickness.
It turns out that 30-gauge wire is pretty thin: 0.01 inch, in fact.
But, the trick with the horse-fence design is using a proper clamping system, you might have 14-conductors which brings the total of diameters up to 0.14 inch and according to the table (online here) that’s the same total of diameters as a single strand of #7 wire. Easily safe for a kilowatt of power.
Here Comes EZNEC
Since we keep the grown-up version of Roy Lewallen’s EZNEC antenna modeling software at the ready, it didn’t take long to find – in certain configurations of dipoles, vees, and OCF’s where there can be 1.5 db, or thereabouts, of additional antenna gain when using extremes of wire size or massively paralleled wires.
Take an OCF dipole, for example. When I compared the model of #14 wire, and moved the wire size up to #0 (“single ought” is as big as EZNEC models as a wire size), I found lobes were the antenna “snapped out” almost 2 db stronger!
That sent me shopping. But not for horse-fence antenna wire….
eBay to the Rescue!
I remembered from somewhere in the dusty memory banks that years ago, I had read an article about somewho who had used a spool of computer printer ribbon cable which was turned into a six-band fan-dipole.
That’s what I wanted because a db or more of gain really matters at the entreme of the hobby.
A bit more than a hundred dollars later, my role of “dream wire” showed up.
It’s 14 conductors of #16 wire.
The spool has 220-feet, so enough for a two-band fan dipole and with what kind of wire dimensions?
#16 wire is 0.0508 inches each. Times 14 conductors gets us 0.7112 inches – so this SHOULD radiate like 7-10ths of an inch copper. Can you imagine how efficient a 125-foot piece of ¾” copper pipe would be if you could figure some way to hold it up? This is nowhere near as light as horse-fence. Twenty pounds per side. A price I’m willing to pay…go ahead and sag a bit.
What’s more, since the ends are the only exposed metal, it should be simple to put “corona dope” all over the ends and around the center insulator. We’ll be using a 1:1 W2FMI 5-kW capable current balun in the middle.
This promises to be an interesting experiment and it shows again how much fun you can have “inventing things for yourself” in ham radio.
One Other “Big Signal” Idea…
One last point: Most people don’t realize that low band antennas can fall way below 50 ohms. Low antennas are often be down in the 25-ohm range though it shows up as a 2:1 standing wave ratio (SWR). Depends on meter. On mine it reveals as a “double dip” SWR..
A fellow in France as started selling a special balun for this case called a 1:2 balun. You put 50 ohms in from your transmitter and the output wants to see 25-ohms. Look for ‘em on eBay in the ham gear section. When the XYL’s not home.
For 30-bucks, it’s the core-only: you supply the box and fittings…But are you kidding? Deal of a lifetime! My last “Super-antenna” (the 746-foot OCF dipole) works fine, but the SWR is habitually wrong. It has the “double bottom SWR” – a giveaway the antenna is under 50 ohms. This get the monster antenna back on track…
Next time you hear me on the air, hold up your coffee cup. I should be able to heat it up for you…all the way from Texas!
Vy 73, ac7x George@ure.net