The Ham Radio Series 34 – One Antenna To Rule Them All – Part 2

I was just going to call this “Multiband Antennas,” because that’s what it’s about, after all, but you’ve got to admit this is a catchier title.  

We started out talking about the EFHW – End Fed Half Wave (on its lowest frequencies) – because of their wide popularity these days.  I referred to these as being decades old, and they really are about as old as radio itself.  “I’m so old…” that I don’t think of these as EFHWs, but as Zepp antennas, named after the long wires hung out of the back of a Zeppelin in the early days of both air flight and radio.  Feeding an antenna at the end instead of in the center, like a dipole, is simply more convenient; especially when the far end is flapping in the breeze like it was for the Zeppelins!  Sure beats putting up three really tall poles for a dipole – two for the end and one for the feed point.  For those original Zepp antennas, the set of reasons for wanting a 50 ohm antenna like we do today just weren’t there.  There were no solid state transmitters, tube transmitters could be tuned for different output impedances than we design for today – 50 ohm coaxial cable hadn’t even been standardized on -  and more.

The drawback of high impedances like the Zepp/EFHW is that for a constant amount of power, there’s a couple of important relationships that point to trouble.  

Relationship 1:  Power is Voltage times Current, P = V*I  (current is usually abbreviated as I)  If you’re used to the way ham and technician study guides write that, they tend to use E for voltage – short for electromotive force – that conveniently turns that into P=I*E, or P=IE and who can forget pie?   

Relationship 2:  Voltage is Current times Resistance,  or V=I*R and substituting back into the first equation, we get P=V*I = (I*R)*I or P=I²*R.  If P is constant, and R goes up, I² (current) has to go down.  Since I has gone down, and the power has remained the same, the voltage V has to go up to keep the product I*E constant.    

So what?  The higher the voltage, the more likely that arcing over becomes.  Another phenomena is called corona, a type of discharge different from arcing but that can sometimes be seen (at night) as a glowing area around the feed point, or highest impedance points.  

Let’s put some numbers here.  A typical HF ham rig puts out 100W.  In a 50 ohm system, the current would be SQRT(100/50) or 1.4Amps, and the voltage is 70.9V (SQRT is "square root of").  In a 5000 ohm system, the current drops to 141 milliamps while the voltage goes up 709V.  At the end of a coax run, due to the loss in the coaxial cable, both voltage and current values will be smaller.  In commercial and NASA specs, they consider voltages above 250V to be the start of extra precautions due to corona.  Because of reduced air pressure at higher altitudes, corona forms at lower voltages than at sea level.  Those of you in mountain states will have more ideas of how much this affects you than I will.  It’s similar to how the air pressure at high altitudes reduces water’s boiling point and affects cooking times.

To borrow a line from an old commercial, so what’s a mother to do?  Use something lower in impedance.  Voltage comes back down, current goes back up, chances of arc or corona go down.   It’s one reason that resonant dipoles and monopoles became popular.  Recall that a dipole in free space has a feed point impedance closer to 75 ohms than 50 and a vertical (monopole) is ½ of that or closer to 36 ohms.  (If you read around enough, you’ll find stories from hams who have put down more radials (quarter wave ground wires) to make a more perfect ground and found that their antenna’s impedance match has gotten worse because they’ve driven the impedance closer to 36 ohms.)   

Still, a dipole or monopole cut for one band is only good for that band and the subject here is multiband antennas so this sounds like a dead end.  To be completely accurate, a dipole or monopole is also good on it’s odd harmonics.  That is, a 7MHz dipole or vertical will work on 21 MHz (3rd harmonic) or 35 MHz (5th harmonic; there’s no ham band but just to show the example).  An 80m antenna can give you 30m on its third harmonic, if tuned for the very bottom of the band.  It turns out that with those two exceptions, this is an almost useless thing to keep in mind.  

This is a subject that has been studied for the history of radio, so let’s cut to it.  There are two main ways to make multiband antennas: nesting together several independent antennas at the feed point or using circuits to make them multiband.  These are called traps.

A trapped dipole uses parallel resonant circuits to isolate sections of the antennas from each other.  Parallel resonant circuits present high impedance - an open circuit - which isolates parts of the antenna. An example from the ‘net. 

By way of example, the innermost dipole would be cut for the highest frequency band; let’s say 10 m.  The trap would be designed to be parallel resonant just below 28 MHz, so it wouldn’t allow the 10m signals to “see” (be affected by) the next sections of wire.   The next section of wire between the traps (plus some residual reactance from the trap, typically inductance which makes the antenna shorter) lets the antenna resonate on the next band down that’s desired; perhaps 12m, 15 or 20.  The second trap would cut off below the bottom of the second band so neither 10m or the next band “see” that next section but allow lower frequencies past it.  Finally, the last section of wire would allow the lowest band.   Designs that resemble this drawing for 10, 15 and 20m are extremely common, and are widely sold as “triband beam antennas.”

There are free pieces of software that help you design and build traps.  Some use coaxial cable, some have you create air wound inductors and use lumped capacitors. A good source of information and ideas about software is at W8JI.

The other main type of antenna is multiple single-band antennas fed at a common point, and often called a fan dipole.  

They don’t have to look like this; in fact, they probably shouldn’t.  This website presents information from the Stanford Research Institute that imposes some physical distances that seem to be necessary.

“The SRI found that the wires at the center feed point had to be separated by at least 5 1/2 inches vertically and the ends separated by 38 inches in the 2 to 18 MHz range. As in any fan dipole construction, all of the dipoles are connected in parallel but in the SRI method, the separation between them at the feed point must be maintained.

By this simple change they found that you could accurately cut the antenna element lengths for given frequencies and eliminate the need for pruning.

In the drawing above, the lowest frequency antenna is on top and is cut 4% short of the standard 1/2 wave length. (Length in feet= 0.96 times 468 divided by the operating frequency in MHz).

The middle frequency antenna (lower in frequency), is cut for an exact 1/2 wave length. (length in feet= 468 divided by the frequency in MHz)

The highest frequency antenna is at the bottom and cut for 1% longer than the 1/2 wavelength (length in feet= 1.01 times 468 divided by the frequency in MHz)

Compared to the construction effort of a standard multiband dipole the only difference is the fabrication of a feed block or center insulator that is about 12 inches vertically by 3 inches wide, so make sure this is made of a good insulating material such as Lucite, Bakelite, fiberglass, or PVC.” 

I’ve run a fan dipole before, although it has been a while.  The SRI information wasn’t available back then, so I trimmed it manually with an SWR meter.  I think the wires were cut for 40, 30, 20, and 10; the antenna also worked on 15m because of the third harmonic of the 40m wires.  There was some interaction with the 30m element and I don't remember how I got around that.  Probably an antenna tuner.  The spreaders were cut from some six inch long, inch wide, 1/16” thick pieces of plexiglass, drilled with four holes on every piece and enough on each side to keep the wires parallel to each other and not touching.  I worked many countries on that antenna, with it mounted like an inverted Vee sloping to ground on both ends and folded in the middle so that the two sides of the antenna were around 90 degrees to each other.

Trapped and fan antennas are widely available and (of course) can be homemade.  Don’t forget that these can be monopoles or dipoles; and the monopoles are very widely sold as multiband or trapped verticals.  Pro tip: if you see a vertical sold as not needing radials, it might well be a vertically mounted dipole that is actually fed well above ground.  A good example is antennas sold by a Florida company, GAP Antenna Products, like their Titan DX antennas.  There are many multiband verticals on the market.  

The major difference between a fan dipole, or different, full-sized antennas and trapped antennas is traps will have some loss associated with them while full-length wires don’t have that loss.

Finally, you’ll note there’s a type of multiband antenna I haven’t mentioned at all, the log periodic dipole array, also called LPDA or just LP.  I’ve talked about these antennas in the past. (Here for example)  They have their place, but are pretty advanced as a home project. 

A Cushcraft (brand) multiband vertical called the R8 – for the number of bands it covers. 

Something that I need to mention is that with both the trapped or fan dipoles, it might not be possible to get every band in the HF spectrum.  Before the bands that we were given in World Administrative Radio Conferences, the amateur bands were separate enough that wires didn't interact.  Now, with new bands like the slivers called 60 and 30meters, and the slightly wider slices at 17 and 12m, it's harder to deal with the interactions.  This guy describes how he addressed it:  two antennas: one trapped dipole for the 10/15/20/30/40/80 bands and another antenna for 17 and 12m.  Not everyone is interested in operating on all bands though, and antennas for the older, larger ham bands might be all some people care about.