The Ham Radio Series 35 – One Antenna To Rule Them All – Part 3

I thought an interesting aspect to this story would be to ask, “what do the professionals do for an all-band HF antenna?”  In this case, consider commercial airliners.  As a setup for talking about the HF antenna system, let me talk about the rest of the system.  

A typical installation would be a couple of HF transceivers (in a rack elsewhere in the airplane, not in front of the pilots or crew) for redundancy.  The transceiver for an air transport aircraft like an Airbus A320 or a Boeing 737, as well as the bigger planes, tends to cover the complete 2-30 MHz HF spectrum with almost exclusively upper sideband only, regardless of frequency.  While ham transceivers are typically rated 100W for a CW signal and up to 200W Peak Envelope Power for a two-tone test, the commercial HF transceivers are rated 400W PEP on four-tone tests.  While the average output power is higher than the ham transceiver, it’s more like 3 dB higher than the 6dB you may be thinking from 100 and 400 Watts.

This shows that the antenna on an air transport plane must cover the entire 2-30 MHz spectrum because while there are discrete aviation bands, they’re able to use any frequency in the case of disaster.  Further, those antennas need to be multiband and the installation is always a fight to save every pound for the paying customer.  You may have seen pictures of old propeller airplanes with wires running from the tail forward – those were largely end fed wires like the Zepp or EFHW that was in part one.  That sort of antenna went away decades ago as airspeed went up.  Modern airliners use an almost random length vertical; that is, electrically short on some bands and electrically long on others.  It’s most often on the leading edge of the vertical stabilizer – or tail fin, as most people call it.  It’s pretty unimpressive-looking if you don’t know what you’re looking for.  It’s the long rectangular shape circled here. 

This replacement antenna is what they look like off the aircraft.

These are essentially monopole antennas – quarter wave verticals – not designed to be exactly ¼ wave on any particular aviation band.  The radio drives 50 ohm coax to an antenna tuner at the base of the vertical that’s essentially an L-Network, like the amateur autotuners we spoke about earlier in this series, except more conservatively rated (because it’s from a “high-reliability” market with those requirements).  

You’ll probably remember that quarter wave antennas depend on a ground network that emulates the other half of the half wave dipole as depicted in this figure.  On an airplane that’s a hundred feet long (or longer) and either all metal or built from a carefully designed composite with enough metal in it to be highly conductive, the fuselage, vertical and horizontal stabilizers, wings and all do an excellent job of providing the ground.  In your backyard, that’s likely to be more difficult to implement.  Most hams provide a ground for their vertical by putting down wires stretched radially outward from the antenna, and therefore called radials.  

Volumeshave been written about how many radials you need, how long they need to be, and how to position them, so I’m not going to try to cover that extensively here.  You can’t go wrong by putting down as many as you can, ¼ wave long on the lowest frequency of operation if it’s a multiband antenna, like this aircraft antenna.  It’s probably OK to summarize that as at least four radials that are at least ¼ wave long and as straight as you can lay them.  More is always better, and more importantly, more ¼ wave radials are better than fewer, longer radials.  

If you told me you had enough wire to make eight 1/8th wave radials or four quarter wave radials, I’m not sure what the right answer is and I bet you’d get both answers if you looked around.  But if your choice was four quarter wave or eight 3/16 wave radials (chosen to be halfway between 1/8 and ¼) I’d put down the eight slightly shorter ones.  AM broadcast stations are known for putting down one or two inch wide copper straps every one or two degrees around each tower (three towers seems most common) and each radial a full quarter wave long (which is 234 feet long at 1.000 MHz).  That’s a LOT of copper.  

If you’re a relative newcomer to ham radio, you may need to sit down for what I’m about to tell you.  The VSWR at the antenna doesn’t really matter.  The problem with the antenna not matching the transmission line impedance is that the reflections of the waves bouncing back and forth in the cable between the antenna and the transmitter causes loss in the cable.  What this says is that if you want to place the antenna tuner to minimize those losses, put it at the antenna end of the coax.  Yeah, tuners in the shack are more convenient, and I love that too, but the physics is undeniable. 

As a practical matter, a vertical can be too short, but not too long. A reasonable number to use as the shortest vertical you’d ever want to put up is 1/8 wave, although the difference builds up as length goes down.  Simply calculate the well-known 234/freq (in MHz) for a quarter wave’s length in feet and divide that by two.  That makes an 80 m vertical still pretty tall – 33’ but 30’ will be hard to tell apart – and 160m antennas unreasonably tall at 65 feet tall.  To me, a 65’ tall vertical is only reasonable if you live in a place where thunderstorms and lightning strikes are nowhere near as common as here in the lightning capital of the US. 

Let’s reduce this to a simple example.  An all band vertical for 80 to 10m could be a 33 foot vertical element fed at the bottom with a tuner like this and good coaxial cable from the base of the wire to the shack (with lightning protection, switching and whatever you want).  33’ could be 1” aluminum tubing in 8’ lengths connected to each other or a #12 wire if you have a handy tree or structure to hang it from.  At the base, and on the ground terminal of the tuner, you’d put out four 66’ long wires as straight as you can, evenly spaced every 90 degrees around the base.  You can bend the radials around fences, garden plants or other obstacles.

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Now, let me drop some electromagnetics for those who are interested, but that might make some people’s head hurt.  If this doesn't sound like something you're interested in, just skip this.  The antenna on the aircraft is not actually a quarter wave monopole like you’re used to; it’s not a length of wire or tube insulated from the rest of the airplane.  It’s a slot cut in the metal of the vertical stabilizer.  There is no metal antenna there; the slot – the missing metal – is the antenna.  The concept is called a dual of an antenna.  The antennas are called slot antennas, and are most often used in UHF and microwave bands simply because most things aren’t large enough to use slot antennas at lower frequencies like HF.

Imagine a dipole in open space (which means ignore how the power got to it to radiate).  In electromagnetics, a conductor in an infinite open space can be replaced by an insulator, in this case a slot or air gap, cut in an infinite ground plane – which the airplane emulates well enough to be useful.  Instead of the radiation being caused by the current in the two halves of the dipole flowing in the same direction, the radiation of the slot is caused by voltage flowing in the same direction in the top and bottom edges of the slot.  The principle is called duality and we see an infinite conductive sheet (the metal of the vertical stabilizer) with a dipole-like slot replacing an infinite insulating space around a wire dipole.  Voltage and current are duals of each other.

If you’d like more technical details, a good write up is at a site called Antenna Theory.com which includes a good video.