Tech Tip: “There Ain’t No Free Lunch!”

10Nov16

There Ain’t No Free Lunch

OK. The English is bad but the title says it all. So many hams are looking for that “all band, does everything” HF antenna.

On VHF and UHF the “tuning” of an antenna is far less critical than on HF. The wavelengths at 144 MHz and above provide a naturally wide bandwidth so that you assemble the antenna and, in most cases, it just works. Nearly all transmitting antennas at VHF and above are resonant types.

There are basically only two classes of HF antennas: Resonant and Non-Resonant. Let’s look at resonant antennas first.

Bushcomm BA Series Monoband Dipole Antennas BA-40B

Bushcomm BA Series Monoband Dipole Antenna

Resonant antennas include (but are not limited to) monoband dipoles, monoband and trapped verticals, mono-band and trapped multiband Yagis, and specialized multiband antennas like fan and parallel dipoles. Resonance may be designed into these antennas by the use of traps, linear loading, stubs, or by the natural resonance of the length of the radiator. With these antennas, resonance occurs only in narrow chunks of spectrum.

Non-resonant antennas include (but are not limited to) long-wires, un-trapped multiband verticals, off-center fed dipoles, and other compromise antennas. These antennas typically require a wide-range antenna tuning unit (ATU).

On HF, the wavelengths are long to very long and resonance becomes more critical. A dipole on 80 meters may have a useful SWR bandwidth of only 60 kHz or so. If you want to work 75 phone with an 80 meter CW antenna, you’ll need an ATU (better referred to as a transmatch) to compensate. All resonant HF antennas – ALL OF THEM – used outside their resonant bandwidth require the use of a tuner. If you are looking for an antenna that will cover 160 through 6 and work efficiently… that hasn’t been invented yet.

Non-resonant antennas may be force-fed using ATUs in conjunction with baluns or feedline current chokes. Baluns and chokes will keep RFI out of your shack and allow the tuner to force-feed the non-resonant antenna so that power is radiated instead of being lost in standing waves or impedance losses. For example, 43 foot verticals are 43 feet long to avoid accidental resonance. In other words, they’re designed to be totally non-resonant. Their balun or unun and associated ATU allow them to work across a very wide spectrum. The lower the frequency, however, the poorer the efficiency of these antennas becomes.

Compromise antennas require compromise solutions and support. There ain’t no free lunch!

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