6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing

6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing

ACMA Exam Focus: Syllabus 8.7 and 8.8. Recall that reducing field strength to the minimum required for effective communication is good radio practice; that balanced antenna systems tend to cause fewer EMC problems than unbalanced ones; and that the transmission line should leave the antenna at right angles to minimise EMC problems. Note the framing carefully - the syllabus calls minimum power good practice, not a numerical legal limit.

---

1. Use the Minimum Power Required

Every immunity failure in a neighbour's equipment has a threshold field strength. Below it, the device works; above it, it does not. That single fact makes power reduction the cheapest and fastest EMC tool you own.

Field strength at a given distance is proportional to the square root of the transmitted power:

$E \propto \frac{\sqrt{P}}{d}$

So dropping from 100 W to 10 W is a $10\log_{10}(100/10) = 10$ dB reduction in power, and reduces field strength by a factor of $\sqrt{10} \approx 3.16$. On a receiver calibrated at the conventional 6 dB per S-unit, that is a little under two S-points at the far end - very often still a perfectly workable contact, while the neighbour's television returns to normal. Distance works the same way: doubling the separation halves the field strength, a 6 dB improvement, for no loss of your own signal to a distant station.

This is why the syllabus phrases it as good radio practice: run the power you need for effective communication, not the maximum your licence allows. It sits alongside the licence condition that a person must not operate an amateur station if it causes harmful interference to other licensed services, and that the ACMA may restrict the operation of a station to avoid harmful interference.

2. Siting: Distance and Height Beat Power

Because field strength falls with distance, where you put the antenna matters more than how much power you put into it.

• Keep the antenna as far from neighbouring houses, boundary fences and the street as the block allows.
• Get it high. Height moves the intense near field away from household wiring, telephone and NBN drop wires, and the neighbour's roof space - and it simultaneously improves your low-angle radiation for DX. Height is the one change that helps your signal and your EMC situation at the same time.
• Avoid running the antenna parallel to and close alongside mains service drops, telephone lines or a metal fence. Parallel conductors couple efficiently; right-angle crossings couple far less.
• Keep the feedpoint - the high-current, high-field region - away from the house.

3. Balanced Versus Unbalanced Antenna Systems

Balanced antenna systems tend to cause fewer EMC problems than unbalanced antennas.

In a balanced system - a centre-fed dipole with a choke balun, a doublet fed with open-wire line, or a full-wave loop with a balun - the currents in the two halves of the antenna are equal in magnitude and opposite in direction. The fields produced by the two feedline conductors cancel, so the feedline does not radiate. The radiated field is symmetrical about the antenna, and it comes from where you intended: the antenna, up in the air.

In an unbalanced system - an end-fed wire worked against an earth or counterpoise, an off-centre-fed dipole with no common-mode choke, coaxial cable connected directly to a dipole with no balun, or a vertical with an inadequate radial system - there is no symmetrical return path for the antenna current. The current returns instead along the outside of the coaxial braid. This is common-mode current, and it turns the feedline into part of the radiating system.

That is the whole EMC problem in one sentence: the feedline now radiates as it runs down the wall, past the meter box, through the roof cavity and into the shack - which is precisely where all the susceptible wiring and equipment lives. You have effectively installed a radiator inside the building. Common-mode current also degrades receive performance, conducting household noise back up to the antenna terminals, and makes the radiation pattern unpredictable.

4. The Feedline Should Leave the Antenna at Right Angles

Recall that the transmission line, balanced or unbalanced, should leave the antenna at right angles to minimise EMC problems, and should continue that way for as far as practical - ideally at least a half wavelength.

The reason is field geometry. The electric field of a dipole is polarised along the wire. A conductor lying parallel to that field has a voltage induced along its length; a conductor lying perpendicular to it has minimal induced voltage, because it is orthogonal to the field lines. Running the feedline at right angles therefore places it in the region of minimum coupling, so very little current is induced onto the outside of the line, whatever balun you have fitted.

In practice this means:

• From a horizontal dipole, drop the coaxial cable vertically away from the feedpoint before running it horizontally to the shack.
• From a vertical antenna, run the feedline horizontally away from the base, along or under the ground, before turning towards the house.
• Never let the feedline run diagonally back alongside one leg of the antenna to save a few metres of cable - that is the worst possible geometry and undoes the work of the balun.

5. Keeping RF Out of Your Own Shack

The station suffering the first symptoms of common-mode current is usually your own. Watch for a "hot" microphone or RF burns, erratic SWR or ALC readings that change when you touch equipment, computer resets and USB dropouts on transmit, a runaway keyer, or a linear that trips.

The remedies follow directly from the theory above:

• Fit a choke balun at the feedpoint and a second common-mode choke where the feedline enters the building.
• Bond all equipment cases to a single station bonding point using short, wide straps. Length is inductance; a long thin wire is not an RF bond.
• Put ferrite chokes on every lead leaving the transceiver - microphone, key, audio, USB, DC and control.
• Remember that an RF bonding system is not a substitute for the mains protective earth. Never disconnect or defeat the mains earth in pursuit of an RF problem.

Good EMC practice is cumulative: minimum power, a well-sited and balanced antenna, a feedline leaving at right angles, and chokes where cables enter equipment. Each measure is modest on its own; together they usually take a station from "the neighbours complain" to "nobody notices".