Braid-breaker
A filter that prevents TV interference from nearby HF transmitters.
A braid-breaker is a device used to filter out television interference (TVI). This type of interference often occurs when a nearby high-frequency (HF) transmitter creates a strong field, and the TV’s aerial down lead acts like a longwire antenna or a simple vertical element. The resulting radio frequency (RF) current flowing through the TV tuner can generate harmonics that disrupt the picture. The braid-breaker stops RF signals picked up on the outside of the coaxial cable from entering the TV, while allowing the intended RF signal from the antenna to pass through the inner conductor.
There are two main design approaches for reducing unwanted signals. One uses a choke filter, which blocks signals in the electrical mode most interference uses. The other uses filters that either admit or block signals based on their frequency. Combining filters of either type—or both—can multiply their effectiveness, so two slightly different filters work better than one alone.
A ferrite ring choke works by creating a high impedance for signals traveling along the cable’s braid only, while passing differential-mode (balanced) currents unchanged. The wanted signal is differential-mode, with equal and opposite currents in the braid and core. The alternating current in the braid generates magnetic fields in the ferrite, adding inductance in series with the braid. For the wanted signal, the equal and opposite currents produce magnetic flux that cancels out. This device is called a choke because the ferrite effectively chokes off the interference path.
The other filter type relies on frequency. Below a certain frequency, inductors (coils) impede higher frequencies more and pass lower ones, while capacitors do the opposite: they pass higher frequencies and impede lower ones. These components can be arranged to block or pass signals based on frequency. A simple high-pass filter design uses two 4.7 pF ceramic capacitors and two air-core inductor coils (four turns of 20 AWG copper wire wound around a 6 mm drill bit). The design is a symmetric network: the cable is cut, and the coils connect the braid to the core at each end, while the capacitors bridge the cut—one capacitor connects core to core, and the other reconnects the outer shield braids. As an extra precaution, a 1.5 MΩ resistor is wired in parallel with the capacitor connecting the shields.
- Capacitor value
- 4.7 pF
- Coil turns
- 4 turns
- Coil wire gauge
- 20 AWG
- Coil form diameter
- 6 mm
- Resistor value
- 1.5 MΩ
- Cable impedance
- 72 Ω
- Frequency thresholds
- below 50 MHz (HF), above 450 MHz (UHF)
Lore & Background
The braid-breaker is designed to address television interference (TVI) caused by a nearby high frequency (HF) transmitter. The interference arises because the aerial down lead plugged into the TV acts as a longwire antenna or vertical element, allowing RF current to flow through the TV tuner and generate harmonics that spoil viewing. The braid breaker prevents RF signals picked up on the outside of the coax from entering the TV, while passing the desired RF signals from the antenna inside the coax.
Designs for braid-breakers are based on two types of filters: a 'choke' filter that blocks signals in the electrical mode most interference uses, and filters that selectively admit or impede signals depending on frequency. Carefully chosen combinations of filters of either one type or both types multiply each other's effects, so that even if only slightly different, two filters are more effective than a single filter or either filter alone.
A ferrite ring choke works by presenting a high impedance to signals traveling along the braid only, while passing differential-mode currents unchanged. The wanted signal is in differential mode with equal and opposite currents in the braid and core. The alternating current in the braid is impeded by magnetic fields in the ferrite, placing a large inductance in series with the braid, while the wanted signal's currents produce equal and opposite magnetic flux that cancel out. The device is called a 'choke' because the ferrite effectively chokes off the signal path for interference.
Reader's Guide
The braid-breaker is significant for amateur radio operators because it provides a practical solution to television interference (TVI), a common problem when operating HF transmitters near TV sets. The source article describes two main filter types: a ferrite choke that blocks common-mode currents on the braid, and a high-pass filter that uses capacitors and inductors to separate HF interference from UHF TV signals. The high-pass filter design uses two 4.7 pF ceramic capacitors and two air-core inductor coils (4 turns of 20 AWG wire on a 6 mm form). A 1.5 MΩ resistor is wired in parallel with the capacitor connecting the shields to prevent static buildup without affecting the 72 Ω signal. The filter's impedance characteristics ensure that HF signals (below 50 MHz) are short-circuited by the coils and blocked by the capacitors, while UHF signals (above 450 MHz) pass through the capacitors with the coils presenting high impedance. The article notes that combining both a choke and a high-pass filter is even better, as the filter alone may not be as effective for common-mode currents, which the choke selectively removes. This combination approach multiplies the filtering effect, making the braid-breaker a versatile tool for reducing TVI.
Did You Know?
- A braid-breaker prevents RF signals picked up on the outside of a coaxial cable from flowing into a TV set.
- The ferrite choke works by presenting high impedance to signals on the braid while passing differential-mode currents unchanged.
- A 1.5 MΩ resistor is used in the high-pass filter to prevent static buildup on the TV aerial.
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