VHFUHF Baluns Fact and Fancy Ian White GM

VHF/UHF Baluns – Fact and Fancy Ian White, GM 3 SEK This is the talk that I presented at the RSGB Convention in October 2018. RSGB will eventually release a video of the talk, but in the interest of publishing the technical content as soon as possible, you’re very welcome to click through these slides. 73 from Ian GM 3 SEK

Contents 1. The changes that we’ve learned from HF – it’s about noise and interference. 2. A fresh look at existing VHF-UHF baluns – including what we now need to know. 3. Which VHF-UHF baluns to choose? 4. Mistakes to avoid 5. Find Out More the. RSGB @the. RSGB

What we’ve learned from HF Textbook model of a balanced antenna: NEVER EXISTS AT HF ! the. RSGB @the. RSGB

What we’ve learned from HF At HF, reality is always asymmetrical: High risk of TVI “Common-mode Feedline RF current” radiates on feedline Induces RF current on TV downlead, mains wiring the. RSGB @the. RSGB

What we’ve learned from HF Same applies on receive: Noise and interference in amateur bands Feedline picks up Common-mode noise house. RFfrom current on feedline the. RSGB @the. RSGB

What we’ve learned from HF New ideas from HF: 1. Think of this as an EMC problem – it’s all about noise and interference. 2. It’s about unwanted Common-mode RF currents – and how to stop them. the. RSGB @the. RSGB

The Private Life of Coaxial Cable 1. The inside of a coaxial cable is private – completely screened from the outside world. 2. Because, at RF, current only flows on the surfaces of conductors. = THE SKIN EFFECT 3. So the outside surface of the coax shield is a separate ‘live’ conductor that can carry unwanted RF currents.

The Private Life of Coaxial Cable Antenna feedpoint Inside the coax, I 1 = I 2 (very strongly coupled) Any difference Coax feedline between I 4 and “Common-mode” RF I 5 spills over to current on the sobecome STOPflows ITI 3 HERE – outside with ansurface. . . RF choke. the one you need to block.

What we’ve learned from HF New ideas from HF: 1. 2. Think of this as an EMC problem – it’s all about noise and interference. It’s about unwanted Common-mode RF currents – and how to stop them. 3. There are two completely different families of baluns: • “Current Balun” = Common-mode RF Choke – blocks unwanted currents • “Voltage Balun” – equalizes voltages. the. RSGB @the. RSGB

What we’ve learned from HF Voltage Baluns Outer surface of coax, radiating the. RSGB @the. RSGB

What we’ve learned from HF Voltage Baluns • A voltage balun does not prevent common-mode currents – it passes them straight through. • At an antenna feedpoint, a voltage balun only works IF the feedpoint is already well balanced! the. RSGB @the. RSGB

What we’ve learned from HF New ideas from HF: 2. 3. 1. Think of this as an EMC problem – it’s all about noise and interference. It’s about unwanted Common-mode RF currents – and how to stop them. Current Baluns are better than Voltage Baluns for most antenna applications. 4. Accurate measurements on baluns are difficult, even at HF. the. RSGB @the. RSGB

What’s different at VHF-UHF? Maybe we should first identify what’s the same. the. RSGB @the. RSGB

What’s the same at VHF-UHF? 1. Noise/EMC – the same problem, but even more important for us at VHF-UHF. 2. Current baluns versus voltage baluns – same principles apply. 3. So current baluns are generally a better choice – exactly the same as at HF. the. RSGB @the. RSGB

What’s different at VHF-UHF? The Big Difference: Feedpoints of VHF-UHF Yagis are already well balanced! Until we connect the coax. . . the. RSGB @the. RSGB

What’s different at VHF-UHF? • Different objective (from HF): How to connect the feedline without spoiling the balance that already exists. • Why? To minimize stray common-mode currents that can increase RX noise. the. RSGB @the. RSGB

What’s different at VHF-UHF? Four more significant differences: 1. You don’t need a high-performance CM choke (In EMC language, this is a “soft” problem, doesn’t need a high choking impedance. ) 2. Shorter wavelengths at VHF-UHF allow some new solutions. the. RSGB @the. RSGB

What’s different at VHF-UHF? 3. Don’t forget the boom! • Common-mode currents on coax and boom will couple together. • CM current along the boom will spoil suppression of minor sidelobes. the. RSGB @the. RSGB

What’s different at VHF-UHF? 4. Accurate balun measurements are almost impossible! • For VHF and above, we have to rely on “best possible practice” • Backed up by good basic knowledge. the. RSGB @the. RSGB

A new look at traditional VHF-UHF baluns • Designs are changing towards the simple split dipole with a direct 50Ω feed. . . • Through a balun, of course. the. RSGB @the. RSGB

Some traditional VHF-UHF baluns Pawsey Stub for direct 50Ω feed Dipole Too many bad copies! the. RSGB @the. RSGB

Some traditional VHF-UHF baluns Pawsey Stub for direct 50Ω feed Dipole Mr Pawsey’s Original Stub – a shorted two-conductor transmission line Coax inside one tube Symmetrical U-shape, λ/2 around (180°) the. RSGB @the. RSGB

Some traditional VHF-UHF baluns Pawsey Stub for direct 50Ω feed Dipole • This is a voltage balun. • The stub provides voltage balance and the 180° phase difference. • 1: 1 impedance match. • But nothing to stop commonmode currents. • Too many bad copies! Coax inside one tube Symmetrical, λ/2 around the. RSGB @the. RSGB

Some traditional VHF-UHF baluns Not-a-Pawsey Stub – one of too many bad examples! • Seriously missing the point about symmetry! (Not really his fault, too much bad info around to copy. ) the. RSGB @the. RSGB

Some traditional VHF-UHF baluns Half-wave loop balun for a 200Ω folded dipole the. RSGB @the. RSGB

Some traditional VHF-UHF baluns Half-wave loop balun or for a symmetrical T-match M 2, INC. the. RSGB @the. RSGB

Some traditional VHF-UHF baluns Half-wave loop • Another voltage balun. • Creates equal voltages with 180° phase difference. • Requires a 200Ω feedpoint impedance. • Complicated, some losses. • Does nothing to stop commonmode currents. the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF = Common-Mode Chokes Purpose: to block unwanted current on the outer surface of the coax. . . and then, flowing along the boom. the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Choke baluns for 50, 70 MHz the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Choke baluns for 50, 70 MHz • Ferrite Toroids – no good for VHF! “Bigger and more turns” is often NOT better in an RF choke (makes the resonant frequency too low) the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Choke baluns for 50, 70 MHz • Air-wound coil – very narrowband ? It may work, but not ‘plug and play’ (just like at HF). the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Choke baluns for 50, 70 MHz • Ferrite Beads – more broadband, more promising ? But again, remember: “more and bigger beads” is often NOT better (can make the resonant frequency too low) the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Now the good news: At VHF-UHF we can use resonant lengths of coax the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Resonant-line choke baluns for 144, 432 (1296 +) MHz the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Resonant-line choke baluns for 144, 432 (1296 +) MHz • Classic quarter-wave sleeve choke • Makes a high impedance HERE for common-mode currents. • Sleeve is open-circuit at top (no connection to coax) . . . so it acts like an RF choke. This is a current balun. • Can also connect to the boom HERE • Solid short-circuiting disc connects base of sleeve to coax shield the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Resonant-line choke baluns for 144, 432 (1296 +) MHz • Quarter-wavelength line choke Makes high Z against CM current HERE λ/4 DE Boom Connect to boom Keep away from boom the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Resonant-line choke baluns for 144, 432 (1296 +) MHz • Quarter-wavelength line choke Quarter-wavelength coax line makes high Z against CM current HERE Connected onto boom Ferrite bead for extra Z ? DG 7 YBN the. RSGB @the. RSGB

Current (choke) baluns for VHF-UHF Resonant-line choke baluns for 144, 432 (1296 +) MHz • Tonna quarter-wave “half loop” • Creates high impedance HERE on outside of shield • Tape coax solidly onto boom – very important • Quarter-wavelength loop of coax in free air G 4 CQM • Maybe add a ferrite bead or two, for backup More effective than it looks! the. RSGB @the. RSGB

Mistakes to Avoid! How NOT to do it. the. RSGB @the. RSGB

Mistakes to Avoid! • The Gamma Match – the Balance Destroyer the. RSGB @the. RSGB

Mistakes to Avoid! • Taping Choke to Boom the. RSGB @the. RSGB

Mistakes to Avoid! • The Dive Through Boom the. RSGB @the. RSGB

Mistakes to Avoid! • Choke wound on the Boom the. RSGB @the. RSGB

Mistakes to Avoid! Always Remember: • At the feedpoint, the outside of the coax shield is a live conductor – so keep it clear of the boom. • And in general: KEEP K IT I SYMMETRICAL, S STUPID! S the. RSGB @the. RSGB

Summary • Today, “baluns” are about controlling interference, reducing noise • Choke baluns are best for doing this. • So aim to: 1. Minimize common-mode RF currents on the outside of the coax and on the boom. 2. Don’t upset the good feedpoint balance that VHF-UHF Yagis already have. the. RSGB @the. RSGB

Summary So. . . • Remember the basic principles • Then watch the practical details! the. RSGB @the. RSGB

Find out more… HF: www. ifwtech. co. uk/g 3 sek Blog: https: //gm 3 sek. com www. dg 7 ybn. de/symmetrising www. rsgb. org

VHF/UHF Baluns – Fact and Fancy the. RSGB @the. RSGB

Mistakes to Avoid! • The Gamma Match – the Balance Destroyer the. RSGB @the. RSGB

VHF/UHF Baluns – Fact and Fancy the. RSGB @the. RSGB

Mistakes to Avoid! • The Dive Through Boom the. RSGB @the. RSGB