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Antenna system Differential and common mode feed line current Balun types and characteristics Insights from NEC models, measurements Example application: HF multiband dipole All rights reserved. 07/10/17 2

80m half wave centre fed dipole on 40m, ATU and 20m of 50Ω coax feed line. Dipole feed point impedance taken to be 4000+j0Ω. Line Loss (db) System Type Zin (Ω) Line ATU Total efficiency RG58 4.95-j13.59 8.7 1.1 9.8 10% RG213 2.42-j13.64 5.6 2 7.6 17% Although RG213 has 3.1dB lower loss, the impedance it presents to the ATU causes 0.9dB higher ATU loss, net benefit is just 2.2dB. All rights reserved. 07/10/17 4

Primary role Transfer of energy from transmitter to antenna. Secondary role Impedance transformer Transform the ratio of V/I (the impedance) at the feed point to something that better suits the transmitter or some other upstream element of the system. Radiator Feed line currents can contribute to radiation on Txor pickup on Rx, can be good or bad. All rights reserved. 07/10/17 6

A pair of conductors in proximity to some other conductors or conducting surface (such as the natural ground) can operate in two modes simultaneously, differential mode and common mode. Differential mode is where energy is transferred due to fields between the two conductors forming the pair, and common mode is where energy is transferred due to fields between the two conductors forming the pair together in common and another conductor or conducting surface. The currents flowing in the two conductors at any point can be decomposed into the differential and common mode components. All rights reserved. 07/10/17 7

Differential current Id is the component that is equal but opposite in direction, it is half the difference in the two complex line currents I1 and I2. Common mode current Icis the component of the line currents common to both conductors, it is half the sum of I1 and I2. Id=(I1-I2)/2 Ic=(I1+I2)/2 I1=Ic+Id I2=Ic-Id So, for example, if I1=2A and I2=-1A: Id=(2--1)/2=1.5A; and Ic=(2-1)/2=0.5A. All rights reserved. 07/10/17 9

In a coaxial line with fully developed skin effect and operating in TEM mode: the current on the inside surface of the outer conductor is equal to the current in the opposite direction on the outside surface of the inner conductor. As a consequence: differential current flows entirely inside the coax; and total common mode current flows on the outside surface of the outer conductor. All rights reserved. 07/10/17 10

Desirable / undesirable Some effects may be desirable in some circumstances and undesirable in others. Inverse square law effects in far field, evanescent modes in near field. Induction into nearby conductors, EMC. Pattern distortion. Pickup of noise energy from nearby conductors. Examples: OCFD; Random length wire. All rights reserved. 07/10/17 12

Definition Types, broad characteristics, identifying. Current Baluns Voltage Baluns Specifications / measurements / relevance Impedance ratio Power handling Insertion loss Insertion VSWR Common mode impedance Broadband / narrowband All rights reserved. 07/10/17 14

A bal-unis a device for facilitating transition between balanced and unbalanced states. In non-ideal (iepractical) systems, it is better to think of a balun as a device facilitating transition between different states of balance. All rights reserved. 07/10/17 15

An idealcurrent balun delivers currentsthat are equal in magnitude and opposite in phase. A good current balun will approach the ideal condition. It will deliver approximately equal currentswith approximately opposite phase, irrespective of the load impedance (including symmetry). Common mode currentwill be small. If the load impedance is not symmetric, then the voltages at each output terminal will not be equal in magnitude and opposite in phase. A parameter often used to quantify the effect of a current balun is its common mode impedance or choking impedance. An ideal current balun has infinite common mode impedance, a good one has very high common mode impedance (typically thousands of ohms for an effective general purpose balun in an antenna system). All rights reserved. 07/10/17 16

An idealvoltage balun delivers voltagesthat are equal in magnitude and opposite in phase. A good voltage balun will approach the ideal condition. It will deliver approximately equal voltages (wrtthe input ground) with approximately opposite phase, irrespective of the load impedance (including symmetry). Common mode voltage((v1+v2)/2) will be small. If the load impedance is not symmetric, then the currents flowing in each output terminal will not be equal in magnitude and opposite in phase. An ideal voltage balun has zero common mode impedance, a good one has very low common mode impedance (ohms). All rights reserved. 07/10/17 17

Baluns are excessively lossy. Current baluns are better because they operate at zero flux. Voltage balun matches every time where current baluns don t. 4:1 baluns are better, that is why they are fitted to most ATUs Baluns do not affect VSWR. Common mode current causes high VSWR. All rights reserved. 07/10/17 19

Changing the length of coax changes VSWR. My VSWR meter does not agree with my antenna analyser. I don t cause RFI/BCI/TVI/EMC problems. Baluns cause harmonics and intermodulation distortion. I don t need no stinkin balun, I have the QSLs to prove it. All rights reserved. 07/10/17 20

Insight through NEC models. The art and science of modelling. More complete models of an antenna system include ground and the common mode current path. Techniques for modelling common mode current include: outer surface of outer conductor of coax modelled as single conductor; model each wire of two wire lines as GW elements. All rights reserved. 07/10/17 21

Model 1: half wave dipole for the 40m band; 10m height (λ/4) above average ground; feed point is 5% off centre to expose the model to some practical outcomes of less than perfect symmetry; feed line is coax, and it runs from a physically small transmitter located at ground height directly up to the feed point; lower end of the feed line and transmitter are isolated from ground; and no discrete balun or common mode choke. All rights reserved. 07/10/17 22

Model 6: half wave dipole for the 40m band; 20m height (λ/2) above average ground; feed point is 5% off centre to expose the model to some practical outcomes of less than perfect symmetry; feed line is coax, and it runs from a physically small transmitter located at ground height directly up to the feed point; lower end of the feed line and transmitter are effectively grounded; and a discrete common mode choke of impedance 1000+j1000Ω at one third the height of the feed line. All rights reserved. 07/10/17 24

Common mode current can be measured, it is not difficult. However, there is plenty of bogus instrumentation out there, a common trap is to compare the amplitude of each line current, ignoring the phase. If the currents are balanced, the amplitudes are equal; If the amplitudes are equal, the currents are not necessarily balanced. All rights reserved. 07/10/17 26

MFJ-835 with proprietary exclusive BalancedBarTM : How do you detect imbalance in open-wire or ladder line? By measuring and comparing each feedlinewire current --your feedlineis balanced if the currents are equal.... MFJ advertising. All rights reserved. 07/10/17 29

A transmission line balance test meter. VK5BR AR Aug 2009: All we need is two identical ferrite toroidalcores to make up current transformers, one placed in series with each leg of the coax line. The outputs from secondaries of the two transformers are fed through a selection switch into a rectifier and filter circuit to give a line current indication on a micro-ammeter. By selecting one, and then the other, of the two positions of the switch, the currents in the two legs of the line can be compared. If near equal, we can be satisfied that the balance is OK. If different, there is a common mode current component on the line and we can expect it to radiate. To get rid of this component, we may have to improve the balancing interface between the line an the antenna. If a choke balun is used this might mean increasing the inductance of the balun unit. All rights reserved. 07/10/17 30

There is no shortage of hams who will tell you that a balun didn t work for them. Likewise, there are plenty who will assure you that a balun was very effective. Conclusions without reliable and meaningful observations and sound analysis are of very limited value. All rights reserved. 07/10/17 32

HF multi band dipole: non-resonant unloaded dipole fed in the centre with low loss open wire feed line. Balun objectives 1.8 30MHz. 1:1 ratio. High common mode impedance. High voltage withstand. Generous power rating given our 400W PEP limit. Practical for Australian constructors All rights reserved. 07/10/17 33

Design Insertion loss Insertion loss does not imply conversion of RF to heat Insertion loss is relatively unimportant for use with an ATU Insertion VSWR Insertion VSWR is relatively unimportant for use with an ATU Voltage withstand 1000W, VSWR(50)<100: Vpk=(2*1000*50*100)^0.5=3100 Vpk Common mode impedance Z >500 Ω from 1.8-30MHz Z >2k Ω from 3.5-15MHz All rights reserved. 07/10/17 34

Large F14 nickel zinc ferrite toroid core from Neosid(Sydney) PTFE insulated silver plated stranded conductor 0.5mm 2 to 0.8mm 2 (ebay) 12 turns twisted pair conductor, wound crossover style (W1JR), ends of each winding not too close. All rights reserved. 07/10/17 35

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