XformHFAnt.pdf

RHP-TRP1 ¨ R H Pearson G4FHU July 2006 Page 1 of 11

Broadband Transformers in HF Antenna Systems

by R H Pearson G4FHU

Introduction

Transformers wirewound on suitable ferrite or powdered iron ring cores are easy to make, and can

give excellent service in HF antenna systems, but sometimes disappoint or even fail completely

when insufficient attention is paid to their operating conditions, and the type of winding and

magnetic core employed. Part 1 concerns impedances, voltages and currents in antenna systems.

Part 2 explores transformer principles and Part 3 the properties of magnetic cores and Part 4 shows

a simple practical transformer and some conclusions..

1.1

Antenna impedance

Most ordinary power transformers are designed for and used in well defined conditions of voltage

and frequency. Those used in amateur radio HF antenna transmitting systems are usually required to

operate over a remarkably wide frequency range and are often subjected to extremely variable,

onerous and uncertain load impedances.

When a transformer is placed somewhere in line between a transmitter and an antenna, it can be

difficult to predict and awkward to measure the operating conditions that the transformer will

experience. For example, suppose it is used at the junction between a feeder and a dipole and that

the operation is always restricted to the 80m HF band. The impedance of a typical dipole designed

for that band is likely to be somewhat similar to that shown in Fig 1. This is very much like that of

a "lumped" LCR series tuned circuit, having a purely resistive impedance at the frequency of

resonance, and a significant reactance over most of the band. The dipole may be visualised as a

kind of tuned circuit but with multiple resonances. Its inductance, capacitance and resistance are

"distributed" i.e. spread out in space, so much so that most of the applied power is radiated and only

a small proportion is wasted as local heating. A significant and inconvenient feature is that the

reactance, though nullified at a frequency of resonance, increases in magnitude very rapidly with

even a small change in frequency and as will be shown in detail later, this can greatly increase the

demands upon a transformer used in the feed path.

Fig 1 Resistance, Reactance and Impedance of a typical wire dipole around its resonant frequency

RHP-TRP1 ¨ R H Pearson G4FHU July 2006 Page 2 of 11

1.2

Impedance Transformation by the Feeder Cable

The transmission line used to connect the antenna to the transmitter, the "feeder", behaves as yet

another kind of distributed LCR network. The impedance at its input end, another place where one

might want to connect a transformer, will often be considerably different from that of the load to

which it connects. Indeed, transmission lines themselves are sometimes used quite deliberately as

impedance transformers, though they only function satisfactorily in that role, over a very narrow

ranges of frequency.

(a) (b) (c)

Fig 2 Impedance at the feed point of an antenna system consisting of a 3.7 MHz 70 ¼ dipole

operating at resonance, via a cable having Characteristic Impedance Z 0

(a) Z 0 = 70 (b) Z 0 = 50 ¼ (c) Z 0 = 300 ¼

In some circumstances the feeder will modify the antenna impedance to advantage, but in many

others it makes the impedance less convenient as shown in Fig 2.

1.3

Antenna System Tuning Units and Baluns

For the above reasons, most amateur radio antenna systems incorporate an adjustable network of

low-loss inductors and capacitors that can be adjusted automatically or manually to achieve the

correct impedance transformation at any desired frequency. Ideally, it ensures that the load

presented to the transceiver is free of reactance and has the optimum resistance required by the

transmitter to enable it to deliver its full normal power.

It tunes the whole system, including itself, and so may be called an "Antenna System Tuning Unit",

but this is usually contracted to "ATU" despite that fact that it does not tune the antenna alone.

Some prefer to use the term "Matching Unit" or "Impedance Matching Unit", but the word

"Matching" can trigger what has from time to time proved a rather unprofitable and sometimes

heated controversy that is best avoided here!

Quite often the antenna is nominally a balanced structure in respect of impedance, i.e. it has equal

impedances between each terminal and earth, or impedances are so high as to have negligible

effects, in which case it is said to be "Floating". When an unbalanced feeder, such as a coaxial

cable, is used with a balanced antenna, a special kind of transformer called a "balun", i.e. a balance

to unbalance transformer, is sometimes used at the junction, so that the balanced impedance

condition of the antenna is not disturbed. The advantages include less risk of feedback down the

feeder sending RF power into the operating room, less received interference from nearby sources,

and less disturbance to the anticipated radiation polar diagram of the antenna, caused by the feeder.

RHP-TRP1 ¨ R H Pearson G4FHU July 2006 Page 3 of 11

A good alternative is to use a balanced feeder and to shift the balance/unbalance connection

problem to the more accessible operating room end of the system. Transceivers are almost always

arranged with unbalanced output terminals, with one side maintained at Earth potential for safety

reasons when connected to the main electricity supply. Many commercial ATUs have unbalanced

ports at both ends, in which case a broadband balun will best be located between the ATU and a

balanced feeder.

One splendid type of ATU though old as the hills is still favoured by experienced constructors and

operators. Often called a ÐLink CouplerÑ, it uses an input winding coupled via mutual inductance to

a tuned winding at the output end which serves both as part of the reactance network, and as a

voltage balun if so arranged. When driving a balanced feeder this completely avoids the need for a

broadband type of balun, and being part of the adjustable network does not suffer at all from being a

narrow frequency band device; indeed it confers useful extra selectivity on both transmission and

reception. Although it has gradually disappeared from books and magazines over the last 30 years

or so, it recently appeared again in an interesting RadCom article by Peter Dodds [1].

Another possibility is to use an ATU that is a balanced circuit at both input and output. Then a

balun interposed between the transceiver and the ATU will always be loaded by a well adjusted and

convenient impedance that enables it to work well with modest and predictable requirements. This

is an ideal arrangement for a QRP station but rarely seen in higher power systems owing to the need

for some extra precautions in the layout and controls of the ATU to ensure that no high voltages

become accessible outside the box.

1.4

The importance of load reactance

It is sometimes thought that there is some inherent reason why a wirewound transformer will not

work properly when feeding a reactive load. That is not really so but reactive loads can indeed be

the cause of a transformer operating problem for related reasons that will now be explained more

precisely.

In most domestic power circuits, the supply voltage V is pretty constant, so that adding some series

reacta nce X to a load resistance R will not only increase the load impedance

Z += but will also reduce the load current I = V/Z. But in an antenna system the ATU is

used deliberately to override this limitation by cancelling the reactance and adjusting the effective

resistance so that full power is transmitted throughout the whole system and is therefore applied to

any and all transformers in that system.

We can calculate exactly what will happen at any point in the system where a power P is passing

via an effective resistance R in series with a reactance X, (though alas in real life, we are most

unlikely to know those figures).

Since P = I 2 R, the current

I =

so that the voltage will be:

( ) (

( ) Ô

)

Ä +

This expression is plotted in Fig 3 as a contour map, showing how the RMS maximum voltage V

depends upon both R and X when transmitting a power of 100 W PEP . For a power of 400 W PEP

double the voltages shown.

RHP-TRP1 ¨ R H Pearson G4FHU July 2006 Page 4 of 11

Fig 3 V RMS that can arise in parts of an antenna system carrying 100 W

To put this into a practical perspective, bear in mind that when feeding a 50 ¼ resistive load, a 100

W PEP transmitter has an output voltage of 70.7 V RMS and a 400W PEP one 141.4 V RMS . As can be

seen in Fig 3, a load consisting of an abnormally low resistance and high reactance can result in a

remarkably high voltage that will arise just as the tuning operation succeeds in producing the

desired impedance transformation to suit the transmitter.

2.1

Broadband Transmission-Line Transformers (TLTs)

To make a wirewound transformer that works at high frequencies it is necessary to try to make

every turn of every winding link with the exactly same magnetic flux. Inadequacy in this respect is

equivalent to putting inductance (so called "leakage inductance") in series with the output. This is

why such transformers almost always use identical windings wound in multi-wire groups.

Ways of analysing the HF behaviour have been devised by regarding the windings as transmission

lines and using the standard transmission line equations to obtain accurate predictions. Though

many articles and books have been published on this rather specialised subject, anyone deeply

interested in TLTs can do no better than begin by reading the most frequently quoted original

papers; by Ruthroff [2] and Guanella [3]. Though more formally presented than later

interpretations, they are "The Real McCoy" and reliable guides when encountering doubt or

confusion. They show how to combine the properties of an ordinary transformer with those of a

transmission line, to produce an impedance transformer that works satisfactorily over a far wider

range of frequency than either a conventional transformer or a transmission line alone could

achieve.

Controversy sometimes arises in amateur radio circles, caused perhaps by over-simplified accounts

of what is to be found in these seminal publications. Though it is true that the low frequency

RHP-TRP1 ¨ R H Pearson G4FHU July 2006 Page 5 of 11

response of a TLT is maintained by ordinary transformer action, and the high frequency response by

transmission line action, it is simply untrue to say that the magnetic core is unimportant for high

frequency signals. That misconception has encouraged countless radio amateurs to attempt the

construction of TLTs using ring cores that are unsuitable for high frequency usage owing to

inadequate permeability and/or excessive power loss at HF.

It may be that vital details of diagrams in the original papers are sometimes overlooked. For

example, in Ruthroff's paper, his Figure 15 (reproduced as Fig 4 here), contains an easily

overlooked condition warning that the reactance of the windings is assumed to be much larger than

the load or source resistances, and in Guanella's paper Figures 1(a & b), (Figure 5 here), the crucial

inductive effect of the magnetic core is shown as an additional part (B) of the circuit diagram.

If one uses circuit simulation software (such as "SPICE") to aid design work it is vital to add

appropriate inductances to the circuit net list to avoid becoming seriously misled.

Fig 4 One of Ruthroff's TLTs Fig 5 One of Guanella's TLTs and its

equivalent circuit

Another very important factor is that in those early papers and in many recent ones, all the theory

and all the practical tests assume that the source and load impedances are perfectly resistive and of a

carefully calculated magnitude. In the simple Ruthroff example shown above, the design and

analysis assumes that the signal source is a pure resistance Rg, the load is a resistance R L = 4Rg and

that the winding is a transmission line with a characteristic impedance Z 0 = 2Rg. Such

circumstances rarely exist in an amateur radio station antenna system.

2.2 Transformer windings: number of turns

When designing an ordinary 50 or 60 Hz power transformer, one can usually specify its operating

conditions very precisely in terms of supply voltage and maximum current. Inescapable in the

design process for any transformer primary winding (or indeed any inductor used in an AC. circuit)

is what is often called "The Transformer Equation". It states in modern terms and relating to AC

circuits, precisely what Michael Faraday so famously discovered; the relationship between an

induced alternating voltage and rate of change of a magnetic flux linking a winding.

RMS

MAX

In this relationship,

V is the voltage applied to the primary winding, or induced in a secondary winding,

f is the frequency ( hertz, Hz),

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: