Many antennas and

antenna designers

neglect the true

cause of loss. The

major problem using

short antennas is

the reactance, not

the length!

## What is

Reactance?

Reactance really

means the voltage

and current at that

point of the system

are no longer

in-phase or 180

degrees

out-of-phase. The

result is, for the

same real work

power, the reactive

power or

Volt-Amperes-Reactive

power is higher. We

can say the

“apparent

power” is

higher while the

“power that

does real work”

remains the same.

Let’s assume we have

a transmitter with

100 watts output.

With a zero

reactance load the

RMS voltage times

RMS current will be

100. Remember the

formula P= I E ? If

the load is 50 ohms,

we would have 70.7

volts times 1.414

amperes.

When we add

reactance to the

system, the voltage

and current are no

longer in step at

that point in the

system. To have 100

watts of real useful

power, we might have

1000 volts and 2

amperes! That’s 2kW

VAR power with a

100W transmitter.

This is really why

feed lines get lossy

and tuners burn up

when an antenna

system is very

reactive.

## What Does

Reactive Power Do?

My first lesson

in reactive power

occurred around 1962

or 63. I was a

Novice with a single

807 in my homebrew

transmitter. It ran

60-75 watts input,

or about 30-50 watts

output. I tried to

load my 40 meter

dipole on 80 meters,

and my feed line

actually melted!

Let’s look at an 80

meter dipole

example. With 100

watts and a modest

height 80 meter

dipole at resonance

we have 1.32 amperes

at 75.9 volts at the

feedpoint. The 50

ohm SWR is 1.15:1

Frequency = 3.72

MHz

Source 1 Voltage =

75.89 V. at -0.1

deg.

Current = 1.318 A.

at 0.0 deg.

Impedance = 57.59 –

J 0.09832 ohms

Power = 100 watts

SWR (50 ohm system)

= 1.152 (450 ohm

system) = 7.813

*Let’s Shorten*

the Dipole….

For the 102-foot

G5RV we have:

Frequency = 3.72

MHz

Source 1 Voltage =

143.5 V. at -61.09

deg.

Current = 1.441 A.

at 0.0 deg.

Impedance = 48.16 –

J 87.19 ohms

Power = 100 watts

SWR (50 ohm system)

= 4.957 (450 ohm

system) = 9.699

Notice if we

multiply 1.441

amperes times 143.5

volts, we have 206.8

watts of reactive

power.

With the dipole

at G5RV length of

102 feet, we have

1.44 amperes and

143.5 volts. The VAR

power is 206.6 watts

with 100 watts of

real power applied.

The VSWR referenced

to 50 ohms is 5:1.

Losses are not

significantly

different, because

VAR power is only

207 watts.

*Let’s Shorten*

the Dipole even

more……..

When we shorten

the antenna to 88

feet, we have a

problem.

Frequency = 3.72

MHz

Source 1 Voltage =

1078 V. at -87.53

deg.

Current = 2.153 A.

at 0.0 deg.

Impedance = 21.58 –

J 500.3 ohms

Power = 100 watts

SWR (50 ohm system)

> 100 (450 ohm

system) = 46.651

What a change in the

system! With a 100

watt transmitter

current is now 2.15

amperes while

voltage is 1078

volts. 50 ohm SWR is

over 100:1. Antenna

feedpoint VAR power

is 2317 watts with

only 100 real watts

applied. So far as

the overall system,

including the

matching, feed line,

conductors, and

insulators are

concerned, it is

like we have

increased power

level 23 times.

### What About an

Extreme?

Let’s look at a

20 meter dipole on

80 meters.

Frequency = 3.72

MHz

Source 1 Voltage =

14570 V. at -89.94

deg.

Current = 6.233 A.

at 0.0 deg.

Impedance = 2.574 –

J 2338 ohms

Power = 100 watts

SWR (50 ohm system)

> 100 (450 ohm

system) > 100

VAR power is now

9.1 kilowatts with a

100 watt

transmitter.

### Conclusion

VAR power is the

real killer in

system efficiency

and component

damage. VAR

power is the

real reason a very

short dipole taxes

system components

and has terrible

efficiency.

We can get away

with a 88-foot

dipole on 80 meters

if we have proper

reactance

compensation. Proper

reactance

compensation brings

voltage and current

back into phase,

reducing the

apparent power

handled by the

system. This reduces

losses.

The components

required must be

sized to handle the

increased voltage

and current. A 100

watt transmitter

operating into a

highly reactive load

can be like running

9kW, so far as the

components in the

reactive part of the

system are

concerned. This

means the feed line,

balun, and tuner

must be

conservatively rated

when using a short

dipole, or you have

to run very low

power. It also means

losses will be high.

We have to use very

low loss components

when reactive power

levels are high,

otherwise we convert

energy into heat.

If we look at the

antenna by itself,

we hardly notice any

change in antenna

performance when

length is decreased

from 130 feet to 50

feet or less on 80

meters. The problem

is the reactive

power of a very

short antenna

stresses other

components,

including the

feed line, balun, and

tuner.

We probably

wouldn’t notice 5dB

of power loss, after

all conditions can

vary many decibel

from day to

day. This is

why QRP operators

often enjoy using

inefficient

antennas. Operators

who place their

antennas in

competitive

situations would get

the feeling

something is not

quite right.

Of course with QRO,

poor efficiency

shows up like a slap

across the face. A

melted tuner, balun,

or feed line will

pique someone’s

interest.

Power loss might

easily slip under

our

“RADAR” at

100 watts or less,

but at high power

the additional

component stresses

will capture our

attention!