e004045.pdf

COMM UNICATIONS

active whip aerial

mini short-wave aerial

Design by G. Baars

Short-wave listeners who like to travel a lot have a particular need for an

aerial that provides a bit more gain than a standard whip aerial. A second

major requirement for a portable aerial is that it ideally should be as

compact as possible. This active whip aerial is only 50 cm long, and it

completely meets all the demands placed on an ideal travelling companion.

In the January 2000 issue, we already

offered short-wave listeners with limited

space an alternative to a long-wire aerial,

in the form of a simple active loop aerial.

This is a very compact aerial, but we fully

understand that it is not ideal for use

while travelling. You can hardly expect to

be able to hammer a few nails into every

window frame that you encounter on your

journey, just so you can wind a few loops

of wire around it. The loop aerial is thus

better for stationary use.

What then is the ideal travelling com-

panion for a short-wave listener? If you

demand an aerial that provides better

reception than a standard whip but is

also as compact as possible, you arrive

almost automatically at the design

described in this article: a short, active

whip aerial whose electronics you can ﬁt

in your vest pocket, powered by a 9-V

battery!

The concept

What most of us would really like to have

is a sort of magic aerial: very small and

compact, and at the same time much bet-

ter than its bigger brothers. Unfortu-

nately, magic aerials do not exist, so in

real life we always have to accept a com-

promise. This is not so bad, as long as the

result is close enough to what we wanted

in the ﬁrst place.

Of all the known possibilities, the most

compact and easy to use type of aerial is

without doubt a short whip aerial. If it is

made in telescopic form, it need not be

4/2000

Elektor Electronics

COMM UNICATIONS

any longer than around ten centimetres when

pushed together. It’s hard to get any more compact

than this.

However, a very short whip aerial has two

important drawbacks. Since it is very much shorter

than the wavelength of the signals that we want to

receive with it (a quarter-lambda whip aerial for the

49-m band would actually have to be 12 m long),

its radiation resistance is very high. This creates an

immediate difficulty with adapting the aerial to the

low-impedance input of the receiver. If we were to

couple such an aerial directly to the input of the

receiver, this mismatch would mean that only very

little of the voltage that the aerial captures from the

ether would actually ﬁnd its way into the receiver.

The second drawback is that a whip aerial with

a length of 50 cm (for example) naturally picks up

a lot less signal than an aerial that is physically

much larger.

These two problems demand an active exten-

sion to the aerial. This should at minimum have the

following characteristics: (a) high input impedance,

(b) low output impedance and (c) a reasonable

amount of gain.

With an eye to the high signal levels that are com-

monly used in the short-wave bands, we must nat-

urally also impose suitable requirements on the

large-signal behaviour of the electronics. Finally, if

we want to be able to continuously scan the short-

wave bands without having to stop to retune the

aerial, the active part of the aerial must also have

a handsomely wide bandwidth.

All in all, this is a fairly impressive list of require-

ments!

10 dB. If desired, the gain can be

increased by increasing the voltage on

gate 2 (by reducing the value of resistor

R2), but doing so causes the problems

due to intermodulation and other forms

of interference to quickly increase. It is

therefore not recommended.

After transistor T1 has taken care of

adapting the input impedance and pro-

viding the gain, the only problem that we

have left is to match the output imped-

ance to the 50-Ω input impedance of the

receiver. This job falls to transistor T2,

which is a VHF JFET that is biased for a

very high drain current so that it can do

its work properly. The signal is coupled to

the output connector K1 via capacitor C4.

The receiver can be connected to K1 via

a length of 50-Ω coaxial cable.

ready-made from Readers Services. If you

use this circuit board, it is very easy to

assemble the circuit, since there are only

a handful of components involved. The

only detail worth noting is that the dual-

gate MOSFET T1 must be soldered to the

bottom side (copper side) of the circuit

board, which is one reason why it is a

good idea to pay careful attention to the

lead arrangement of this transistor. With

the type J310 (or E310) transistor that is

used for T2, there are also versions avail-

able in an SOT-23 SMD package; these

have ‘SST’ or ‘PMBF’ as a preﬁx. The cir-

cuit board layout is designed so that

these SMD devices will also fit on the

board, but in this case T2 must also be

soldered to the bottom side of the board,

just like T1. A J310 in a standard TO-92

package can be mounted in the usual

way on the top side (component side) of

the board.

A board-mounted female receptacle is

used for the output connector K1. The

The printed circuit board

The printed circuit board is shown in Fig-

ure 2. It is unfortunately not available

ANT1

≈ 40mA

9...12V

100n

The realisation

Although our assignment was not exactly easy, if

you look at Figure 1 you can see that the active

portion of the aerial is nevertheless a very simple

design. It contains only two semiconductor devices

and a few resistors and capacitors, nothing more.

However, we must remark right away that we

intentionally chose to use FETs for the active com-

ponents, in order to prevent the non-linearity of the

amplifier from causing too many problems with

intermodulation products. The quadratic input

characteristic of a FET gives better results in this

regard than the exponential characteristic of a bipo-

lar transistor. In addition, the linearity of the ampli-

ﬁer can be even further improved by operating the

FETs with fairly high drain currents.

Let’s return to the schematic diagram. The very

short whip aerial (50 cm long) is connected to

gate 1 of the dual-gate MOSFET transistor T1 via

capacitor C1. The unusually high input resistance

and low input capacitance of this transistor place

almost no load on the aerial. In addition, this

MOSFET, which can be used up to very high fre-

quencies, also provides the gain of the amplifier.

Since excessive gain does more harm than good,

the voltage gain is here limited to around 5 to

2V2

7V3

220n

10p

2V8

33p

J310

BF961

100n

000027 - 11

BF961

J310

SOT-23

TO-92

Figure 1. Two FETs provide a proper impedance match for the whip aerial, as well

as the necessary gain.

Elektor Electronics

4/2000

COMM UNICATIONS

000027-1

Figure 2. The active circuitry for the antenna can be quickly built using this tiny printed circuit board.

COMPONENTS LIST

Ω

R2 = 100k Ω

R3,R5 = 47k

Ω

R4 = 560 Ω

R6 = 56

Ω

Capacitors:

C1 = 10pF

C2,C5 = 100nF

C3 = 33pF

C4 = 220nF

Semiconductors:

T1 = BF961

T2 = J310 or E310 (SMD versions:

SST310, PMBF310)

Miscellaneous:

K1 = phono (RCA) socket for

board mounting

Enclosure: e.g. Hammond type

1590A (90x38x30mm)

manner in which the 50-cm whip aerial is

connected to the input pins depends on

the particular aerial that is used.

cast aluminium enclosures made by

Hammond are a very good choice. The

circuit board together with a 9-V battery

will just fit in a type 1590A enclosure. If

you use a type 1590B enclosure, you will

have room to spare, but this enclosure

lacks the charm of the smaller format.

There are of course also enclosures avail-

able from other manufacturers in which

the small circuit board and the battery

can be housed in a neat manner.

more suitable solution. It should have a well regu-

lated output voltage between 9 V and 12 V, with no

interference or noise.

There is one final remark. As we noted earlier

on, it is not a good idea to choose too high a value

for the gain, due to the associated intermodulation

products and other forms of distortion. For exactly

the same reason, it is also not a good idea to make

the whip aerial much longer than 50 cm. With a

longer aerial, the increase in the signal strength is

marginal, but the distortion level increases quickly.

(000027-1)

The enclosure

Once you have assembled the circuit

board, the ﬁrst thing you should do is give

it a good inspection. After this you can

check the voltage measurements marked

on the circuit diagram, using a digital

voltmeter. These values were measured

using a supply voltage of 9 V. The values

that you measure may be different, due to

the normal tolerance range of the FET

characteristics. Deviations from the

marked values of up to 25% should be no

cause for alarm.

Once everything seems to be in order,

it’s time to look for a suitable enclosure.

A metallic enclosure is preferable. The

Conclusion

The current consumption of the circuit is

around 40 mA. Since the capacity of

modern alkaline batteries is fairly large,

a 9-V battery is in principle a very satis-

factory source of power when you use the

aerial while travelling. For long-term

usage, a mains adapter appears to be a

Design editing: K. Walraven

Text (Dutch original): S. van Rooij

4/2000

Elektor Electronics

Resistors:

R1 = 1M

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