Ultima loopstick VLF antenna.pdf

‘Ultima’

loopstick VLF antenna

Design:

Richard Q. Marris G2BZQ

With its 30 cm long, 3.2 cm

diameter (12 x 1 1 ⁄ 4 inch) ‘bun-

dled’ ferrite core, the ‘Ultima’

high-performance loopstick

antenna covers much interesting

activity between 50 kHz and

195 kHz. The ‘Ultima’ is ideal

for home, portable, holiday and

overseas travel. It also covers

the new 136 kHz band with

excellent results, but for recep-

tion only .

An increasing number of radio

amateurs in the UK and several

European mainland counties are

now actively using a new VLF

(Very Low Frequency) band at

136 kHz (135.7 – 137.8 kHz).

The big challenge at these

extremely long wavelengths is to

make transmitter antennas with

reasonable efficiency . In fact,

anything greater than 1 per cent

is considered a feat! As to

receiver antennas, the emphasis

is on noise elimination .

Recently, distances of almost

2,000 km have been covered by

amateurs using CW and modest

transmitter powers (EI0CF –

OH1TN, 2-way CW QSO on

137.2 kHz). To amateurs in the

UK, the station DA0LF in Ger-

many is a good ‘DX target’. Lots

of useful information on VLF

Dx-ing may be found in Rad-

Com , the magazine published by

the Radio Society of Great

Britain (RSGB). In the US, a

group calling themselves

‘Lowfers’ has been active for

many years collecting valuable

information on the quirks of

‘their’ 1,750-metre band.

The ‘feel’ of the radio spectrum

below 150 kHz or so (approx.

2,000 metres) is totally different

from anything you may have

experienced on higher frequen-

cies. Although you will not fail

to notice a complete lack of AM

broadcast stations, the most

prominent feature is the huge

noise level which can, on occa-

sion, have a devastating effect

on reception.

For reception, a specially-

designed type of receiving

antenna will be found desirable,

especially in urban areas where

levels of man-made noise can be

diabolical, especially when using

a long-wire antenna. Noise expe-

rienced below about 150 kHz is

either internal or external. Internal

30cm (12") long x 3cm (1 ") dia

1 4

nickel- z i nc f er rite core

cially in urban areas, man-made

noise can be from just about any

electrical/electronic source such

as TVs, computers, calculators,

thermostats, light dimmers, vac-

uum cleaners, lawn mowers,

electric power tools, traffic, power

lines and may other sources in

the immediate neighbourhood.

Atmospheric noise, including

electric storms, is a natural phe-

nomenon, which, at its worst, can

obliterate reception.

Space to many amateurs is at a

premium, and a multi-turn

tuned frame loop antenna of,

say, 1.2 m x 1.2 m will be the

absolute size limit. Despite all

its obvious advantages, includ-

ing sensitivity and good selec-

tivity, such a directional antenna

can be a cumbersome brute, and

inconvenient to store when not

Z = 50

800p

984108 - 11

Figure 1. Schematic of the Ultima ferrite-rod VLF antenna

noise is either generated in the

receiver, or enters via the AC

mains power wiring. Remedial

action can be taken. External

noise is different kettle of fish.

Entering by way of the antenna,

it is either man-made or atmos-

pheric. Man-made noise, in its

simplest forms, is QRM from

another station. Otherwise, espe-

30cm (12")

28cm (11")

3cm

(1 1 / 4 ")

o/d

L1 = 28cm (11") wide close wound turns

of 24 SWG en. Cu. wire

2x PVC tails

plastic or card tube

7.6cm (3")

3.65cm

(1 7 / 16 ")

3.8cm

(1 1 / 2 ")

o/d

L2 = 3.65cm (1 7 / 16 ") wide close wound turns

of 24 SWG en. Cu. wire

2x PVC tails twisted

loosely together

to COAX

984108 - 12a

slide L2 over L1 and secure

ferrite core

to COAX

984108 - 12b

Figure 2. (a) winding L1 and L2; (b) L1 and L2 assembly.

108

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30cm (12")

9.5mm ( 3 / 8 ")

15cm (6") rod

superglue

adhesive

6 ferrite rods as above

adhesive

3cm

(1 1 / 4 ")

o/d

insert core into 3cm (1 1 / 4 ") o/d

card/plastic tube

1 - 6 = ferrite rods

= wood dowel / rod

L1 + L2 wound on here

wood rod extension handle

(optional)

984108 - 13

Figure 3. Build the Ultima in this order. Forming a single 30 x 1 cm (a); and a single 30 x 3.2 cm ferrite core (b). Drawing (c) shows the 30 x 3.2 cm ferrite

winding assembly, and drawing (d) the format of the final ferrite core.

in use.

An alternative is a ferrite-rod

loop (‘loopstick’), which in its

original form is less sensitive

than a frame loop. However a

highly sensitive ferrite loop can

be designed with careful selec-

tion of ferrite core materials and

dimensions. To put it simply, the

ferrite loop then has to be large

at, say, 30 x 2.5 cm (12 x 1

inch). Unfortunately, such ferrite

rods are not only few and far

between but also astronomically

expensive. The size of affordable

manganese-zinc or nickel-zinc

ferrite rods stocked by radio

parts retailers is usually either

20 x 1 cm (8 x 3 ⁄ 8 inch) or 20 x

1.25 cm (8 x 1 ⁄ 2 inch). It was

decided to home-brew a 30 cm

long, 3.2 cm diameter (12 x 1 1 ⁄ 4

inch) rod by bundling a number

of smaller zinc-nickel rods. The

target inductance was 26+ mH.

In the basic loopstick circuit

shown in Figure 1 , coil L1 is

brought to resonance by variable

capacitor C1. The core of L1 is

the above mentioned ‘bundled’

giant loopstick. Over the centre

of L1 is wound coupling coil L2,

which provides the coaxial con-

nection to the receiver or VLF

up-converter. The coil assembly

(Figure 2) is wound on a 30 cm

long x 3.2 cm diameter thin wall

cardboard tube (Clingfilm tub-

ing!). L1 has a width of 28 cm,

and consists of an estimated

466 close-wound turns of

24 SWG (0.6 mm) enamelled

copper wire ( Figure 2a ). The

winding is terminated with lead-

outs of PVC covered hook-up

wire. In practice the winding

ends were held in place with a

spot of SuperGlue, with other

spots every 2.5 cm or so along

the winding. This is necessary

as winding the coil is a lengthy

process. The coupling coil, L2,

is wound on a 7.6-cm (3-inch)

length of cardboard tube of a

diameter which just slips over

L1. L2 is a 3.6-cm (1 7 ⁄ 16 inch)

wide close-wound winding of

24 SWG enamelled copper wire,

terminated with hook-up wire

leads lightly twisted together.

The whole coil assembly is

shown in Figure 2b .

The next step is to build the

bundled ferrite core — this is

illustrated in Figure 3 . It con-

sists of six 30 cm long, 1 cm

diameter ferrite rods glued

together to form one solid 30 cm

x 3.2 cm (approx.) core. MMG

F14 grade nickel-zinc material

was used. An alternative would

be the US type 61 material. At

these low frequencies, the dif-

ference in performance between

the F14 and 61 materials is

small. Each 30-cm rod is made

from two 15-cm rods, secured

end-to-end with SuperGlue. The

rod ends should first be lightly

rubbed down with a fine glass

paper in order to remove any

grease, etc. (see Figure 3a ).

This technique effectively pro-

duces one long rod from two

shorter ones. Other combina-

tions of length could be used

such as 20 + 10 cm, the 10-cm

section being cut from a 20-cm

rod using a hacksaw. In this way,

three 20-cm rods would make

two 30-cm rods.

The solid 30-cm long, 3.2 cm

diameter ferrite rod consists of

six 30-cm rods wrapped around

a wood dowel, and temporarily

held in position with a couple of

elastic bands, see Figures 3b,

3c and 3d . Next, the rods and

dowel are adhered together to

form one solid core, by cemen-

tation with a 15-minute setting

adhesive such as Uhu. The

adhesive is run along between

all mating rod and dowel sur-

faces, by easing them gently

apart with a thin blade. It is

important to ensure that the sur-

faces have the adhesive between

them. Several strong elastic

bands are put around the rods,

making sure that the circular

rod formation is maintained. The

assembly should be left in a

warm place for at least 24 hours

to make sure that the adhesive

is thoroughly cured. The elastic

bands are then cut away.

The core is then inserted into

L1, with any slight looseness

being eliminated with masking

tape. On the prototype, the wood

dowel was made a few centime-

tres longer than the rods, so that

the core can be extracted from

the coil if and when necessary,

for example, for experiments.

The simple final assembly is

clearly shown in Figures 4a,

4b and 4c . Three identical

strips of wood are fastened

together with wood screws to

form an inverted ‘U’ shape chas-

sis. Coil L1/L2 is mounted on

the top with two narrow type

plastic coated Terry clips fas-

tened at the chassis ends. The

twisted ends of L2 are dropped

through a hole drilled in the

chassis top, and taken to the co-

axial socket mounted on a piece

of thin board, screwed to the

chassis end.

The 800-pF tuning capacitor,

C1, is mounted on the chassis

side as shown in Figure 4. On

the prototype, a rigid air-spaced

800-P tuning capacitor

(392+11+392+11 pF AM/FM

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Terry clip

knob

COAX socket on plate

in plastic box

coupler insulated shaft

alu bracket + panel bush

984108 - 14b

30cm (12")

Terry clip

knob

COAX socket on plate

coupler

insulated shaft

alu bracket + panel bush

984108 - 14a

2.5cm (1") plastic coated

Terry clip

L1, L2

COMPONENTS LIST

(see text) with shaft coupler,

insulated extension shaft,

metal bracket, panel bush

and knob

1 off 38 cm (15 inch) length of

1 cm ( 3 ⁄ 8 inch) o/d wood dowel

1 off coaxial socket

2 off narrow 2.5-cm (1 inch)

plastic coated Terry clips

Reel of 24 SWG (0.6 mm)

enamelled copper wire.

12 off F14 or 61 grade ferrite

rods, 15 cm long, 1 cm diam-

eter ( 6 x 3 ⁄ 8 inch)

1 off 3.2 cm (1 1 ⁄ 4 inch) thin wall

cardboard tube (e.g., Cling-

film tubing)

SuperGlue

Slow setting glue, e.g. Uhu

1 off 800pF tuning capacitor

spindle end of C1

(without knob)

alu bracket

30 x 12mm

(1 1 / 4 " x 1 / 2 ")

wood

Figure 4. Details of the final assembly.

984108 - 14c

type wired in parallel) is

attached to the chassis side. It

is fitted with a shaft coupler and

an insulated shaft passing

through a panel bush in a small

bracket to the control knob. A

1,000-pF (500 + 500 pF) tun-

ing capacitor would also be sat-

isfactory. The tuning capacitor

is enclosed in a plastic dust-

cover box. The ends of L1 are

taken to the tuning capacitor

connections.

The tuning range of the proto-

type was 50 kHz to 195 kHz.

The ‘Ultima’ is used with a Palo-

mar VLF up-converter whose

output frequency is in the 80-m

(3.5-MHz) amateur radio band.

A simple turntable is an advan-

tage to be able to turn the loop,

which is directional. The fre-

quency range was carefully

selected. At the LF end is the

60 kHz MSF Rugby Standard

Time/Frequency station, which

produces a mighty signal as

might be expected. Moving up-

frequency the tuning range

passes through the 73-kHz band

where various Time/Frequency

Standard stations can be

received, and much else of

interest. Next comes the 2,000-m

European Amateur band around

136 kHz which has recently

arrived.

As compared with a traditional

20 x 1.25 cm diameter loop-

stick, the ‘Ultima’ features dra-

matically increased signal

strength, with atmospheric and

man-made noise being mostly

eliminated, or reduced to an

acceptable level by simple loop

rotation.

(984108-1;W)

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