HOM rev. new!Heathkit of the Month #42 - HD-1422-A Antenna Noise Bridge
Heathkit of the Month #42:
by Bob Eckweiler, AF6C
Heathkit HD-1422-A
Antenna Noise Bridge
Introduction:
If you work with antennas, an antenna noise
bridge can be a very handy tool. Table 1 lists
some of the uses of an antenna noise bridge as
described in the Heathkit manual:
Finding Antenna Resonant Frequency.
•
Finding Non-resonant Characteristics.
•
Making Tuned Circuit Measurements.
•
Making Component Measurements.
•
Finding Transmission Line Resonant Fre-
•
quency.
Tuning Transmission Line Length 1/4ƛ or
•
1/2ƛ.
Antenna Tuner Adjustments.
•
Table 1: Bridge Uses (from Heathkit Manual)
An antenna noise bridge measures an unknown
impedance connected to the bridge. Two controls are on the front panel; these controls are
adjusted until the noise nulls in an external detector. The front panel resistance control then
shows the resistive component of the impedance. The front panel center zero reactance
control reads the reactance, usually in ±pF. To
find the true reactance you must convert capacitance into reactance using the well known
formula:
X = 1/(2πfC)
where f is the frequency of the detector and C is
the capacitor front panel reading. Negative C
values represent inductive reactance.
An antenna bridge, sometimes called an R-X
bridge, is made up of three sections: a wideband noise generator, sometimes keyed at an
audio frequency, a bridge circuit, and a detec-
Figure 1: Heathkit Antenna Noise Bridge
Model HD-1422-A - Front View
tor. The best detector (and the one most commonly used) is the station receiver; it is sensitive, selective, stable and accurately reads the
frequency.
The Heathkit HD-1422 Noise Bridge:
Heathkit first released the HD-1422 in late
1985. In 1989 it was upgraded to the HD-1422A, changing the paint scheme to a small degree,
as well as making a small improvement to the
circuit. The HD-1422-A was priced at $49.95 in
1989.
The HD-1422 has a range of 0 to 200 ohms resistive component and ±60 pF of reactance.
This 60 pF calculates from 1.5KΩ at 1.8 MHz to
90Ω at 29.7 MHz.
The front panel of the HD-1422 (Figure 1) contains three controls and an LED pilot light. On
the left is the OFF-ON slide switch with a red
LED above it; the other two controls, left to
right, are a potentiometer scaled 0 – 200 that
represents the resistive (R) component in
ohms. A 120 pF variable capacitor that is scaled
60 XL – 0 – 60 XC that represents the reactive
(X) component in pF . Negative pF converts to
inductive reactance.
On the rear panel of the HD-1422 (Figure 2)
are, left to right, RECEIVER (SO-239 UHF
connector), UNKNOWN (SO-239 UHF con-
nector), External +9VDC power jack (1/8” mini
phone jack) and a #8 GROUND stud. The unit
Copyright 2012, R. Eckweiler & OCARC, Inc."Page 1 of 5
Page 2
Heathkit of the Month #42 - HD-1422-A Antenna Noise Bridge!HOM rev. new
is normally powered by in internal standard
NEDA #1604 9-volt battery. The external
power jack presents an interesting problem;
the external power connector is a male 1/8 plug
and is prone to shorting when lying on the table unplugged, or while plugging into the HD1422 with the power on. This requires a rather
high impedance or current-limited external
supply to prevent sparks when the plug shorts.
The optional PS-2350 wall wart power supply
provides a nominal 9V at 100 ma max. that can
power the antenna noise bridge. The HD-1422
requires 45 ma at 9 to 11 VDC.
The Heathkit HD-1422-A Noise Bridge:
Externally the “A” model of the HD-1422 appears identical to the original except for the
paint scheme. The cabinet color has changed
from a two-tone dark brown to a two-tone dark
gray. The nomenclature remains in white except for the Heathkit logo in the upper left that
is red on the original and is pale yellow on the
“A” model.
Circuit-wise the HD-1422-A is very similar to
the older model. A bypass capacitor, C108, has
been added across the emitter resistor of Q102,
the second stage of the noise amplifier. Also,
the schematic has been corrected to agree with
Figure 2: Heathkit Antenna Noise Bridge
Model HD-1422-A - Rear View
the actual circuit. In the original schematic
R107 and R108 are shown connected between
the 9-volt line and the base of Q101 and Q102
respectively. However the circuit board x-ray
view shows the resistors correctly connected
between the base and respective collector. See
figure X; this figure also shows the added C108.
On the later model R109 is mounted directly to
the leads of added C108, and C108 is inserted
into the circuit board holes negating the need
for an updated circuit board. There is another
minor schematic error that was not fixed in the
later schematic. The “A” and “B” circuit board
connections are mislabeled; “B” should be the
ground lead with the BLK wire and “A” should
be the connection from R101 with the RED
lead.
Figure 3: R107 & R108 are misplaced on the HD-1422 schematic (left) but corrected on the
HD-1422-A schematic (right). Both units are wired identically except for added C-108.
Page 2 of 5!Copyright 2012, R. Eckweiler & OCARC, Inc.
Page 3
HOM rev. new!Heathkit of the Month #42 - HD-1422-A Antenna Noise Bridge
The Heathkit HD-1422A Circuit Description:
The Heathkit HD-1422A circuit can be broken
into 4 parts: A tone oscillator, a noise generator, a noise amplifier and a bridge circuit. The
schematic is shown in figure 5.
The tone oscillator uses an NE-555 timer integrated circuit (U101) to produce a nominal
1,000 Hz square wave that drives the noise
generator. This effectively puts a 1000 Hz tone
on the noise signal making it easy to distinguish. It is a feature not found in many antenna
noise bridges.
The square wave output at pin 3 of the U101
passes current through zener diode D103 to
ground. Zener diodes are notorious for generating noise. This noise is wide-band and in use
where the noise is not desirable is immediately
bypassed to ground with a capacitor. Here the
noise is wanted and is coupled to a two stage
noise amplifier by C106.
The noise amplifier is two stages of simple amplification. The first stage (Q101) is a simple
amplifier coupled via C104 to the second stage.
The gain of the second stage (Q102) was increased in the later model by bypassing the
emitter with C108. The output of the second
stage excites the primary lead of T101, the
bridge transformer.
back, which is your station receiver. The bridge
is pretty simple. When the R of the unknown
impedance is identical to the R of the potentiometer AND the reactance of the unknown impedance is also identical to the reactance of the
reactance capacitor then the bridge balances
and the noise in the receiver nulls. In order to
measure inductive reactance as well as capacitive reactance a 68 pF capacitor (C107) is place
in series with the unknown leg of the bridge.
This skews the reactance part of the adjustment leg so that zero reactance occurs in the
known leg when the variable capacitor is at 68
pF which is about mid-scale.
HD-1422-A Assembly & Test:
Heathkit classed this kit as a “one evening project”. Most of the components mount on a
small printed circuit board. The internal 9V
battery is held to the board by a clip; one lead
of the battery connector connects to the board;
the other lead is connected to the external
power jack so that when external power is
plugged in, the battery is disconnected.
Transformer T101 must be wound by the
builder. Three 9” color-coded wires are
grouped and wound next to each other on the
small green color-coded toroid core. A total of
seven turns per winding are made. I could not
find the core model in an old Amidon catalog.
T101 is part of the bridge circuit; the heart of
the HD- 1422-A Noise Bridge. The circuitry up
to this point just produces white noise over the
HF frequency at a level of 50 to 100 µV. Heathkit added U101 to give the noise a nice 1,000Hz
tone and make it distinguishable, but the 50µV
noise should register at least S9 on most receivers. The secondary of the bridge circuit has
four legs. Two of the legs are identical windings
in a RF toroid transformer. The third leg is
coupled through a 68 pF capacitor to the UN-KNOWN UHF connector on the back. The
fourth leg is made up of a 7 - 120 pF variable
capacitor in series with a 250Ω potentiometer.
The center tap of the secondary windings is
connected to the detector connector on the
Copyright 2012, R. Eckweiler & OCARC, Inc."Page 3 of 5
Only the front and rear panel controls and
connectors are not mounted on the circuit
board. These are mounted and wired up after
the board is completed. Finally the board is installed by #6 hardware to the chassis and leads
from the board are attached where necessary.
Checkout is done by first making some simple
ohmmeter readings and then installing the battery (or plugging in the external power supply)
and making one voltage measurement.
Calibration requires only your receiver, a short
piece of wire and a 51Ω resistor (supplied with
the kit). First the unknown connector is
shorted internally with the piece of wire and
Page 4
Remember,ifyou aregettingridof anyold HeathkitManualsorCatalogs,pleasepass them along to me for my research.
Heathkit of the Month #42 - HD-1422-A Antenna Noise Bridge!HOM rev. new
the receiver is connected with a short piece of
coax to the RECEIVER connector. Then the
Noise Bridge controls are adjusted for a null.
This should happen with the two controls near
zero. If necessary the knobs can be loosened
and set right on. Then the short is replaced
with the 51Ω resistor. Again the Noise Bridge
controls are adjusted for a null. This time the
pot should indicate 50Ω and the reactance
should remain at zero. Again the knobs can be
realigned. This should be repeated if necessary
until the calibration is as close as possible.
Summary:
The Heathkit HD-1422, while a basic Antenna
Noise Bridge, provides numerous features that
make antenna measurements easier.
In the late seventies I built a noise bridge from
an article in the February 1977 issue of Ham
Radio Magazine. The authors W6BXI & W6NKU
gave a presentation at our club, providing a lot
of additional information. I still have a copy of
their presentation if anyone would like to see it.
Their bridge is designed to operate over a
larger reactance span then the Heathkit bridge
- 0-250Ω and ±180 pF and they designed
adapters to further increase coverage on the
lower ham bands. A computer program I wrote
for the old Apple ][ and later in “C” for the Apple IIgs made this a versatile piece of test
equipment. Unfortunately porting it to a more
modern computer is still on my to-do list.
After building the HR featured noise bridge, I
picked up a Palomar Engineers “R-X Noise
Bridge” which has an R-X range of 0-250Ω and
±70 pF. And some years back I was given the
Heathkit by Elmer - WA6PFA during a shack
Figure 4: HD-1422-A From the Winter 1991
Catalog. Note closeout price of $39.95
cleaning he was conducting.
For Next month I hope to present another
component of the Heathkit SB Amateur Line.
73, from AF6C
Page 4 of 5!Copyright 2012, R. Eckweiler & OCARC, Inc.
Page 5
HOM rev. new!Heathkit of the Month #42 - HD-1422-A Antenna Noise Bridge
Figure 5: Heathkit HD-1422-A Schematic"Q101-102:#2N5770 Si NPN Transistor
#D101-102:#1N4149 Signal Diode#T101:#7 turns trifilar wound
#D103:#VR6.8 Zener Diode#U101:#555 Timer IC
Copyright 2012, R. Eckweiler & OCARC, Inc."Page 5 of 5
Loading...
+ hidden pages
You need points to download manuals.
1 point = 1 manual.
You can buy points or you can get point for every manual you upload.