RCA BPA-10 Guide

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ANTENNA
TUNING UNIT
Type BPA-10
INSTALLATION-OP~RATION-MAINTENANCE
INSTRUCTIONS
Printed in U. S. A.
1B-30168-3-21-46

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TABLE OF CONTENTS
Title
Technical Summary ......... .... , ........................................................... .
Electrical Characteristics
.... ................... ........ .. .................. ...... ..... ... .
Tube Complement . .
.. ... .. ............ ... .. . .... ........ ... .. .. . ....... .... .. .......... . ·
Mechanical Specifications
................................... ·.· ........................... .
Description
......................................•.....................••...................
Purpose .... , ....................•.......••.... . ..... . .........•• ..••.• ...........•....
Construction .. ... ............. ....•.... . • .•..... .. .. . . .. ..........•........••..........
Circuit .........................•.....••...............................................
Installation
Mounting
...................................................................•..........
R-F Connecfions .........•........................................................... , ..
Remote Metering and Audio Monitoring ................................. ·•· . . ... .... ... ..... .
Static Drain
... . ......•....•..................•...........................................
Tower Lighting, .. ~ ..................................................................... .
Tuning
............ .. ....... .... .. . ..... . .....•..• .... . .. . .. .... .. .. ............ . . .........
General Considerations .. ...... .. .. .........•.. .... ... . ....... ...... ... ......... .........
Tuning Procedure with R-F Bridge ... ...... ......... . .... .. . .. .............................. .
Tuning Procedure without
R-F Bridge . .. ................ ............ ... . .. . ...... ....... . : . . .
Final Check
......................••....••.••...........................................
Remote Metering Equipment ...........•.••................................................
Maintenance ................•........••••...•. , , ....... .. ............. , •...................
Parts List ......... . .. .......................•..•.•...........•......... ... .... . .............
ILLUSTRATIONS
Figure
2
1-Type BPA-10 Antenna Tuning Unit (Exterior View)
2-Simplified Circuit Diagram (K-861732) .... . ...... , ...............................•..........
3-Coupling Circuit and Tuning Unit Network (K-861757) ......................................... .
4-Antenna Tuning Unit (Interior View) .............•..... . .. ...... ... . ........................
5-Antenna Tunin·g Unit (S;hematic M-428316) ...... .. ... ....... ..... ...... . .. . .. .. ....•.....•.
6-Antenna Tuning Unit (Outline M-428610) .........•............ .·. ...... . .................... .
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TECHNICAL SUMMARY
ELECTRICAL CHARACTERISTICS:
Operating Limits:
Carrier Frequency • • . • • •• • . • • • • • •• • • • • • • • . • . • . . . . . . • • . • . • •. . • . • . . . • • . . . • • 540
kc to 1700 kc
Transmitting Power (maximum) ...•••••.. • •...•....•••••.••.•..••••••.••..•...... , . . . 10 kw
Antenna Resistance:
Output 5 kw or less . • . • . . • • •. • • . . . •• . • •. . . . . . . . . . . . . • . . . . . . . . . . . • . . . lO ohms to 1100 ohms
·Output 10 kw or less . . . • . . . . . • •. . . . •• . . . • . . . . . • . . . . . . . . . . . . . . . . . . . • . . 20 ohms to 1100 ohms
Line Impedance ••....................•....•............................ 50 ohms to 350 ohms
Antenna Reactance ......•............ , .••.. . ...... . ...... . ............. +j500 to -j500 ohms.
Can be extended in a positive direction by the addition of a series capacitor; and in a negative
direction
if operating from a line of lower impedance than the antenna resistance.
Monitoring Rectifier:
Output Impedance
...••.......•........•.......... . .. to operate into 20,000-ohm bridging load
Output
Level (program level-including bridging Joss):
At 5-10-kw output . . . . . . • • . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 9 vu
At 1-kw output .........•..•.......•........................................ -15 vu
Rectified current .........•........•••.••...•..... 75 ma d.c. maximum into a maximum of l 000 ohms
Frequency Characteristic .....•...••.•...•••.•..............•...... substantially flat to l 0,000 cycles
Power Supply . • • . . • •.• . • • • • • . • • • • • • • • •. • • • • • . . . . . . . . . . . . . • . • . . . • . ... • . . . • . . . 230 volts, 60 cycles
TUBE COMPLEMENT:
Rectifier . . . . . . . . • • • • • • • •• • • • • • • . • • • • • • • • • • • • . . . . . . . . . . . • • . • . . • • • • . . • • . • . • . . • . . one RCA-5V4G
MECHANICAL SPECIFICATIONS:
Dimensions:
Height
...••....•.•...••.•...•.•••••.••................•.•..•.••..•...•....•... 44 inches
Width .•.•..•..•......••••.••..•••••.•..........................•............. 34 inches
Depth .. . . . . . • . •. . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . .
23 inches
Weight (net) ••..•••••.•.•••••••••••••••••••..........•...•.•.••.•.•••..•.•.....•. 330 pounds
3

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4
Figure
1-Type
BPA-10 Antenna Tuning Unit (Exterior View)
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Purpose
Description
Circuit
The
Type BPA-10 Antenna Tuning Equipment serves
the
double
purpose
of
matching antennas
of
widely
di-
vergent characteristics
to
either toncentric
or
open-wire
transmission lines
and
of
suppress·ing
carrier
harmonics
on transmitters up
to
ten kilowatts
(kw)
output
.
Construction
All
parts
of
this equipment
are
enclosed in a
weather
-
proof
metal housing
equipped
at
the
front
with
a
door
affording
ready
access
to
the interior. This
door
is
pro-
vided with a lock. The antenna ammeter may be read
through a circular
window
in
the
door
and
is
protected
from lightning surges
by
a short-circuiting switch, which
is
open,ted
by
means
of
a knob extending through the
side
of
the housing. A monitoring rectifier unit
*(
Ml-
7488-A)
is
contained
within
the housing
to
furnish,
if
de-
sired, audio-frequency
voltage
for
program
monitoring
and rectified carrier current
for
remote antenna current
indication.
*Ml-28902-B
only
.
Ml
-28902-A does
not
contoin
monitoring
rectifier.
The circuit
of
this antenna tuning unit essentially con-
sists
of
a single T-section low-pass filter which reduces-
the number
of
elements
to
a minimum. Referring to the
schematic
diagram,
Figure 5, there will be observed
two
series inductors (L
1,
L2
) which
are
employed
to
adjust independently the respective terminating im-
pedances
of
the transmission line and the antenna cir-
cuit. The capacitive shunt leg, which
is
common
to
the
two
branches,
is
fixed
at
a value determined
by
the
operating
frequency
of
the station.
Signal energy
for
operation
of
the monitoring recti-
fier
is
obtained from a tuned pickup coil (
L3
) which
is
coupled
to
the antenna
loading
inductor (
L2
).
This
energy
is
rectified
in
a full-
wave
circuit using an
5V4G
tube
and
the
output
is
balanced
to
ground
for
excitation
of
a monitoring
amplifier
. Terminals also
are
provided
for
connection to a remote antenna ammeter and inter-
lock
relay
located in the transmitter house. A 230-volt,
60-cycle
power
supply
is
required
for
energizing the
rectifier filament transformer (
Tl
).
Installation
Mounting
The unit
is
designed
for
mounting on a wooden
plat-
form
or
a steel
angle
cradle
by
means
of
the side flanges
at
the bottom
of
the housing. Rear mounting strips also
are
provided
to
permit mounting the unit on
two
up-
right
posts. Dimensions
are
given in the outline
drawing,
Figure 6.
Care
should be taken
at
installation
to
select a posi-
tion where the antenna lead
will
be as short as possible.
It
is
also
important
to
insure
adequate
grounding
by
connecting the housing
to
the
ground
system through a
heavy conductor
or
a
copper
bus.
R-F
Connections
The antenna lead-in post
is
located on the
top
of
the
unit,
and
provision
is
made
for
mounting a similar post
(Ml-19413-1
bowl
insulator) on the left-hand side
of
the
housing in case an
open-wire
line
is
used. Concentric
line when employed should _be
brought
in through a hole
in the bottom
of
the cabinet and connection made
to
the
upper terminal
of
coil L1. In cases where a remote an-
tenna ammeter
is
not used, terminals
No.
5
and
No.
8
should be connec_ted
together
by
means
of
a jumper.
Remote
Metering
and
Audio
Monitor1ng
An a-c supply
of
230
volts,
60
cycles
will
be required
to
operate
the rectifying equipment
for
remote meter-
ing. The associated filament transformer
is
tapped
for
operation
at
190, 210,
230
or
250
volts
and
'should be
adjusted
to
the
tap
nearest the existing line voltage.
Terminals
No
. 2
and
No.
3
are
used
for
connection
of
the
power
supply.
Static
Drain
No
provision
for
static
drain
is
made in this unit.
If
no conductive
path
to
ground exists elsewhere, a static
drain
should be mounted across the antenna horn
gap,
or
at
some
other
suitable place.
Tower
Lighting
No
complication as
to
tower
lighting
is
introduced
as the shunt arm
is
open circuit
to
power
frequencies.
Tuning
CAUTION
-
REMOVE
THE
TRANSMITTER
PLATE
VOLTAGE
PRIOR
TO
EACH
ADJUSTMENT
OF
THE
ANTENNA
AND
TRANSMISSION-LINE
CIRCUITS.
FULL
POWER
SHOULD
NOT
BE
APPLIED
TO
THE
LINE
BEFORE
PROPER
ADJUSTMENTS
HAVE
BEEN
COM-
PLETED.
DANGEROUS
VOLT
AGES
MAY
OCCUR
THROUGH
IMPROPER
TERMINATION
AND
RESULT
IN
DAMAGE
TO
THE
LINE
AND
EQUIPMENT_
6enaral
Conditions
Although
the network used in this unit serves the
two
functions
of
impedance matching
and
antenna tuning
concurrently,
it
is
desirable
to
consider them separately.
For antenna tuning, the coil
L2
can be used
to
series-
resonate the reactive component
of
a capacitive
an-
tenna; or,
if
the antenna
is
inductive, the reactive com-
5

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ponent' can be thought
of
as
being absorbed into the
antenna tuning coil.
In
either case, only the
resistive
portion
of
the antenna impedance
is
left,
and
the im-
pedance matching function can be regarded
as
taking
place between purely resistive impedances, inasmuch
as
the characteristic impedance
of
most lines
is
resistive.
Under these conditions, the values
of
reactance em-
ployed are determined
by
the values
of
the impedances
to be matched, with the phase shift through the network
as
-a parameter. Since the circuit
is
a section
of
a low-
pass
filter, the phase shift may be anywhere between
zero and
-180
degrees, moking possible a wide range
of
reactance values. However, it
is
not advisable
to
work
too close to the
-180
degree (or cut-off) point
of
the
section,
or
with
such
low
values
of
phase shift
that
the
second harmonic will not be sufficiently attenuated,
hence a value near
-90
degrees will usually be found
most suitable.
These
statements will become clearer
upon examination
of
the equations re'lating
to
the re-
actances
of
the arms
of
the network and the antenna
and line impedances. Referdng to Figure
2,
which
shows
a simplified circuit diagram, the reactances are:
~r
1-
~cosP]
sin
P
X2
= -
~
[
1-
v'i½
cos
P]
~
~
Xa=
+ - -
sin
P
sin
p
x,
(1)
Line impedance
Antenna resistance
=
Phase
shift through network
It will be appreciated
that
choice
of
p
near zero
or
-180
degrees leads to large values
of
reactance
In
all
arms since
sin
p
approaches zero
for
these values.
On
the other hand, if we take
p =
-90
degrees,
these
equations simplify to:
X1
=
+
~
X2=+~
Xa=-~
(2)
If this leads to inconvenient
sizes
of
X
1,
X2,
or
Xa,
a new value
of
p
can be chosen
to
yield better values
for
the desired reactance.
Tuning
Procedure
with
R-F
Bridge
The
use
of
a radio-frequency
bridge
is
recommended
to
insure accurate tuning adjustment and the applica-
tion
of
this instrument will be assumed during the ensuing
treatment
of
the tuning procedure. In
cases
where a
bridge
is
not available, measurements can be made
by
a substitution method, also
to
be described.
To
determine the values
of
inductance and capaci-
tance
to
be
used
for
proper line matching ond antenna
tuning, it
is
essential
to
know the impedance
of
the
antenna
at
the operating frequency. For purposes
of
comparison, the resistances and reactances
for
various
heights
of
typical insulated towers
of
the guyed-mast
and self-supporting types
are
shown in Table
I.
Xz
BOWL.
INSUI..ArOR
X3
X-S6173Z
-
-
-
Figure 2-Simplified Circuit Diagram
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TABLE
I
Antenna
Height•
Self-Supporting Guyed-Mast
in
·
Electrical Type Type
Degrees
G
R
jx
R
jx
50
7
-jlOO
8
-j220
60
9
-j
70
13
-j170
70
14
-j
25
19
-j
75
80
20
+ j
11
28
-j
28
90
40
+j
35
36
+j
0
100
60
+j
80 80
+j140
110
90
+j
90
140
+j320
120 175
+j
80
220
+J500
130 190
+J
15
370
+J600
140 165
-j
70
660
+j480
150
130
-j
85
1100
+j
0
160
82
-J
55 550
-J250
170
60
-J
25
280
-J450
180
40
-J
5
180
-J500
190
28
+J
25
120
-J430
200
23
+J
50
80
-J400
• Height in electrical degrees =
Height
in feet
X frequency
in kilocycles
X
1.016
X
10-0
X 360.
Substitution
of
the resistance components
of
line
and
antenna impedances in the equations (
1)
gives the values
of
X
1,
X
2
and
X3 necessary
for
the impedance m·atch-
(ng function. Examination
of
the reactive components
indicates the reoctonce necessary
for
tuning. For ex-
ample, suppose
we
hove a 60-ohm line
and
a 120-
degree antenna
with
175 ohms
of
resistance
and
+j80
ohms
of
reoctonce. Substitution
of
the values
60
and
175 ohms in equations (2) gives the
value
of
102.5
ohms
for
X
1,
X2
and
X3.
To tune
out
the antenna reoctonce in this case,
it
is
only
necessary
to
assume
that
this reoctonce
is
a
port
of
the required
value
of
X2. Subtracting the
80
ohms
of
antenna reoctonce from 102.5 ohms leaves 22.5 ohms
to
be
obtained
in the coil
L2
.
When
the other arms
of
the network hove been adjusted to the
proper
v
alue
of
the 102.5 ohms, there will exi
st
a condition
of
im-
pedance match between the line and the antenna re-
sistance
and
the antenna reoctonce will hove been
removed
as
a cause
of
loss
.
In
making these adjustments, the line should be di
s-
connected
and
_the impedance
bridge
connected across
the input terminals
to
determine when the desired value
of
60
ohms hos been obtained. When measurements
show
that
the input impedance
of
the tuner with the
antenna connected
is
60
ohms resistive, the line may be
reconnected
for
a final check before turning on .full
power
.
Calculations
of
the current in the capacitive branch
ore
made to insure
that
the
rot
ing
of
the capacitor
is
not exceeded. Proper capacitors
are
supplied on the
basis
of
information received with the
order
.
To
enable
intelligent estimates
to
be mode
of
the
inductances
obtained
by
topping
down on coi
ls
L1
and
L2,
it should be mentioned
that
their maximum induct-
ance
is
120 microhenries.
Tuning
Procedure
Without
R-F
Bridge
If
no impedance
bridge
is
available,
a simple sub-
stitution method
may
be used
to
determine when a
proper
adjustment has been obtained.
To
do
this, it
is
necessary
to
arrange
to
switch the line from the tuner
to
a resistance equal
to
the line impedance, noting the
change in line current accompanying the switching.
When
no change occurs, the tuner
is
in
proper
adjust-
ment. Because it
is
not desirable
to
apply
full
power
to
the test resistor, which will usually have a roting
of
a relatively
few
watts, connection should be made
to
a
low
power
stage in the transmitter during this adjust-
ment.
To
make the adjustment simple, a coupling circuit
and a series tuning
capacitor
con be used, as shown in
Figure
3.
LZ
c1,,c2
(AS
REQUIRED}
2
K-86/757
Figure
3-Coupling
Circuit and Tuning Unit
7
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With
'
the
switch in
the
resistor position, capacitor C
can be tuned
for
a maximum current reading in meter
G.
Then, on switching
to
the tuner input, the direction
in which capacitor
C
must
be
turned
to
increase the
current
again
to
a maximum indicates the sign
of
the
reactance in the antenna circuit. If
the
capacitance must
be increased, the
load
is
capacitively reactive;
if
the
capacitance must
be
decreased, the
load
is
inductively
reactive;
if
no change
is
necessary,
the
load
is
resistive.
Similarly, if the current reading
at
resonance
is
greater
than before, the
load
resistance
is
less
than Z0;
if
that
reading
is
less
than before, the
load
resistance
is
greater
than
Z
0;
if
there
is
no change, the
load
resistance
is
equal
to
Zo-
CAUTION-Remove
unused jumpers.
This method may be used
to
determine unknown re-
sistances simply
by
using a
calibrated
test resistor; like-
wise,
unknown reactance values may be determined
by
using a
calibrated
condenser
at
C.
Final
Check
Upon completing the tuning procedure as outlined in
the preceding paragraphs, the adjustments should be
checked before full power
is
applied
to
the line
and
antenna. The recommended method
of
check
is
de-
scribed in the
following
paragraph.
With
the measuring equipment disconnected from the
· tuning unit, attach the transmission line
and
insert a
low-range thermal milliammeter in the ungrounded side
at
each end of the line.
Apply
sufficient
power
to
pro-
vide a
readable
deflection on each meter and note the
current values. These values should
agree
within 15
per
cent when the tuning adjustment has been correctly per-
formed. Under
such
conditions, full
power
may be
ap-
plied
to
the line
after
removing the millammeters.
Upon
application
of
fvll
power, the current through
each
of
the tuning capacitors
(Cl,
C2) should be
measured under conditions
of
full modulation. The
maximum permissible current values
for
these capacitors
at
three nominal frequencies
are
shown on the name-
plates.
At
intervening frequencies, the maximum values
will
be
approximately
proportional
to
those listed. If
such
currents
are
found
to
be
excessive, the capacitors
should
be
rearranged in the circuit.
Remote
Metering
Equipment
The antenna tuning unit embodies the necessary
equipment
to
enable the installation of a remote meter
for
measuring antenna current and also furnishes audio-
frequency energy
for
operation
of
a monitoring ampli-
fier. The method
of
remote antenna-current indication
as
outlined herein has been approved
by
the Federal
Communications Commission.
The remote meter should require 25
to
50
ma
direct
current
for
full-scale deflection and should have a scale
corresponding
to
that
of the antenna ammeter (M
1)
.
It should be equipped with a shunt adjusted
so
that
the
deflections
of
both meters
are
identical.
In
most cases, a
5-ohm
variable
shunt will be satisfactory
for
this pur-
pose.
As
shown
by
the schematic
diagram
(Figure 5), ter-
minals No.
5
and
No
.
8
are used
for
connection
to
the
remote meter and transmitter interlock relay. Sufficient
output
for
proper
deflection of the remote meter may
be obtained
by
adjusting the coupling between the an-
tenna
loading
inductor (
L2
} and the monitoring pick-up
coil (
L3
} and
by
tuning the latter
to
the carrier fre-
quency. A
wide
tuning range
is
afforded
by
the six
taps on the pick-up coil and
by
the
use
of
two
capaci-
tors (C4, C5) whi
ch
may be employed singly, in series,
or
in
parallel.
Jumpers are provided
to
facilitate inter-
connection of these capacitors.
Maximum output will be secured
as
the pick-up coil
is
tuned
to
resonance. It
is
not advisable, however,
to
approach
resonance
too
closely since the increasing
selectivity of this circuit
will
seriously impair the audio-
frequency response characteristic.
At
resonance, the re-
sponse
at
10,
000
cycles will be
down
approximately
4
db
. Under no conditions should the current through the
series
resistors (
Rl,
R4)
be
allowed
to
exceed
75
ma
d.
c.
When an
audio
moni
tor
is
to
be used,
an
output
level of
approximately
+17 dbm
is
available
from
this source, the circuit
of
which
is
balanced
to
ground
and may be used to feed a 500-ohm
load
.
The
load
in
this case must be
capable
of
handling 25
ma
of
direct
current. It
is
desirable, therefore,
to
feed a 20,000-ohm
or
greater
bridging
load
. If the monitoring ampfifier has
only
a 500-ohm input, a 20,000-ohm carbon resistor may
be inserted in series with the 500-ohm transformer.
Under this condition, the direct-current
flow
is
negligible
and the output level from the rectifier
is
reduced
to
-1
dbm
at
5-kw operation and to
-7
dbm
at
1-kw opera-
tion.
Maintenance
The antenna ammeter shorting switch
(Sl)
should
be
kept closed except when readings
are
being taken.
All
connections, especially the coil connector clips,
8
should be inspected
regularly
to
insure tightness and
thus
avoid
undue heating
at
such
points. Screens and
ventilation openings should be unobstructed
to
permit
free circulation
of
air.
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Symbol
No.
Al
Cl
C2
C4
C5
C6
Ll,
l2
l3
Description
Rectifier, monitor
Copocitor (frequency deter-
mined)
Capacitor
(frequency deter-
mined)
Capacitor,
300
mmf
Capacitor, 200
mmf
Capacitor, 1000
mmf
Coil
Coil
PARTS
LIST
Stock Dwg. No,
Symbol
Description Stock Dwg. No.
No. No.
No.
K-891875
Ml
Meter (frequency determined)
See
Ml-19465
See Ml-7444-C
Rl
Resistor,
6.4
K,
90 W 17899 K-890162-2
R2
Resistor,
IO
K,
90
W 17900 K-890162-3
R3,R5 Resistor, 250 ohms, 10 W 17901 K-891887-2
See Ml-7444-C
R4
Resistor, 125 ohms, 10 W 17902 K-891887-1
69860
P-32221-532
69864 P-32220-683 SI Switch,
250
V.,
30
amp.
90038 K-890957-1
42335
P-722009
-5
47 Tl
Transformer 17897 K-900433-501
T-611579-501
17898 M-415903
XI
Socket, vacuum
tube,
octal 18007 K-844041-50 I
Figure
4-Antenna
Tuning
Unit
(Interior
View)
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