Sencore FS134 Service Manual

FS

1 3 4

FIELD STRENGTH

METER

MANUAL

INSTRUCTION MANUAL

SENCORE FS134 FIELD STRENGTH METER

instrument designed to identify and measure the frequency and signal strength of
FM stations and all VHF and UHF TV stations. Because of the increa se in popu
lari ty of "FM Stereocasting" and the UHF boom, it was realized that a great need
for this type of instrument existed. Sencore engineers did considerable res earch
to develop a Field Strength Meter which is rugged, lightweight, portable and accu
rate and yet has that "P rofessional" appearance.

Strength Meter.

1. Completely solid state construction provides the utmost in portability, light

weight compactness and battery power economy.

2. Printed circu it construction on unbreakable board material gives dependability
and long life.

3. Completely battery operated for field use. Uses standard "C" cells available
anywhere. Provisions have been made for installation of a battery charger accessory.

4. Choice of 75 ohm or 300 ohm input from the front panel.

5. Uses most popular type 75 ohm input jack for direct connection to antenna dis
tribution systems, thereby eliminating messy adapters, which can introduce addi
tional SWR losses.

6. Has built-in matching transformer for 300 ohm input. 300 ohm input terminals
are of the thumb nut type for complete v ersatil ity of any twin lead termination.

7. Built-in 20db (X10) and 40db (X100) attenuators provide control of high level in
put signals, especiall y those signals encountered in distribution systems. The
attenuators can be used with the 75 ohm or 300 ohm input.

TheSen core FS134 Field Strength Meter is a completely solid state portable

Listed below are some of the Special Features of the Sencore FS134 Field

8. Separate VHF and UHF tuners are provided formiximum stability and minimum
losses.

9. Three stage 42 .8 MC high gain IF system controlled by amplified AGC stages
gives stabili ty and good control of both weak and strong input sig nals.

10. Large four inch meter allows operator to read signal strength from several
feet away.

11. Extended logarithmic microvolt range from 30 to 30,000 microvolts permits
antennas to be installed without constantly changing the sensitivity range of the meter .

12. Built-in, 3 1/2 inch speaker, with six transistor audio system to drive it, is
essential when monitoring FM and TV audio signals.

13. Signals as low as 5 microvolts can be heard in the speaker (the meter sta rts
indicating at 30 microvolts) thereby permitting weak fringe area signals under 30
microvolts to be "found" and built up with proper antenna orientation to a level vis-
able on the meter.

14. Extremely good shielding prevents strong signals from being received directly,
they must be received by the antenna, thus making the peaks and valleys of signal
strength more pronounced as the antenna i s rotated. This, of course, makes it
easier to pinpoint signal sources.

15. Detector output jack on the front panel permits external VTVM or oscilloscope
connections when monitoring.

16. Front panel CAL control compensates for decreasing battery voltage and affords
constant accuracy at all times.

SPECIFICATIONS
Tuning Ranges

53 MC to 109 MC - TV Channels 2 to 6 ; FM Channels 201 to 300
173 MC to 218 MC - TV Channels 7 to 13
465 MC to 895 MC - TV Channels 14 to 83

Sensitivity
53 MC to 109 MC - 30 microvolts + 3DB
173 MC to 218 MC - 30 microvolts + 3DB
465 MC to 895 MC - 30 microvolts + 3DB

Selectivity (Bandwidth)
500 KC & 3DB points

Intermediate Frequency

42. 8 MC
Input Impedance

75 ohms - 300 ohms with Built-in Matching Transformer
Image Rejection

53 MC to 109 MC - 40DB
173 MC to 218 MC - 40DB
465 MC to 895 MC - 30DB

IF Rejection

WDB

Audio Power Output
150 milliwatts

Power Consumption and Requirements
24 milliamps (8> 12 volts on VHF (no signal)
35 milliamps @12 volts on UHF ( no signal)
2 milliamps @-1.5 volts

Nine "C" cell batteries are used for power source. An accessory battery charger,
used with a rechargeable battery is available.

Dynamic Meter Range
30-30,000 microvolts (60DB) on logarithmic scale

Physical Specifications
Height - 9 1/2"
Width - 10"
Depth - 5"
Weight - 9 lbs.

Temperture Range

-5 to +105°F Operating Range

-2 0 to +140°F Storage Range

TRANSISTOR AND DIODE COMPLEMENT
Ref. No.
TR1

TR2
TR3
TR5, TR6, TR7 2N1745 IF Amplifiers
TR8
TR9
TRIO
TR11
TR12 2N1304

TR13

TR14 2N1304
TR15
TR16 2N1304

CR3, CR4

of your local TV stations, or from your signal generator. If you do this when the

The following blank spaces are for you to record the signal strength leve ls

FS134 is new, you will always have a handy reference for later use.

Channel

__________

Type

2N2362
2N2361 VHF, Signal Mixer - UHF, IF Amplifier
2N2362

2N2923
2N1304
2N1304
2N404

2N404
2N1304
1N34

Microvolts

__________

VHF, RF Amplifier - UHF, IF Amplifier
VHF, Oscillator
1st Audio

Audio Amplifier
Audio Driver
Audio Driver
Audio Output
Audio Output
AGC Amplifier
AGC Emitter Follower
Meter Balance
AM and Slope Detector

Input Used

Function

__________

Type Antenna

CONTROLS ON THE FS134

as any fine rec eiver. In addition to normal controls found on a recei ver, TUNING,
BANDswitch, OFF-ON switch and VOLUME control, the FS134 has a CAL control
to adjust for changing battery voltage. A front panel met er calibrated in microvolts
and DB and a series of input jacks complete the controls on the panel. Following is

a brief description on how each of these controls are used.

The FS134 is as easy to use as a radio and basically has the same controls

OFF-ON SWITCH. The OFF-ON switch, as the name implies turns the FS134 off

or on, but in addition has a center CAL position which is used in conjunction with

the CAL control and the meter to set the B+ voltage to the FS134 circuits.
CAL CONTROL. The CAL control is adjusted prior to taking a signal strength

measurement by placing the OFF-ON switch in the CAL position and adjusting the
CAL control until the meter indicates at the CAL line. This set s the voltage on the
RF, IF and AGC circuits to 10 volts resulting in constant signal strength accuracy
regardle ss of the battery condition. When it becomes impossible to make this ad

justment the battery voltage has dropped too low and the batteries should be replaced.

BAND SWITCH. The BAND switch selects one of three frequency bands; the low
VHF band, channels 2-6 and FM; the high VHF band, channels 7-13 ; and the UHF
band, channels 14-83. It i s located directly above the frequency dial.

TUNING. The TUNING control at the lower right of the frequency dial turns the
frequency dial and, of course, the internal tuning capa citor at a 6:1 ratio for "fine"
tuning adjustment of the selected signal. It is used for all three bands.

VHF bands each block representing a channel starts at a point representing the
car rier frequency (low frequency end) and stops at a point representing the sound
car rier frequency (high frequency end). This was done to simplify field use of the
FS134.

VOLUME. The VOLUME control adjusts the sound level from the speaker. In nor
mal use the sound level should be kept as low as possible to prevent excessive drain
on the batteries. The VOLUME control has been designed so that it can never be
completely reduced to ze ro to serve as an audible reminder that the unit in ON.

METER. The panel meter has two scales for determining signal strength - one
calibrated in microvolts, the other in decib els (DB). The decibel scale is used pri
marily when checking out an antenna distribution system for the various losses en
countered in pads, feedthroughs, cables et c. The zero DB reference is 1000 micro
volts across 75 ohms.

reducing the possibility of error by reading a wrong s cale.

The tuning dial is calibrated in frequency and by TV channels. On the two

The single microvolt sc ale is used for all VHF and UHF frequencies thus

INPUT JACKS. The basic input on the FS134 is the XI, 75 ohm jack. The signal i s
fed directly thru a pa ss filter to an RF amplifier in either of the VHF bands and in
the UHF band is fed directly to the UHF tuner. When signal levels are high the
signals can be attenuated by connecting into the X10 jack (20DB loss) or the X100
jack (40DB loss). Of course, when using the attenuator jacks the meter indication
microvolts should be multiplied by 10 or 100 respectiv ely, and on the DB indication

20DB or 40DB added respectively. The best meter accuracy is obtained between 30
and 1000 microvolts, therefore when you d esire the most acc urate indication and
the meter reads above 1000, additional attenuation should be used.

trans former. In use, the 300 ohm twin lead from the antenna would be connected
to the thumb screw terminals and the output of the matching transformer would be

coupled through the jumper cable to the appropriate 75 ohm input jack. When using
the 300 ohm input, the meter reading (microvolts) should be multiplied by two and
for best accura cy this product should be multiplied by the conversion f actor (See
page 7 ), which takes into account the losse s in the matching tranformer for the

various frequencies encountered.

The 300 ohm balanced input is changed to 75 ohm unbalanced by a matching

THE FS134 OFF-ON INDICATION. There are no pilot lights on theFSl34, but there
are two indicators, one is visual and the other an audible indication that the FS134
is on. The meter on the FS134, when the unit is on and no signal is fed to the input,
will read negative or below the 30 microvolt line on the scale. Just glancing at the

meter will tell if the unit is on. If a signal is present of course, the meter will
read up scale and again you will be able to tell if the FS134 is on. The volume con
trol on the FS134 cannot reduce the volume to ze ro, so there will always be either
the signal sound or low background noise from the speaker. In this way Sencore
has eliminated the power consuming pilot light and has provided more economical
battery life.

OPERATING INSTRUCTIONS

NOTE: THE METER WILL INDICATE BELOW THE 30 MICROVOLT MARK WHEN
NO SIGNAL IS PRESENT.

of signals receiv ed by an antenna or from an antenna distribution system is that the
output impedance of the antenna or system is properly matched to the field strength
mete r. If there is any mismatch, standing waves a re developed, which reduce the

amount of signal actually received by the meter and, of course, the meter indica
tion will not be a ccurate. ALWAYS BE SURE THAT YOU USE THE CORRECT
IMPEDANCE LINE OR CABLE FOR THE ANTENNA OR SYSTEM YOU ARE WORK
ING WITH AND THAT YOU CONNECT TO THE CORRECT INPUT ON THE FS134.
Cables with 75 ohms impedance (from a straight dipole antenna, for example)

should always be connected to the 75ohm input jacks (XI, X10 or X100). Three
hundred ohm twin lead from a folded dipole antenna should always be connected to
the matching transformer input and the short 75 ohm jumper cable used to connect
the output of the transformer to the appropriate 75 ohm input jack .

push the OFF-ON switch to CAL and with the tuning dial set to a point where no
signal is pres ent, adjust the CAL control until the m eter reads at the CAL line.
Push the OFF-ON switch to ON, and you are now ready to measure signal strength.

Measuring Signal Strength With the FS134. Rotate the tuning control to the approx
imate frequency of the signal to be measured and observe the meter indication.
Rotate the tuning control about this point slowly to obtain a maximum meter indica
tion. If the indication is past full scale on the meter, feed the signal into the next
higher attenuator, XI0 or XI00 to obtain a lower meter reading. Then repeat the
tuning procedure until a maximum meter indication is obtained. You can read the
meter directly if 75 ohm coaxial cable is used or multiply by 2 if 300 ohm twin lead

is used. Then this reading must be multiplied by the attenuator used. For exam
ple, 75 ohm coaxial cable plugged into the XI0 jack produces a reading of 245 micro

volts. This will be 245 X 10 or 2450 microvolt s. If 300 ohm twin lead is used giv
ing a signal strength of 350 microvolts in the XI jack, the signal would be 350 X 2
X 1 or 700 microvolts, total signal. When using 300 ohm twin lead connected to
binding posts, the meter reading must be multiplied by 2 to get the cor rect value
of signal strength. For greater accuracy the product must be multiplied by the
conversion factor found in the chart below, which takes into account the losses of
the matching t ransformer. For the average antenna installation, however, it is not
necessary to multiply by the conversion factor. On 75 ohm coaxial cable input,

read the meter direct.

The fir stand most bas ic requirement when attempting to measure the strength

With the input properly connected turn the BAND switch to the desired band,

Low VHF Band (Channels 2-6-FM) Multiply by 1.1
High VHF Band (Channels 7 -13) Multiply by 1.4
UHF Band (Channels 14-8 3) Multiply by 3.0

danger of damaging the meter, because the AGC system will hold the me ter current
at a safe level.

UHF Signal Strength Measurement. UHF signals are measured the same as above
except that all lead-ins, terminations, etc. are much more critical that at VHF
and the finest care must be taken to see that the cables are terminated properly,
cable connectors are tight, cables and lead-ins held well away from metal surfa ces
etc. It is much more difficult to obtain accurate measurements in the UHF band,
so every precaution you take will make the reading that much more accurate.

come by, noise is also down, resulting in good quality pictures at UHF at lower
signal levels than the same quality picture at VHF re quires.

If by chance in the XI jack some signals read beyond full sc ale there i s no

One big advantage at UHF is that although high signal levels are harder to

Tuning the FS134 to Monitor Sound Signals. Most RF signals can be heard through

the speaker on the FS134 although the sound may not be that of voice or music. For

example, the signals from the video portion of a TV signal contain a low frequency

(60 cycle) buzzing sound. However, the speaker is mainly intended for identifying

intelligible sounds such as voice or music. When tuning in an FM station or the

sound of a TV signal, the best sound will occur as the meter indication drops off

slightly and will occur at either side of the peak reading. This is a normal con

dition because slope detection is used in the FS134 sound system. On the video

carrier of a TV signal the maximum 60 cycle buzzing sound will be loudest when

the meter is at its peak indication, because the vide6 carrier is amplitude modu

lated (AM).

Turning the volume control clockwise will increase the sound output. It is

suggested that you keep the sound output as low as useable, or off, when not using

the sound signal to keep the drain on die batteries as low as possible.

Spurious Responses. Although the image rejection ratio for the FS134 is over 100

to 1 on the VHF TV bands it is possible for a "local" signal at the image frequency

to be of sufficient strength to be heard in the speaker and produce a meter reading

when feeding the signal through the XI input. Therefore, if possible, feed the

signal through the X10 or X100 input j ack s and spurious responses, such as this,

will be virtually non-existant. This also applies for UHF signal measurements.

Noise Level in the FS134. The FS134 circuits were designed to keep inherent noise
at a minimum, however extern al noise pickup by the antenna will be amplified and
received the same as an RF signal. Th erefore in extr emely noisy are as the FS134

meter may produce random meter indications caused by noise. The only a lterna 
tive is to move the antenna to a les s noisy location. In doing so, this will also im

prove the FM, VHF, or UHF reception.

Determining the Frequency of a Received Signal. The frequency of any signal may
be read directly from the tuning dial. Set the tuning control for maximum meter

indications and read the frequency at the point on the tuning dial where the cross
hair passes through the frequency indication marks on the tuning dial. The FM
band and the VHF TV bands are on the top of the dial, and the UHF TV band is on

the lower half of the dial.
Use of Detector Output Jack. The DET OUT ja ck on the front panel of the FS134 is

provided so that the detected video signal may be monitored with an oscilloscope or
external m eter if desired. It is especially useful when checking boosters or antenna
amplifiers to see if these units are overloading on one or more channels causing a
loss of sync or sync compression or if they are causing cross modulation in the

other weaker channels.

FACTS YOU SHOULD KNOW FOR BEST USAGE OF THE FS134

the tran smission and receiving of VHF and UHF TV signals, and FM signals. The
whole business seems quite confusing when you hear such terms as: microvolt sig
nal strength, field intensity in microvolts per met er, antenna systems with so many
DB gain, DB, DBM, DBJ, 75 ohm coax, 300 ohm twin lead, mismatch, standing wave
ratio (SWR), pads, losses, matching transformers, and many others. Let 's see if

we can straighten some of this out.
to the input of a receiver and talked about SWR losses if this were not done properly.

As you know a straight dipole antenna has a characteristic impedance of 75 ohms
and a folded dipole antenna has a characteristic impedance of 300 ohms. Most of
the antenna arrays that have been manufactured over the years were designed for
300 ohm impedance, although recently some new arrays are being designed for 75

ohm impedance.

same impedance as the antenna, and the impedance of the lea d-in must match the
input of the receiv er, or points of mismatch will occur. Connections that are mis
matched will not pass the en tire signal, but rather will r eflect a part of the signal
back up the line. If there are two or more mismatched connections signals can
actually bounce back and forth several times. When some of the signal is ref lected
due to mismatch, standing waves occur. This condition is referred to in terms of
the standing wave ratio (SWR) which can be calculated by dividing the sum of the
two signals (original signal and reflected signal) when they are in phase by the sum

of the two signals when they a re out of phase. The clo ser that the SWR ratio is to

1.0 the bette r the match.
match, by using pads or matching transformers. A matching transformer gener

ally called a Baiun is actually an impedance transforming device that will convert a
300 ohm BALanced input to a 75 ohm UNbalanced output or vice versa. It consists
of two short lengths of 150 ohm twin line that are connected in series on one end
and in p arallel on the other. The 150 ohm lines may be wound around a coil form,

Before putting the FS134 to work let's review briefly some fa cts concerning

We stated earlier the importance of matching the impedance of an antenna

The importance of all this is that the lead-in from the antenna must have the

It is possible to change from one impedance to another with very little mis

or in some cases are wound on a ferrite core.
another, although you may be more familiar with them as attenuation pads with the

same input and output impedances designed for a specific DB attenuation. It is well
to note that any pad, whether it is designed specifically for attenuation or for
matching two impedances, will always introduce some loss. Impedance matching

pads are generally designed for a 6DB loss for ease of calculations in use, since

1/2 of the input voltage will appear at the output.

Pads are resistive networks that can be designed to match one impedance to

FIELD INTENSITY AND SIGNAL STRENGTH

field intensity pattern in terms of microvolts per meter fo r the surrounding country
side. This field intensity figure i s the voltage induced in a conductor one meter
long as the magnetic flux of the transmitted wave passes through the conductor at
the speed of light. Field intensity measurements can be made with the FS134 (see

procedure on page 12).

strength of a received signal is dependent on the antenna array , and how much sig
nal the antenna can pickup from the field intensity pattern. A well designed array
will be able to pick up much more signal than a straight dipole, for example.

had to be devised. The straight dipole was chosen as the reference, and all other

antennas are compared against it. Therefore an antenna arr ay with 20DB gain would

"collect" ten times the amount of signal voltage that a straight dipole would collect.
volts or DB. Microvolts by itself has very little meaning unless you als o consider

the impedance that the voltage is developed acro ss. .In other words, it is the re

ceived power that is important, but since we are always dealing with either 75 ohm
or 300 ohm impedance, we can and do get by with just the voltage term. For ex
ample, consider a 300 ohm antenna array that i-s receiving a 1000 microvolt signal.

The signal power would be E R and would equal .0033 micro-watts. Now let's
assume that you would like to use a 75 ohm coax lead-in. You would place a match

ing transf ormer on the antenna mast to convert 300 ohms to 75 ohms. Assuming
no losses in the matching transformer you would find that the output voltage of the

transf ormer would equal the square root of .0033 m icro-watts times 75 ohms or

500 microvolts. Notice that the microvolts at 75 ohms is just half of the microvolts

at 300 ohms, yet in both cases the power is the same. The FS134 is cali brated for

a 75 ohm input, therefore, when using the built-in matching transformer for 300

ohm input you must multiply the microvolt reading by two.

in cables, pads, etc., the voltage figures resulting become so large and cumber

some that the more convenient DB term is used. A big advantage is that gains and

losses of a system expressed in te rms of DB can be added or subtracted directly

making calculations more simple.

industry has agreed that 1000 microvolts across 75 ohms would be a good zeroDB

reference, since a signal of this strength will produce a good quality picture. Terms

such as DBJ and DBM are expressions of this reference.

output voltages, keeping in mind that the input and output impedances must be the

same.

Transmitted signals fill the air around us. Each station has a plot of their

Do not confuse signal strength in mic rovolts with field intensity above. The

Since some antennas are more effective than others a means of rating them

The strength of received signals is generally refe rre d to in terms of micro

Once a signal is received and is amplified in a booster or experiences losses

To establish a DB scale, however, some reference must be used. The TV

You may recall the formula for calculating DB when you know the input and

DB = 20 log E out

E in

So that you won't have to dig out your old logarithm book you can simplify your ca l
culations with the following DB chart.

Charts and useful fo rmulas:

Coaxial Cable Losses in DB/100 ft.

Type Cable

RG59/U
RG6/U

RG6/U Foam
RG11/U
RG11/U Foam

Ch. 2

2. 8 3.2

2.1

1.7

1.6

1.1

Ch. 4

2.3

1.9

1.8

1.3

Ch. 6

3.6

2.6

2.1

2.0 2.2

1.4

FM

4 .0 5.3

2. 7

2.2

1.5

Ch. 7

4 .0

3.2

2. 7

1. 6

Ch. 10

5.6

4.2

3.3

2.9 3.0

1.9

Ch. 13

5.9

4.3

3.5

2.3