Bird Technologies 28-88-04B User Manual

Instruction Manual

Vari-Notch

Manual Part Number

YOU'RE HEARD, LOUD AND CLEAR.

® Duplexers (4” Cavities)

7-9176

8625 Industrial Parkway, Angola, NY 14006 Tel: [phone] Fax: [phone] [e-mail] www.bird-technologies.com

Warranty

This warranty applies for one year from shipping date.

TX RX Systems Inc. warrants its products to be free from defect in material and workmanship at the time of shipment.

Our obligation under warranty is limited to replacement or repair, at our option, of any such products that shall have
been defective at the time of manufacture. TX RX Systems Inc. reserves the right to replace with merchandise of
equal performance although not identical in every way to that originally sold. TX RX Systems Inc. is not liable for dam-
age caused by lightning or other natural disasters. No product will be accepted for repair or replacement without our
prior written approval. The purchaser must prepay all shipping charges on returned products. TX RX Systems Inc.
shall in no event be liable for consequential damages, installation costs or expense of any nature resulting from the
purchase or use of products, whether or not they are used in accordance with instructions. This warranty is in lieu of all
other warranties, either expressed or implied, including any implied warranty or merchantability of fitness. No repre­sentative is authorized to assume for TX RX Systems Inc. any other liability or warranty than set forth above in con­nection with our products or services.

TERMS AND CONDITIONS OF SALE

PRICES AND TERMS:

Prices are FOB seller’s plant in Angola, NY domestic packaging only, and are subject to change without notice. Fed­eral, State and local sales or excise taxes are not included in prices. When Net 30 terms are applicable, payment is
due within 30 days of invoice date. All orders are subject to a $100.00 net minimum.

QUOTATIONS:

Only written quotations are valid.

ACCEPTANCE OF ORDERS:

Acceptance of orders is valid only when so acknowledged in writing by the seller.

SHIPPING:

Unless otherwise agreed at the time the order is placed, seller reserves the right to make partial shipments for which
payment shall be made in accordance with seller’s stated terms. Shipments are made with transportation charges col­lect unless otherwise specified by the buyer. Seller’s best judgement will be used in routing, except that buyer’s routing
is used where practicable. The seller is not responsible for selection of most economical or timeliest routing.

CLAIMS:

All claims for damage or loss in transit must be made promptly by the buyer against the carrier. All claims for shortages
must be made within 30 days after date of shipment of material from the seller’s plant.

SPECIFICATION CHANGES OR MODIFICATIONS:

All designs and specifications of seller’s products are subject to change without notice provided the changes or modifi­cations do not affect performance.

RETURN MATERIAL:

Product or material may be returned for credit only after written authorization from the seller, as to which seller shall
have sole discretion. In the event of such authorization, credit given shall not exceed 80 percent of the original pur­chase. In no case will Seller authorize return of material more than 90 days after shipment from Seller’s plant. Credit
for returned material is issued by the Seller only to the original purchaser.

ORDER CANCELLATION OR ALTERATION:

Cancellation or alteration of acknowledged orders by the buyer will be accepted only on terms that protect the seller
against loss.

NON WARRANTY REPAIRS AND RETURN WORK:

Consult seller’s plant for pricing. Buyer must prepay all transportation charges to seller’s plant. Standard shipping pol­icy set forth above shall apply with respect to return shipment from TX RX Systems Inc. to buyer.

DISCLAIMER

Product part numbering in photographs and drawings is accurate at time of printing. Part number labels on TX RX
products supersede part numbers given within this manual. Information is subject to change without notice.

Bird Technologies Group TX RX Systems Inc.

Manual Part Number 7-9176

Copyright © 2004 TX RX Systems, Inc.

First Printing: September 1997

Version Number Version Date

1 09/19/97

2 04/09/98

3 04/26/99

4 06/23/04

Symbols Commonly Used

WARNING

CAUTION or ATTENTION

High Voltage

Use Safety Glasses

ESD Elecrostatic Discharge

Hot Surface

Electrical Shock Hazard

NOTE

Important Information

Bird Technologies Group TX RX Systems Inc.

GENERAL DESCRIPTION

Vari-Notch® duplexers are used to provide simulta­neous operation of a transmitter and receiver (or
two transmitters) which are at different frequencies
while connected to a common antenna. These
duplexers are frequently used in radio repeater
systems. This instruction manual (part# 7-9176-1)
covers the installation, tuning, and maintenance of
Vari-Notch duplexers constructed from 4” diameter
cavities.

Tabl e 1

shows the model numbers and
electrical specifications of the duplexers covered by
this manual. Vari-Notch duplexers are composed of
two groups (or sets) of daisy-chained resonant cav­ity filters, which couple signals to and from the
shared antenna. This creates two signal paths, a

high frequency channel and a low frequency chan­nel. The minimum frequency separation between
the channels, as well as the isolation in dB’s (per
channel and between channels) is listed for each
model in table 1.

The cavity filters used in a transmit channel will
reduce transmitter noise components at the
receive frequency, thus preventing noise desensiti­zation of the receiver. Conversely, the cavity filters
used in a receive channel will isolate the receiver
from the transmitter carrier preventing carrier
desensitization of the receiver.

Model

Number

28-38-03A/D 132 - 174 150 0.5 1.9 80 45

28-37-07A/C 144 - 174 400 3 0.7 85 30

28-37-06A/C 144 - 174 125 1 1.2 75 35

28-37-04A/C 144 - 174 125 0.5 1.8 65 30

28-37-81227-A-R2 144 - 174 125 0.5 2 65 30

28-52-02A 215 - 250 250 1.6 1.2 90 55

28-56C-02A 380 - 420 250 5 0.7 100 50

28-65-07A/B 406 - 430 250 3 1.25 85 60

28-65-02A/B 406 - 430 350 3 0.8 80 30

28-66-02A/B 442 - 450 350 5 0.6 100 46

28-70-09A/B 450 - 470 250 5 1.25 100 70

28-70-02A/B 450 - 470 350 5 0.6 100 46

28-69-06A/B 470 - 512 250 3 1.25 86 60

28-69-02A/B 470 - 512 350 3 0.8 80 30

Frequency

Range

(MHz)

Power

Rating

(Watts)

Min. Freq.

Separation

(MHz)

Insertion

Loss

(dB)

Isolation

(dB)

Per Chan. Bet Chan

28-89-01A/B 806 - 866 125 45 0.8 90 60

28-88-04A/B 890 - 960 125 39 0.8 90 50

28-88-01A/B 890 - 960 125 3.6 1.25 90 40

28-97-01A/B 1215 - 1300 125 12 1 100 50

Tabl e 1 :

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 1

Vari-Notch Duplexer electrical specifications.

Resonant cavity filters are the basic building blocks

a

of the system. Also important, are the interconnect
cables between each filter which have cut length's
equivalent to either 1/4 λ or 3/4 λ of that channels
pass frequency. The exception is the antenna cable
that couples each channels final filter to the
antenna port, which is cut to 1/2 λ of the other

(or

remaining) channels pass frequency. This effec­tively places a relatively large impedance in parallel
with the antenna, insuring a good impedance
match between the other

(or remaining) channel
and the antenna. This technique of impedance
matching allows both channels to be connected to
the same antenna with very little loss due to mis­matching. The antenna cables are permanently
soldered and crimped to the antenna junction. The
combination of the antenna junction and the
attached antenna cables is referred to as an
"Antenna Junction Assembly".

Figure 1

shows the functional block diagram of a
typical four-cavity Vari-Notch duplexer system. Six
and eight cavity systems are similar except for the

extra filters in each channel. The photograph
shown in

Figure 2

is the front view of a typical four­cavity Vari-Notch duplexer. Each of the physical
components in the system is labeled with the field
adjustable parts shown in emboldened italics.

UNPACKING

Care should be used when removing the duplexer
from it's shipping container to avoid unnecessary
damage. It is important to visually inspect the
duplexer for any shipping damage as soon as pos­sible after taking delivery. It is the customers

responsibility to file any necessary damage claims
with the carrier.

Vari-Notch duplexers are rugged devices but may
become detuned if jostled or dented during ship­ping. The most easily damaged parts of the
duplexer are the tuning rods. These rods are
marked where they exit from the locking nut with a
dab of red varnish or other color/type of paint. If
this seal appears to be broken it may indicate that
the system has been detuned in transit.

High
Frequency
Equipment

Low
Frequency
Equipment

RG214 or RG142
Double-Shielded Cable
(Supplied by customer)

Rejection notch tuned to low frequency

Rejection notch tuned to high frequency

Interconnect Cable

λ

or 3/4 λ of this)

(1/4
(channels pass freq.)

Vari-

Notch

Filter

Passband tuned to high frequency

Vari-

Notch

Filter

Passband tuned to low frequency

Vari-

Notch

Filter

Vari-

Notch

Filter

Antenna Cable

(1/2 λ of the other)

(channels pass freq.)

Antenna
Junction

To
Antenn

TYPICAL FOUR CAVITY VARI-NOTCH FILTER

Figure 1:

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 2

Block diagram of typical four-cavity Vari-Notch Duplexers.

INSTALLATION

Vari-Notch duplexers should be securely installed
in a dry, vibration-free environment. Attachment of
the cavity shells to a ground bus is recommended
in order to maximize lightning protection. A light­ning protection device placed in the antenna feed­line, preceding the duplexer, is recommended.
High quality double shielded coaxial cable termi­nated with quality connectors (N-type) are recom­mended for connecting the transmitter and receiver
to the duplexer and are available from TX RX Sys­tems Inc. It is also important to observe the power
handling ratings of cables in transmit systems.

Mount the duplexer in its permanent operating
position using suitable hardware. Connect the two
transmitters (or transmitter/receiver) and the
antenna feedline to the duplexer making sure to

connect the correct equipment to the correct port.
Labels are affixed next to each port (port labels) to
help you make the right connections. In addition, a
specification tag will be found in a plastic bag
attached to one of the tuning rods. The frequency
that each cavity group is tuned to will appear on
either the port labels or the specification tag. The
duplexer is now ready for normal operation. No tun­ing is required if the frequencies (high frequency
and low frequency) found on the port labels/specifi­cation tag matches the actual operating frequen­cies.

MAINTENANCE

No special maintenance is required. Vari-Notch
duplexers are passive devices of rugged electrical
and mechanical design. They are tuned at the fac­tory for the original design requirements and

Equipment

Por t

Interconnect

Cable

Var ia ble

Capacitor

Antenna

Cable

1/4” Shaft

Locking Nut

Antenna

Cable

Loop

Assembly

Tu n i n g

Rod

Mounting

Bar

Equipment

Por t

Antenna

Por t

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 3

Interconnect

Figure 2:

Cable

Typical four-cavity Vari-Notch Duplexer.

Cavity

Resonator

require no further adjustment or maintenance.
These devices will stay properly tuned unless they
have been physically damaged or are tampered
with. Check for loose or corroded connectors on
the interconnect cables whenever an inspection is
performed on other station equipment.

Because duplexers are passive devices, field
repairs are rarely required. Field repair of duplexers
is limited to the replacement or repair of damaged
cables. Cavity damage, when it occurs, is usually
due to catastrophic failure from lightning or power
far in excess of the duplexers rating. If cavity prob­lems are suspected, the unit should be returned to
the factory for repair. Due to the critical alignment
of parts inside of the cavity resonators, field repair
is not recommended.

Required Equipment

Due to the sensitivity of the adjustments, it is
strongly recommended that the proper equipment
be used when tuning the individual filters, other­wise the filter should be sent to the factory or an
authorized representative for retuning. The follow­ing equipment or it's equivalent is recommended in
order to properly perform the tuning adjustments
for the Vari-Notch duplexer.

1. IFR A-7550 spectrum analyzer with optional
tracking generator installed.

2. Return Loss Bridge

(Eagle model RLB150N3A).

3. Double shielded coaxial cable test leads
(RG142 B/U or RG223/U).

TUNING

Vari-Notch duplexers are originally pre-tuned at the
factory to the customers specification. To retune
the duplexer, each resonant cavity must be sepa­rated from the group and adjusted individually.
Then the individual cavities are re-connected and
each channel is fine tuned to peak it's overall
response. When reconnecting the assembly, it is
mandatory that each filter and cable be replaced in
it's original position.

There are two adjustable parameters in a Vari­Notch filter; the pass frequency and the rejection
notch. Adjustment of the tuning rod will allow the fil­ters passband to be centered at the desired fre­quency. The rejection notch frequency is adjusted
by turning the variable capacitor located on the
loop assembly.

The insertion loss of each cavity is determined by
the design of the loop assembly and is not
adjustable.

The loop assembly on a 4" resonant

cavity should never be loosened or moved

field

from

it's factory preset position.

It is also important to note that the insertion loss
specification in table 1 for each of the different
models, is the total insertion loss for each channel
of that model. For instance, the specification for
model 28-38-03A/D is 1.9 dB, this means both the
high and low frequency channels will each have a
total of 1.9 dB of insertion loss. The total insertion
loss is the sum of losses from each cavity in the
channel as well as the interconnecting cables
between the cavities.

4. 50 ohm load with at least -35 dB return loss
(1.10:1 VSWR). The JFW Industries model
50T-007 or equivalent.

5. Female union (UG29-N or UG914-BNC).

6. Insulated tuning tool (TX RX Systems Inc.
part# 95-00-01)

7. 1/4" open-ended wrench.

Tuning Procedure

Tuning of the filter requires adjustment of the pass­band and the rejection notch. The passband is

adjusted while observing the return loss response
and the rejection notch is adjusted by monitoring
the output of a tracking generator, after it passes
through the filter.

All Vari-Notch filters should be temporarily removed
from the system and tuned on the bench using test
instrumentation only. Do not adjust the filters while
they are under transmit power. To insure proper
tuning of the 4" Vari-Notch filter, all adjustments
should be performed in the following order:

1. Rough tune the passband.

2. Rough tune the rejection notch.

3. Final tune the passband.

4. Final tune the rejection notch, always the last

adjustment made.

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 4

WARNING

Tuning while under transmit power can result in
damage to the duplexer.

7. Connect the "load" port on the RLB to the input
of the loop assembly, make sure the output of
the loop assembly is connected to a 50 ohm
load, refer to figure 4. The display will now
present the return loss curve for the 4" Vari­Notch filter being measured.

The passband is
that frequency range over which the return
loss is 15 dB or greater.

PASSBAND

The peak of the passband will correspond very
closely to the point of minimum reflected energy
from the filter and maximum forward power through
it. A transmitter connected to the filter will operate
best when the reflected energy is lowest, therefore
the return loss response will be used to set the
passband. The passband can be checked and
adjusted using the following procedure.

Checking the passband

1. A zero reference for return loss must be estab­lished at the IFR A-7550 prior to checking the
passband frequency, this is done by connect­ing the return loss bridge to the analyzer / gen­erator as shown in

Figure 3

.

2. Set-up the analyzer / generator for the desired
frequency (center of display) and for a vertical
scale of 10 dB/div.

3. Do not connect the return loss bridge (RLB) to
the cavity, leave the "load" port on the bridge
open. This will supply the maximum amount of
reflected energy to the analyzer input.

4. Insure that the IFR A-7550 menu's are set as
follows:
DISPLAY - line
MODE - live
FILTER - none
SETUP - 50 ohm/dBm/gen1.

Adjusting the Passband

Adjust the passband by setting the peak (maximum
negative value) of the return loss curve at the
desired passband frequency (should be the center­vertical graticule line on the IFR A-7550's display).
Refer to

Figure 4.

The resonant frequency is adjusted by using the
tuning rod, which is a sliding adjustment (invar rod)
that rapidly tunes the response curve across the
frequency range of the filter. Resonant frequency is
increased by pulling the rod out of the cavity and is
decreased by pushing the rod into the cavity. For
ease in making adjustments, rotate and slide the
rod while gently tapping on it with a screwdriver

dBm

40

30

20

10

0

-10

-20

-30

-40

ANALYZER

INPUT

200

KHz/DIV

dB ATT GEN

40

MHz

dBM

0

300

KHz RES

10

GENERATE

MSEC

OUTPUT

5. The flat line across the screen is the return
loss curve. Select the "MODE" main menu item
and then choose the "STORE " command.

RLB - 150 BRIDGE

6 Next select the "DISPLAY" main menu item and

choose the "REFERENCE" command. This will
cause the stored value

to be displayed at the

Reflected

Load

Source

center of the screen as the 0 dB reference
value.

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 5

Figure 3:

Setting the return loss reference.

handle or other small tool. This will break the sur­face tension on the probe contact fingers and allow
smoother movement of the tuning rod.

Cavity Tuning Tip

When tuning a cavity that has been in service for
some time it is not unusual to find the tuning rod
hard to more in or out. This occurs because TX RX
Systems Inc. uses construction techniques bor­rowed from microwave technology that provide

dBm

40

30

20

10

0

-10

-20

-30

-40

ANALYZER

INPUT

500

KHz/DIV

dB ATT GEN

40

MHz

dBM

0

300

KHz RES

10

GENERATE

MSEC

OUTPUT

large area contact surfaces on our tuning probes.
These silver plated surfaces will actually form pres­sure welds which maintain excellent conductivity.
The pressure weld develops over time and must be
broken in order for the tuning rod to move. This is
easily accomplished by gently tapping the tuning
rod with a plastic screwdriver handle or small ham­mer so it moves into the cavity. The pressure weld
will be broken with no damage to the cavity.

Once the desired response is obtained using the
tuning rod, it is "locked" into place by tightening the
1/4" shaft lock nut.

Failure to lock the tuning rod

will cause a loss of temperature compensation and
detuning of the cavity.

REJECTION NOTCH

The rejection notch will track with the tuning of the
passband and therefore should be the last adjust­ment made to the 4" Vari-Notch filter. The rejection
notch is adjusted by changing the amount of
capacitance in the loop assembly. The capacitor is
a variable tubular-piston type.

Checking the rejection notch

1. The rejection notch is checked by connecting the
tracking generator to the input of the cavity fil­ter while the spectrum analyzer is connected to
the output, as illustrated in

Figure 5.

4" DIAMETER
VARI-NOTCH

FILTER

Figure 4:

RLB - 150 BRIDGE

Reflected

Load

Source

Checking the passband.

50 OHM LOAD

2. Insure that the IFR A-7550 menu's are set as
follows:
DISPLAY - line
MODE - live
FILTER - none
SETUP - 50 ohm/dBm/gen1.

Adjusting the rejection notch

The notch is adjusted by turning the variable
capacitor. Because of the filters sensitivity to tool
contact, an insulated tuning tool must be used to
make the adjustment. Access to the capacitor is
obtained by removing the small screw or rubber
button on the side of the loop assembly.

FINE TUNING THE CHANNELS

Once all of the individual filters have been tuned,
each of the channels as a whole must be fine
tuned. First the passband for both channels and
then the rejection notches. The following is a step
by step procedure for fine tuning the channels and
completes the re-tuning of the duplexer.

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 6

1. Reassemble the duplexer by reinstalling the cav­ities and interconnect cables in their original
locations.

4. Set-up the analyzer / generator to the desired
frequency (center of display) and for a vertical
scale of 10 dB/div.

2. The passband for the channels are fine tuned
first, in a manner very similar to tuning a single
cavity.

3. A zero reference for return loss must be estab­lished at the IFR-7550. Connect the RLB to the
analyzer / generator as shown in figure 3.

dBm

10

0

-10

-20

-30

-40

-50

-60

-70

ANALYZER

INPUT

500

KHz/DIV

dB ATT GEN

40

MHz

dBM

0

300

KHz RES

10

GENERATE

MSEC

OUTPUT

5. Do not connect the RLB to the duplexer at this
time, leave the "load" port on the bridge open.
This will supply the maximum amount of
reflected energy to the analyzer input.

6. Insure that the IFR A-7550 menu's are set as
follows:
DISPLAY - line
MODE - live
FILTER - none
SETUP - 50 ohm/dBm/gen1.

7. The flat line across the screen is the return
loss curve. Select the "MODE" main menu item
and then choose the "STORE " command.

8. Next select the "DISPLAY" main menu item
and choose the "REFERENCE" command.
This will cause the stored value

to be displayed
at the center of the screen as the 0 dB refer­ence value.

9. Connect the "load" port on the RLB to the
equipment port of the channel to be fine tuned.
Terminate the duplexers antenna connector
with the 50 ohm load. The equipment port of
the remaining duplexer channel is left discon­nected, refer to

Figure 6.

10. The display will now present the combined
return loss curve for all of the cavities in the
channel. The channels passband is that fre-

4" DIAMETER
VARI-NOTCH

FILTER

quency range over which the return loss is 15
dB or greater.

11. Fine tune the passband (for maximum return
loss) for the entire channel by gently adjusting
the positions of the tuning rods, moving
between cavities as required. Once the desired
response is obtained "lock" the tuning rods into
place by tightening the 1/4" shaft lock nuts on
each filter.

12. Move the cable from the RLB's "load" port to
the equipment port of the other channel. This
will allow the remaining duplexer channel to be

Figure 5:

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 7

Checking the rejection notch.

fine tuned. Reset the analyzer / generator cen­ter frequency. Repeat steps 10 and 11.

dBm

dBm

40

30

20

10

0

-10

-20

-40

-40

ANALYZER

INPUT

500

KHz/DIV

dB ATT GEN

40

MHz

300

KHz RES

Vari-

Notch

Filter

High Frequency Pass Channel

dBM

0

MSEC

10

GENERATE

OUTPUT

Reject the Low Frequency Channel

Vari-

Notch

Filter

50
Load

Ω

Reflected

RLB - 150 BRIDGE

(RLB)

Figure 6:

Source

Load

Equipment setup for fine tuning the passband of each channel.

13. The rejection notch for each of the channels
must be fine tuned next.

14. Terminate the antenna connector with a 50
ohm load. Connect the output of the tracking
generator to the equipment port of one of the
duplexer channels and the spectrum analyzer
input to the equipment port of the remaining
channel as shown in

Figure 7.

Vari-

Notch

Filter

Low Frequency Pass Channel

Reject the High Frequency Channel

Vari-

Notch

Filter

MODE - live
FILTER - none
SETUP - 50 ohm/dBm/gen1

17. Set the analyzer's attenuation control so that
the "peak" or lowest value on the rejection
notch is displayed. The "peak" should be
around -100 dB depending upon which model
of duplexer you are tuning.

15. Set-up the analyzer / generator to sweep
across the rejection notch frequency of the
channel being tuned. The center of the display
should be set to the desired center frequency
of the rejection notch being adjusted. Set the
vertical scale of the analyzer / generator to 10
dB/div.

18. The cavities rejection notches are adjusted (for
maximum rejection) by gently turning the vari­able capacitors in the loop assemblies. Move
between filters as needed.
Because of the filters sensitivity to tool contact,
an insulated tuning tool must be used to make
the adjustment. Access to the capacitors is
obtained by removing the small screw or rub-

Keep in mind that the high frequency channel

ber button on the side of the loop assemblies.

has its rejection notch set to reject the low fre­quency signal and vice-versa for the rejection
notch of the low frequency channel.

19. Adjust the rejection notch of the remaining cav­ities by changing the sweep frequency of the
analyzer / generator to match the new rejection

16. Insure that the IFR A-7550 menu's are set as
follows:

notch frequency. The equipment stays con­nected as it is.

DISPLAY - line

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 8

20. Repeat step 17 and 18 for the remaining chan­nel (cables and equipment stay connected
where they are). Remember to replace the
small screws or rubber buttons on the side of
the loop assemblies.

21. With the tuning completed, reconnect the
equipment cables and antenna feedline. Test
the system for proper operation.

dBm

dBm

-30

-40

-50

-60

-70

-80

-90

-100

-110

ANALYZER

INPUT

50

KHz/DIV

dB ATT GEN

30

MHz

dBM

0

Figure 7:

300

KHz RES

Vari-

Notch

Filter

Vari-

Notch

Filter

High Frequency Pass Channel

Reject the Low Frequency Channel

MSEC

10

GENERATE

OUTPUT

Vari-

Notch

Filter

Vari-

Notch

Filter

Low Frequency Pass Channel

Reject the High Frequency Channel

Equipment setup for fine tuning the rejection notch of each channel.

50
Load

Ω

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 9

DUPLEXER PROBLEMS AND REMEDIES

Duplexers are passive devices requiring little or no service once installed in a system. The proper design and application of a given Duplexer will give
years of trouble free service. When problems do occur in a duplex system it is necessar y to identify as many abnormal conditions as possible to zero in
on the specific cause of the problem. Unfortunately,there are only a few measurable or observable performance indicators at the disposal of the field
serviceman, and any number of conditions may exist, even simultaneously, which are responsible for the observed phenomena. Most Duplexer installa­tion problems fall into three categories. Each of these three conditions will be treated separately, using the typical cause and remedy approach.

A. High input VSWR
B. Excessive loss
C. Desensitization of the receiver

PROBLEM

A B C

•• Reverse labeling of Tx and Rx terminals. 1

• • Unit tuned to wrong frequencies. 2

• Bad antenna or interconnect cables. 3

• • Use of between series adapter, especially UHF types. 4

•••Duplexer detuned in shipment. 5

• • Water has entered the Duplexer antenna connector from the antenna feed line. 6

••

Spurious Tx output is being reflected by the selective Duplexer input terminal and observed on the wattmeter, the
wattmeter being unable to discriminate between on-frequency and off-frequency energy.

Bad joint in a cable or antenna system beyond the antenna of the Duplexer. All lines may show zero reflected power,
but noise can still be produced when a corroded or indefinite metal-to-metal contact is exposed to RF energy. When

•

this occurs beyond the Duplexer, it cannot be filtered out, and the noise backs up into the receiver.

Adverse cable length between Duplexer and transmitter using varactor or broadband hybrid combining type trans-

•

mitter outputs. Even though the Duplexer VSWR is flat on frequency, the reflected impedance of the Duplexer off
resonance, transformed by changing cable lengths, can cause parasitics to be generated.

Duplexer transmitter mixing with another outside transmitter, producing intermodulation on or near the receiver fre-

•

quency.

Transmitter cable leading to Duplexer in close proximity to Duplexer antenna or receiver cable. This is usually only a

•

problem on close separation Duplexers, (1.0 MHz or less) where the 85 to 100 dB isolation is decreased by adverse
coupling, created by running these cables too close together for too great a distance.

• Inadequate shielding of transmitter and receiver modules in the repeater. 12

The number at right corresponds to the appropriate numbered remedy paragraph

POTENTIAL CAUSE

7

8

9

10

11

• Insufficient duplex isolation for the application. 12

A spurious transmitter response outside of the normal Duplexer isolation band or inadequacy of notch filter type

•

Duplexers to suppress a wide enough band of Tx noise to protect the receiver.

Impedance change in antenna due to icing. VSWR increase may be sufficient to reflect back through the Duplexer

•

and upset transmitter tuning, causing parasitics, which are not suppressed by the Duplexer.

The addition of a broadband power amplifier to a low power transmitter. The noise floor of the low power radio is
raised by an amount equal to the gain of the power amplifier, and in addition, the power amplifier will contribute its

•

own noise. Power amplifiers are just as prone to the generation of parasitics as transmitters, and may be triggered
by an adverse cable length between power amplifier and Duplexer, a problem covered above.

• Excessive loss with changing temperature and apparent Duplexer detuning. 17

14

15

16

REMEDIES

1. Tune a signal generator to the receive frequency and inject it into the antenna terminal, sampling for the signal at each equipment terminal.
Reverse the labels if necessary. It may be that the unit was ordered to the reverse frequencies. If so, the label will indicate this. If the duplexer is
symmetrical in design (usually indicated by the same number of Tx and Rx sections) just reverse the equipment labels and operate. Generally, no
damage will be done to the duplexer when operated in reverse for a short period. If other adverse symptoms appear, contact the factory.

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 10

2. Check the unit label. If needed, the duplexer may be field tuned. Consult the instructions and/or the factory if the duplexer is still under
warranty or beyond field tuning capability.

3. Check cable, by substitution, using a termaline wattmeter, or a thruline wattmeter into a known good load. Check the antenna line input
for reflected power.

4. To eliminate high input VSWR reduce the number of between series adapters by making up proper interconnect cables. UHF connec­tors are non-constant impedance, and certain combinations can transform a 1.1:1 VSWR into a 2.0:1, or vice versa.

5. Consult the instruction manual for field tuning procedures, or the factory, if the unit is still under warranty or beyond field tuning capabil­ity. (We trust that our products will not be prone to this problem).

6. Consult the factory. The affected antenna cables may be field replaceable, or a “baking out” process may be possible.

7. To prove this condition, place a bandpass filter between the Tx and duplexer to clean up the spurious, and put the wattmeter between
the bandpass filter and the duplexer to measure reflected power from the duplexer. The bandpass filter selectivity should be equal to or
better than that of the duplexer at about the 3.0 dB points.

8. Operate the duplex system into a dummy load. If no desensitization occurs, check out all line, antennas, and look for potential bad joints
close to the radiation antenna where re-radiation of noise may be possible back into the antenna system receiver. Loose metal-to­metal contacts on tower guying systems have also been known to create system noise. Note the effect of vibrating tower guys on sys­tem noise.

9. Change the length of cable between the transmitter and duplexer, traversing through half wave in increments of between 1 and 2 inches
until the desensitization ceases or is minimal. A ferrite isolator will also cure this condition when it is installed between the transmitter
and duplexer. However, this is a much more expensive remedy.

10. If the IM is in the duplex transmitter, a ferrite isolator in the duplex transmitter line (NOT antenna line) will show this by either reducing
or eliminating it. More isolation can be obtained by cascading isolators if needed. However, IM of this magnitude indicates the system
should be studied for possible revision to reduce the production of the IM.

11. Cables such as RG-8a/u and RG-213/u should be kept at least 3 - 4” apar t over 5”-6” runs. Use of double shielded cable will reduce the
susceptibility to this problem.

12. Consult the radio manufacturer. This condition can be verified by operating the transmitter into a dummy load while injecting a minimum
quieting signal into the receiver. Some radios require special modifications before they are suitable for repeater operation.

13. If this problem is suspected, contact the radio manufacturer for recommended duplex isolation for Tx noise suppression and carrier
suppression. Duplexer isolation should be measured first per instruction manual to verify rated specifications are present. If more
duplex isolation is required, contact TX RX SYSTEMS for recommended filtering.

14. Consult the factory. Bandpass filter tests can be made to confirm this. In extreme cases, adjustments to the transmitter may b e
required.

15. Either de-ice the antenna, or use an antenna less sensitive to ice. A ferrite isolator can also be put at the transmitter output to improve
the impedance match. Ferrite isolators cannot be put in antenna lines, as they will attenuate Rx signals.

16. A mismatch may possibly be reduced by lengthening the cable which runs between the power amplifier output and the duplexer input
until the receiver desensitization disappears, as follows:

30 MHz to 512 MHz RANGE;

1/2 wavelength, until desensitization disappears.

800 MHz to 1.3 GHz RANGE;

pear, repeat with cables each 3/4” longer than the previous length, not to exceed 1/2 wavelength.

17. We find that this cause most commonly relates to shifting impedance of the transmitter or power amplifier with temperature. The
duplexer appears detuned, since a “conjugate match” (canceling reactance, and matching resistance component) is approached by
shifting the duplexer passband above or below the 50 ohm point, as determined by an increase in output power on the wattmeter. In
this case, temperature control of the room is the only answer, other than upgrading the transmitter.

BNC or N type adapters may be inser ted in the original cable, one at a time and not to exceed a total of

Prepare a cable length 3/4” longer than the original cable and inser t. If desensitization does not disap-

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 11

)

POWER IN/OUT

t

e

VS.

INSERTION LOSS

The graph below offers a convenient means of determining the insertion loss of filters, duplexers,
multicouplers and related products. The graph on the back page will allow you to quickly determine VSWR. I
should be remembered that the field accuracy of wattmeter readings is subject to considerable variance du
to RF connector VSWR and basic wattmeter accuracy, particularly at low end scale readings. However,
allowing for these variances, these graphs should prove to be a useful reference.

INSERTION LOSS (dB)

500

400

300

250

200

150

125

INPUT POWER (WATTS)

100

6.5

7.0

5.5

5.0

6.0

4.5

3.5

4.0

2.5

3.0

2.0

1.5

1.0

.25

.50

75

50

50

75 100

125 150 200

250

300

400

500

OUTPUT POWER (WATTS

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 12

500

400

300

200

100

50

40

30

20

POWER FWD./REV.

VS.

VSWR

V
S

W

R

1.1:1

1.15:1

1.2:1

10

FORWARD POWER (WATTS)

5.0

4.0

3.0

2.0

1.0

0.5

40

20

10

8.0 6.0

4.0

2.0

REFLECTED POWER (WATTS)

FOR OTHER POWER LEVELS, MULTIPLY

BOTH SCALES BY THE SAME MULTIPLIER

1.0 0.8

0.6

0.4

1.25:1

1.3:1

1.4:1

1.5:1

1.6:1

1.8:1

2.0:1

2.5:1

3.0:1

0.2

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 13

Power Ratio and Voltage Ratio to Decibel

Conversion Chart

Loss or Gain Power Ratio Voltage Ratio

+9.1 dB 8.128 2.851

-9.1 dB 0.123 0.351

- dB +- dB +

Voltage

Ratio

1 1 0 1 1

0.989 0.977 0.1 1.012 1.023

0.977 0.955 0.2 1.023 1.047

0.966 0.933 0.3 1.035 1.072

0.955 0.912 0.4 1.047 1.096

0.944 0.891 0.5 1.059 1.122

0.933 0.871 0.6 1.072 1.148

0.923 0.851 0.7 1.084 1.175

0.912 0.832 0.8 1.096 1.202

0.902 0.813 0.9 1.109 1.23

0.891 0.794 1 1.122 1.259

0.881 0.776 1.1 1.135 1.288

0.871 0.759 1.2 1.148 1.318

0.861 0.741 1.3 1.161 1.349

0.851 0.724 1.4 1.175 1.38

0.841 0.708 1.5 1.189 1.413

0.832 0.692 1.6 1.202 1.445

0.822 0.676 1.7 1.216 1.479

0.813 0.661 1.8 1.23 1.514

0.804 0.646 1.9 1.245 1.549

0.794 0.631 2 1.259 1.585

0.785 0.617 2.1 1.274 1.622

0.776 0.603 2.2 1.288 1.66

0.767 0.589 2.3 1.303 1.698

0.759 0.575 2.4 1.318 1.738

0.75 0.562 2.5 1.334 1.778

0.741 0.55 2.6 1.349 1.82

0.733 0.537 2.7 1.365 1.862

0.724 0.525 2.8 1.38 1.905

0.716 0.513 2.9 1.396 1.95

0.708 0.501 3 1.413 1.995

0.7 0.49 3.1 1.429 2.042

0.692 0.479 3.2 1.445 2.089

0.684 0.468 3.3 1.462 2.138

0.676 0.457 3.4 1.479 2.188

0.668 0.447 3.5 1.496 2.239

0.661 0.437 3.6 1.514 2.291

0.653 0.427 3.7 1.531 2.344

0.646 0.417 3.8 1.549 2.399

0.638 0.407 3.9 1.567 2.455

0.631 0.398 4 1.585 2.512

0.624 0.389 4.1 1.603 2.57

0.617 0.38 4.2 1.622 2.63

0.61 0.372 4.3 1.641 2.692

0.603 0.363 4.4 1.66 2.754

0.596 0.355 4.5 1.679 2.818

0.589 0.347 4.6 1.698 2.884

0.582 0.339 4.7 1.718 2.951

0.575 0.331 4.8 1.738 3.02

0.569 0.324 4.9 1.758 3.09

Power

Ratio

dB

Voltage

Ratio

Power

Ratio

Voltage

Ratio

0.562 0.316 5 1.778 3.162

0.556 0.309 5.1 1.799 3.236

0.55 0.302 5.2 1.82 3.311

0.543 0.295 5.3 1.841 3.388

0.537 0.288 5.4 1.862 3.467

0.531 0.282 5.5 1.884 3.548

0.525 0.275 5.6 1.905 3.631

0.519 0.269 5.7 1.928 3.715

0.513 0.263 5.8 1.95 3.802

0.507 0.257 5.9 1.972 3.89

0.501 0.251 6 1.995 3.981

0.496 0.246 6.1 2.018 4.074

0.49 0.24 6.2 2.042 4.169

0.484 0.234 6.3 2.065 4.266

0.479 0.229 6.4 2.089 4.365

0.473 0.224 6.5 2.113 4.467

0.468 0.219 6.6 2.138 4.571

0.462 0.214 6.7 2.163 4.677

0.457 0.209 6.8 2.188 4.786

0.452 0.204 6.9 2.213 4.898

0.447 0.2 7 2.239 5.012

0.442 0.195 7.1 2.265 5.129

0.437 0.191 7.2 2.291 5.248

0.432 0.186 7.3 2.317 5.37

0.427 0.182 7.4 2.344 5.495

0.422 0.178 7.5 2.371 5.623

0.417 0.174 7.6 2.399 5.754

0.412 0.17 7.7 2.427 5.888

0.407 0.166 7.8 2.455 6.026

0.403 0.162 7.9 2.483 6.166

0.398 0.159 8 2.512 6.31

0.394 0.155 8.1 2.541 6.457

0.389 0.151 8.2 2.57 6.607

0.385 0.148 8.3 2.6 6.761

0.38 0.145 8.4 2.63 6.918

0.376 0.141 8.5 2.661 7.079

0.372 0.138 8.6 2.692 7.244

0.367 0.135 8.7 2.723 7.413

0.363 0.132 8.8 2.754 7.586

0.359 0.129 8.9 2.786 7.762

0.355 0.126 9 2.818 7.943

0.351 0.123 9.1 2.851 8.128

0.347 0.12 9.2 2.884 8.318

0.343 0.118 9.3 2.917 8.511

0.339 0.115 9.4 2.951 8.71

0.335 0.112 9.5 2.985 8.913

0.331 0.11 9.6 3.02 9.12

0.327 0.107 9.7 3.055 9.333

0.324 0.105 9.8 3.09 9.55

0.32 0.102 9.9 3.126 9.772

Power

Ratio

dB

Voltage

Ratio

Power

Ratio

Bird Technologies Group TX RX Systems Inc.

Return Loss vs. VSWR

Watts to dBm

Return Loss VSWR

30 1.06

25 1.11

20 1.20

19 1.25

18 1.28

17 1.33

16 1.37

15 1.43

14 1.50

13 1.57

12 1.67

11 1.78

10 1.92

9 2.10

Watts dBm

300 54.8

250 54.0

200 53.0

150 51.8

100 50.0

75 48.8

50 47.0

25 44.0

20 43.0

15 41.8

10 40.0

5 37.0

4 36.0

3 34.8

2 33.0

1 30.0

dBm = 10log P/1mW

Where P = power (Watt)

Insertion Loss

Input Power (Watts)

50 75 100 125 150 200 250 300

3 25 38 50 63 75 100 125 150

2.5 28 42 56 70 84 112 141 169

2 32 47 63 79 95 126 158 189

1.5 35 53 71 88 106 142 177 212

1 40 60 79 99 119 159 199 238

Insertion Loss

.5 45 67 89 111 134 178 223 267

Output Power (Watts)

Free Space Loss

Distance (miles)

.25 .50 .75 1 2 5 10 15

150 68 74 78 80 86 94 100 104

220 71 77 81 83 89 97 103 107

460 78 84 87 90 96 104 110 113

860 83 89 93 95 101 109 115 119

940 84 90 94 96 102 110 116 120

Frequency (MHz)

1920 90 96 100 102 108 116 122 126

Free Space Loss (dB)

Free space loss = 36.6 + 20log D + 20log F

Where D = distance in miles and F = frequency in MHz

Bird Technologies Group TX RX Systems Inc.

8625 Industrial Parkway, Angola, NY 14006 Tel: [phone] Fax: [phone] [e-mail] www.bird-technologies.com