Tuning begins well before any adjustments are made; it starts with system awareness, knowing what is in place and what is being added, and what impact those changes will have on the signal environment. Airline Junction Combiners are tuned by cavity, not by system, and every cavity behaves differently depending on frequency spacing, layout, and physical interaction with the line. 

This guide outlines the procedures and principles necessary to retune or expand a TX RX Airline Junction Combiner operating between 764 and 960 MHz.

Identify the System Configuration

All tuning begins with confirming which type of combiner is in use. Systems may operate on 700 MHz, 800 MHz, or 900 MHz channels, or in some cases, a combination of 700 and 800 MHz. Mixed-band systems (700/800) require two airline junctions. Single-band systems use only one. 900 MHz cannot be combined with 700 or 800 MHz bands under this architecture. Model numbers will indicate band and channel count. Frequency designators include 83B (764–806 MHz), 86A (806–869 MHz), 83E (764–869 MHz), and 96 (896–960 MHz).

Inspect and Prepare the Hardware

Before tuning begins, the system must be verified. Visual inspection should focus on the cavity filters. Each tuning rod is sealed with a visible dab of red varnish. If the seal is broken, assume the unit may be out of alignment. Detuning during shipment is rare but possible, especially in systems that have been re-racked or transported multiple times. Filters must remain mechanically stable and free of corrosion. 

Combiner frames must be installed in vibration-free environments with solid grounding. Avoid heat sinks from direct sunlight. Confirm all coax is double-shielded and terminated with N-type connectors. Signal loss and impedance mismatch typically begin at cable ends, not cavities.

Understand the Signal Path

Transmit signals pass through a unidirectional isolator, then into a cavity filter, and finally to the antenna via the airline junction. The isolator provides forward power routing while absorbing any reflected power through load terminations. The bandpass cavity filter follows, shaped to suppress transmitter noise and reject adjacent signals. The cable length between the filter and junction is precisely measured and should not be modified. Tuning is never performed on the isolator or the airline junction itself; it is only performed on the cavities. 

Electrically Remove the Filter

Filters must be disconnected from the isolator and the airline junction before tuning and full rack removal is unnecessary. Signal path disconnection is sufficient, but physical disturbance during adjustments should be minimized. All transmitters connected to the system must be powered down to prevent the risk of damage to test equipment or the filter under adjustment.

Set Pass Frequency Using a Network Analyzer

A cavity filter’s passband is defined by its coarse and fine tuning rods. The coarse rod (an invar plunger) controls the broad position of the resonant peak. The fine rod (silver-plated brass) controls fine alignment. The coarse rod increases frequency when withdrawn and decreases it when inserted. The fine rod has the opposite response; pushing it in raises frequency, pulling it out lowers it.

A calibrated network analyzer is required. The filter’s radio frequency (RF) input and output are connected directly between the analyzer’s source and receiver ports. Set a 10 MHz span centered on the desired frequency. A medium system bandwidth with transmission and reflection at 10 dB/div is recommended. Adjust the coarse rod until the minimum insertion loss aligns with the center frequency. Use the fine rod to finalize the peak. Lock both rods firmly. The coarse rod is secured with a 10-32 cap screw. The fine rod is held by a knurled thumb nut. Unlocked rods will shift frequency due to thermal drift or mechanical vibration.

Reconnect and Re-Verify Alignment

Reconnect the tuned filter to the airline junction and recheck alignment. System interaction, like minor shifts in response caused by the proximity of the airline, may occur. These are typically less than 100 kHz and can be corrected with a final fine adjustment. Avoid overtuning and allow the system to stabilize for a few moments before the final lock-down.

Set Insertion Loss Based on Frequency Separation

Insertion loss is not fixed. It is a function of loop plate position and must be adjusted depending on how tightly the transmit frequencies are spaced. Greater selectivity (required for closer spacing) introduces greater insertion loss. Factory index labels are marked at each cavity. An index value of 10 corresponds to 1.25 dB insertion loss. Each cavity plate has calibration marks that correspond to the recommended settings.

To adjust, loosen the hold-down screws on the loop plates, rotate to the target index, and tighten. Do not re-torque beyond spec. Lower band systems (700/800 MHz) generally require more loss to maintain channel integrity at narrow separations. After adjustment, reconnect and verify the resonant frequency has not shifted; minor touch-ups may be necessary.

Return Filters When Required

Field tuning may no longer be practical in cases of damaged cavities, seized rods, or degraded contact between rods and walls. TX RX Systems provides retuning services with our field services team. In these cases, a short circuit stub can be installed in place of the cavity to preserve impedance and allow continued operation of the remaining channels. Jumper cables should be removed and stored for later reinstallation.

Final Checks and Signal Integrity

When checking power output or VSWR, account for wattmeter inaccuracies due to connector reflection. Stable tuning requires all mechanical components to be locked and all RF paths to be electrically clean. Detuning is rarely caused by the cavity itself, as it is more often the result of environmental vibration, connector degradation, or improper rod adjustment.

Precision Tuning Backed by Field-Proven Support

TX RX systems are designed to hold alignment under pressure, but when it’s time to make changes, expand capacity, or troubleshoot performance, the team that built the hardware is ready to help. Whether it’s a question in the field or a filter that needs factory retuning, support is only a call away.