Understanding Ferrite-Filter Transmitter Combiners
By: William F. Lieske, Sr., Founder, EMR Corporation
Table of Contents
EMR Corporation has been designing and manufacturing transmitter combiner equipment since our inception in 1980. The Ferrite-Filter type of combiner has become the mainstay of operations in commercial and public safety systems. In 1983 developments began in the 800-960 MHz. ranges resulting in an unprecedented growth in the need for 5, 10 and 20 channel transmitter combiners to serve “SMR” systems.
This type of combiner is most often referred to as Filter – Ferrite since the major components are cavity filters and ferrite based isolators. Selected cables, connectors and hardware round out the component list. We will review the nature of filter-ferrite combiners as an overview, then show how the various elements of this type of combiner contribute to the overall assembly.
History Of The Design
The use of cavity resonators to tie two or more transmitters to a common antenna is more than 70 years old. Where two transmitters were spaced from each other by several MHz., it was possible to place one or more band pass cavities between each transmitter with a “tee” connector connected to the common antenna. Previously, hybrid couplers had been used to combine pairs of transmitters to a common output.
Another bulletin titled Hybrid Coupled Transmitter Combiner Systems will be found in this group of technical papers.
When high quality, low loss R. F. isolators became available the idea of using cavities and isolators together to combine multiple transmitters evolved.
Since the hybrid coupler has more than 3 dB of loss in each branch, combining four, six or eight transmitters results in losses up to 10 dB in practice. By comparison, the filter ferrite type has much lower losses, leading to it being called “low loss” type by land mobile engineers and technicians.
Figure 1 shows a basic two channel filter-ferrite combiner circuit.
A dual R. F. isolator feeds a band pass cavity in each branch and the outputs of the cavities are tied together with a “tee” connector for combining to a common antenna.
Provided that certain minimums of frequency spacing are observed, groups of five to more than ten transmitters can be combined in this manner, using a common junction with the antenna.
The characteristics of the band pass cavities used in filter-ferrite combiners will determine the practical frequency spacing. As spacings become less the circuit “Q” of the cavities must be increased to provide narrower selec-tivity curves.
The term “Q” denotes the figure of merit (eg: Quality) of the cavity. At a given radio frequency, the higher the “Q” the sharper will be the selectivity with a given amount of coupling through the cavity. Note: A comprehensive discussion on cavity resonators will be found in the bulletin “Understanding, Maintaining and Re-tuning Duplexers” elsewhere in this folder.
The filtering response and return loss of a typical 7” square or 8” round 3TEM(1) band pass cavity is shown in Figure 2. The cavity was adjusted for 1.2 dB loss. Note that at 1 MHz. above or below the 860 MHz. frequency the cavity rejects signals by about 17 dB.
(1) 3TEM is a way to write 3rd transverse electromagnetic mode. This denotes a cavity with a resonator element length of ¾ wavelength.
In Figure 3 we have superimposed a second filter system response over that in Figure 2, displaced by 1 MHz. Note that 17 dB of rejection is provided for both signals and that the return loss of each of the two cavities, when viewed from the antenna port, has 20 dB or better of return loss. The points where the response curve of one cavity crosses the pass frequency of the other cavity is important to consider. 17 dB down from a given power level is about 2% thus, with 100 watts applied to one cavity only 2 watts will be coupled into the other one.
Without further attenuation, however, this 2 watts mixed with the opposing carrier and or harmonics in the transmitter power amplifier stages would generate intermodulation products.
Elsewhere in this folder you will find a write-up called: “The Care And Feeding Of R. F. Isolators.” The theory of operation and performance character-istics of isolators is covered in that write-up. You are urged to review it at this point.
Referring again to Figure 1, assume that 17 dB of attenuation is provided between channels at the 1 MHz. spacing. Each dual stage isolator will provide 75 dB or more of additional attenuation, for a total of 92 dB or greater of channel to channel isolation. Figure 4 displays the response of a given cavity resonator at three different coupling settings.
With a loss through the cavity of 0.75 dB, about 12 dB of attenuation is provided at 1 MHz., 16 dB at 1.5 dB and 25 dB at 2.0 dB coupling factors. It is important to determine that a reason-able compromise between adjacent channel coupling versus insertion loss is found to arrive at minimum loss along with needed intermodulation control.
Good practice in designing and tuning up filter ferrite combiners suggests that 3 dB of combining loss, from the isolator input to the antenna port, is acceptable, independent of band of operation. Although 3 dB represents ½ of the input power lost in the combining process, the benefits of combining tend to make up for the effects of power loss.
Figure 5 shows the relative responses of the cavity “nose” characteristic curves at three coupling factors. The vertical resolution in this case is 1 dB per division for “ b” channel and 10 db per division for the “A” channel. Note that even with this resolution the selectivity narrows as coupling insertion loss increases.
Figure 6 shows the responses of a five channel cavity manifold adjusted for 0.5 MHz. channel to channel spacing.
The Star Junction
The layout shown in Figure 7 employs a five-way coaxial junction with a common antenna port. Since the junction is pentagonal in shape, it has come to be referred to as a “Star Junction.” Used with proper cable lengths, according to frequency, cable type and connector characteristics a precise match between all cavity outputs and the antenna port may be accomplished.
This method of combining individual filter-ferrite channels together provides the most uniform insertion loss through all channels and a reasonably broad range of frequency spread. Tune-up is very straightforward once the cable lengths have been optimized.
The majority of transmitter combiners delivered by EMR Corporation use the Star Junction design.
Figure 7 Star Junction Combiner
Line Segment Combiners
As shown in Figure 8, critical lengths of transmission line can be used to combine a number of filter – ferrite branches together. In this case a “tee” connector or a specially made rotatable coupling loop provides the means to chain the cavity outputs together.
It can be seen from the layout in Figure 8 that power accumulates along the string of line segments which are fine tuned by a shorted stub at the end opposite to the antenna. Usually the channel losses are slightly higher at the frequency nearest the stub end of the line due to the accumulated line and connector losses. Moreover, the individual channel input powers accum-ulate and attention to cable and connector ratings must be paid to prevent connector failures. Special adapting devices are sometimes built to handle larger cable and carry the accumulated R. F. currents present when all channels are active.
Figure 8 Line Segment Combiner
Star and Line Segment Comparisons
When properly designed and tuned both combiner types have similar perform-ances when the same types of isolators and cavity resonators are used. The Star Junction type will usually have a more uniform per channel insertion loss and through the use of a special connector such as a 7/16” DIN or other high power rated connector it is quite possible to safely handle up to 200 watts per channel into a five channel, 800 MHz. band SMR combiner.
Need More Channels?
Both combiner types can be expanded to add one or two channels, provided that frequency spacings are suitable. This is mostly a matter of mechanics, since most popular licensed frequency allocations are in groups of five, spaced uniformly at 0.5 or 1.0 MHz. increments. Some systems use non-sequential frequencies, as Authorized under FCC rules. The minimum spacing between adjacent channels must be considered in the combining plan.
Since, particularly in the 800 MHz. band, the most suitable cable lengths are relatively short (8 -10 inches in length) in both designs, a practical arrangement of the cavities and isolators is often very difficult to devise when adding channels to an existing system.
Some combiner manufacturers feature “field expandable” combiners. It turns out, however, that unless the user’s technical capabilities and instrument-ation is adequate, the combiner must be returned to the factory for expansion and re-optimization to insure proper long term performance.
More About Channel To Channel Spacings
Looking back at Figures 4, 5 and 6 it is seen that there is some practical minimum transmitter to transmitter frequency spread that can be used. There are several reasons for this. As channel spacings become smaller, the cavities must be de-coupled to provide sufficient selectivity to prevent coupling more than just a small amount of power to adjacent channels.
As the cavities are de-coupled for higher loss the “nose” selectivity becomes sharper as does the return loss curves. When a multi-channel combiner is properly optimized, each combined channel will “see” an acceptable impedance match at the antenna port.
Antennas and transmission lines change over time. Ice loading on antennas, moisture or even the wind swaying a side mounted antenna near a tower leg can cause changes in the antenna’s VSWR. Every antenna begins to de-grade the day that it is installed and its performance is reduced over time by the ravages of wind, rain, ice and exposure to the sun’s rays. Even the coaxial cable to the antenna will have some shift in velocity factor over time and the dielectric constant of the insulation will change measurably from below zero to over 100 degrees F.
When these changes occur the true impedance of the driven end of the transmission line can and will vary enough to reflect a change in loading on the combiner. When channel frequency spacings are very close, a change in impedance match will be manifested in reflected power in a phase relationship that results in an unsuitable match. This will “pull” the cavity resonators out of tune, moving their response peaks toward adjacent channels, or actually on the channel frequency in extreme situations.
For this reason, filter ferrite combiners that are set too close in channel to channel spacing become erratic, require constant maintenance and often cause failure of connectors, cavities and isolators resulting from changes in the antenna and transmission line.
An appendix is provided at the end of this write-up in which we suggest the minimum practical frequency spacings according to operating band, trans-mitting power and number of combined frequencies.
Practical Minimum & Maximum Frequency Spacing
We have discussed minimum spacing between combiner frequencies. Maxi-mum frequency spacing must also be considered. Antenna bandwidth must be considered along with the frequency distribution of the channels to be combined.
Figure 9 shows what happens as more and more channels are combined. As frequency spacing becomes less, loss goes up. Also, as channels are added, losses will increase.
Often in public safety VHF systems we find irregular groups of channel frequencies to be combined. The rules to follow include:
- Determine that the antenna to be used has sufficient bandwidth to provide a usable VSWR from the lowest through the highest channel frequency. Try to avoid antennas that are rated to a VSWR of 1.5:1 by their manufacturer.
- If possible, select frequencies for combining that are spaced at least 200 kHz in the 150-174 MHz. range, 300 kHz in the 450-470 MHz range, and 500 kHz in the 800-960 MHz range.
- Where a given group of frequencies are spaced such that combining all of them to a single antenna will result in unacceptable losses for one or more channels It is better to use a second antenna, choosing channels according to best spacing.
Some Combining Examples
Suppose that we have a group of 12 UHF frequencies that are to be combined in the best, most suitable manner. The frequencies are:
T1-461.025 MHz. T7-463.925 MHz.
This frequency list is typical of a group of “business Radio” channels that are to be trunked under the new “re-farming” rules. We want to keep the per-channel signal levels as uniform as possible to ensure equal channel access.
We can sort the twelve frequencies into three sub groups, each group having at least 300 KHz. of offset from adjacent frequencies. We determine that three antennas and transmission lines will be used. The combined groups are:
The logic used in this combining plan is the result of using the least numbers of channels meeting reasonable (under 3 dB) losses in each group. Using this deployment of frequencies, the losses would range from 2.4 to just under 3 dB, providing transmit levels that are with 0.6 dB in strength. Losses will be under 3.0 dB per channel.
Combiner Antenna Adjacency
In the 12 channel frequency combining example, above, where do we locate the three antennas with respect to each other? In EMR Corp’s and many other catalogs you will find nomographs that show isolation vs: separation between antennas spaced both vertically and horizontally.
We will use the vertical (also called collinear) spacing to good advantage to separate receiving antennas from transmitting antennas in combining-multicoupling arrangements. It is most desirable to have all transmitting antennas that can be accessed by a particular mobile unit on the same, or nearly the same plane to maintain a proper height-gain relationship. locate These antennas may be horizontally opposed from each other at distances that will result in 20 dB or more of coupling between them. This isolation adds to that provided by the isolator-cavity combination in each combiner path, usually adding up to more than 100 dB of isolation which is usually adequate to protect from I. M. problems.
For mufti-element and collinear “gain” type antennas, the following horizontal spacing distances are suggested:
Frequency Band Spacing (*)
66-88 MHz. 48” min.
140-175 MHz. 32” min.
450-470 MHz. 20-24”
806-864 MHz. 16-18”
827-960 MHz. 15-17”
It must be remembered that reflections from towers, guy wires and other antennas will modify the coupling factor. It is a good idea to actually measure and record each coupling to be assured that at least 20 dB of de-coupling is present. If not, steps must be taken to separate the antennas enough to secure the 20 dB or more of de-coupling.
With certain exceptions, all filter ferrite combiners are shipped from the factory in a fully tuned and optimized condition. The exceptions might be assemblies that are held as spares or stocked by distributors, in which case their technicians perform the necessary tuning services. If not subjected to shipping damage, most combiners are ready to cable up and place in service. Unless specifically instructed by the manufacturer, no field adjustments should be necessary and none should be attempted.
Maintaining Combiners and Systems
We have often had combiners of our manufacture returned to us for re-tuning to different frequencies after daily operation for 8 to 10 years and found them to be within tenths of dB’s in performance from original specifications. Unless something untoward occurs, this type of combiner does not need a “tweaking” or tune-up every so many weeks or months, provided that minimum frequency spacings are observed. This is true of all designs found on today’s market.
If there is evidence of system problems, such as isolator load terminations running warm or hot the antenna system is probably in trouble, reflecting power back to the combiner. To verify this, place a portable watt-meter between the combiner output and the antenna feed line and key up each transmitter (one at a time) and observe both forward and reflected powers. It is quite possible for an antenna to partially fail such that different reflected powers are present within the range of the combined frequencies.
If a small amount of reflected power is present and the power out of the combiner is sub-standard on a given channel, either the cavity has drifted, was inadvertently de-tuned or was damaged by lightning (Yes, this can happen!). Isolators can also fail from exceeding their safe power input rating or long operation with high reflected power that damages load terminations. Connectors and cables can also be damaged by lightning.
An on-site power monitoring system will be an asset in system management, particularly one that can remotely provide monitoring of important power levels via a modem and computer read-out.
It is often found that one or two channels out of a combined group must be re-tuned due to frequency change. Channel frequencies of filter-ferrite combiners may be changed provided that certain guidelines are followed. These include:
- Make sure that the new frequency for the channel meets the minimum spac-ings (See Appendix 1).
- If you will be changing to another frequency that is within 2% or less of the old one and the be frequency is at least 250 KHz. from any other existing channel frequency you should be able to simply apply low power to that channel input and re-tune the cavity for maximum output to the antenna. Example: Old frequency was 153.200 MHz. and new frequency to be 155.025 MHz. Difference is 1.825 MHz. or about 1.2% of the new channel frequency.
- Isolator responses are relatively broad, however, if a frequency change of up to 2% of band frequency is done, a touch-up of isolator tuning can be done.
Appendix #1: Minimum frequency Spacing Between Combined Channels
|Frequency Range||Cavity Size, Type||Minimum Spacing||Maximum Spacing(3)||Comments|
|66-88 MHz.||4” Sq. or 5” Round||0.150 MHz.||4 MHz.||100 watt input power|
|150-174 MHz.||4” Sq. or 5” Round||0.200 MHz.||6 MHz.||150 watt Input power|
|150-174 MHz.||7” Sq. or 8” Round||0.175 MHz.||6 MHz.||To 250 watts input(1)|
|406-455 MHz.||4” Sq. or 5” Round||0.450 MHz.||10 MHz.||Fed. Gov’t. & Amateur|
|406-455 MHz.||7” Sq. or 8” Round||0.275 MHz.||12 MHz.||Fed. Gov’t. & Amateur|
|450-470 MHz.||4” Sq. or 5” Round||0.400 MHz.||10 MHz.||Business & Industrial|
|450-470 MHz.||7” Sq. or 8” Round||0.300 MHz.||12 MHz.||Business & Industrial|
|851-869 MHz.||4” Sq. or 5” Rd.TEM1||1.500. MHz.||15 MHz.||Low power SMR Control Sta.|
|851-869 MHz.||7” Sq. or 8” Rd.TEM3||0.500 MHz.||18 MHz.||SMR – Trunking Systems|
|928-931 MHz.||7” Sq. or 8” Rd.TEM3||0.600 MHz.||5 MHz.||High Power Paging(2)|
|940-945 MHz.||7” Sq. or 8” Rd.TEM3||0.700 MHz.||5 MHz||SMR – Trunking Systems|
(1) Requires 250 watt isolators and cooling package for high duty cycles.
(2) Models up to 500 watts input per channel. Thermal management necessary.
(3) Subject to antenna bandwidth limitations.