A Primer on FM Combiners

Combining FM signals onto a single feedline and antenna saves money and maintenance
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The author is deputy director of All India Radio in Vadodara, Gujarat, India. She has looked after the installation and maintenance of a number of transmitters and facilities in Ahmedabad and surrounding cities. She is the recipient of the Akashwani Annual Award for the article “Tuning of FM Combiner” and Broadcast Engineering Society (India) Award for “Audio Noise Reduction Techniques.”

The world is witnessing high growth of active FM channels, and the methods adopted to propagate in the FM band demand adequate antenna height for good coverage. So it follows that the costliest part of an FM installation is its transmitting tower.

To efficiently use tower “real estate,” a broadband antenna and single feedline can be used to serve multiple FM stations. To accomplish this signal mixing, the broadcast industry uses combiners of various types.

In addition to the obvious cost savings, regulatory requirements in some countries make it compulsory to reduce the number of broadcasting sites. In India, for example, to maintain the same effective radiated power coverage for all operating channels, it is mandatory for FM operators to run their services from the same location. This necessitates combining RF signals so a single antenna and feed line serve all the FM channels. This approach has proven effective, not only using tower “real estate” economically but spreading tower costs over many users.

Though combining technology is not new — it has been used since the 1950s — in the last few decades its uses have become widespread. These days, FM combiners are an integral and crucial part of many FM broadcast setups.

Transmitting several frequencies from a single broadband antenna system requires the use of a combiner, which is composed of frequency-selecting components such as filters, stretch lines with nodes and connecting elements like directional couplers.

When more than one signal is broadcast over a single antenna, the signals may backfeed into each other’s transmitters, where spurious intermodulation products or “spurs” may be generated within the final amplifier stages of the transmitters. These spurs will then be broadcast, causing interference to other FM stations, and so to other bands. Therefore, the signals must be combined in such a way that no chance exists for the signals to feed back into each other’s transmitter. The closer the spacing of the frequencies between transmitters, the more filtering is required to attain the isolation. The farther the spacing, the less filtering is required.

At present, two major topologies are being used. The simpler one is known as starpoint and other is constant impedance combiner, also known as balanced combiner.

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Fig. 1: A simple starpoint combiner.

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Fig. 2: A two-station combiner at All India Radio, Ahmedabad. Stations AIR FM (96.7) and IGNOU FM (105.6) are combined using a simple starpoint combiner.Starpoint combiners
Starpoint combiners consist of N number of band pass filters to combine N number of channels. Each bandpass filter is tuned to the respective channel frequency. As shown in Fig. 1, output of all the filters are connected to a common star point. Here, the isolation is limited up to the rejection of the filter, which rejects all frequencies falling outside the passband. The length of the connecting cables between filters and the star point is selected appropriately to provide very high impedance to other frequencies coming from other branches. This ensures the power only flows towards the antenna from each filter.

In this arrangement, it is very difficult to combine more than four channels because adjusting the lengths of interconnecting cables becomes more complex and impractical with the increase in channels.

These combiners are popular for combining two FM channels. They are inexpensive and don`t require huge space, but the downside is that these do not offer same level of performance as balanced impedance combiners. Fig. 2 shows a two-station combiner used by All India Radio.

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Fig. 3: A 3 dB hybrid coupler. Its outputs are reduced by 3 dB and one output lags the other by 90 degrees. It can also be used to combine signals.3 dB hybrid coupler
A 3 dB hybrid coupler is a broadband device that is an element in more complex combiners. Fig. 3 shows a conceptual overview of the hybrid coupler.

When the signal appears at Port 1, it will come out 3 dB lower at the two output ports, with the signal at one output port lagging the other output by 90 degrees. The fourth port will have isolation typically in excess of 30 dB from the input.

The hybrid coupler can also be used to combine signals; when two signals of equal amplitude and 90-degree phase shift appear at adjacent ports, they will be combined and appear at the output port. The fourth port should receive very little energy, with typical isolation in excess of 30 dB.

Balanced combiners
The balanced combiner is based on a 3 dB hybrid ring. Each leg of the ring contains identical set of either band-pass or band-reject filters. Both the filters are tuned to have closely identical curves.

In each module, one input is narrowband (NB), corresponding to the resonant frequency of the filter. The other input is wideband (WB), which may be at any frequency outside the reject-band or pass-band of the filter and corresponding to the operating range of the 3 dB hybrid coupler.

The greatest benefit of using the balanced combiner is the extra isolation provided through 3 dB hybrid couplers. It also allows easy extension of existing combiners by adding new modules. These combiners are physically larger compared to starpoint combiners.

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Fig. 4: A notch filter balanced combiner. The notch filters reject narrowband frequencies.Notch filter balanced combiner
A notch filter balanced combiner builds on the 3 dB hybrid coupler and balanced combiner by adding two notch filters tuned to reject NB frequencies. Fig. 4 shows the conceptual signal flow through the combiner. The signal fed to NB input (Port 1) is split into two by a 3 dB hybrid coupler, reflected by notch filters, and then exits from Port 2. The WB signal entering at Port 3 is also split by a hybrid coupler, passed through notch filters with minimum loss and combines at Port 2.

The isolation of WB channel from NB frequency is the sum of isolation provided by the 3 dB hybrid coupler and notch filter. However, the isolation of NB frequency from WB frequency is only as good as the coupler, so additional notch filters tuned to the wideband frequency are used at the NB input to ensure that no spurious signals are generated in transmitter connected to Port 1 (NB).

A notch filter balanced combiner has some limitations: If the two filters within any module are not identically tuned, an imbalance will occur that reduces the isolation and a spurious signal can be generated within a transmitter. And once a spur has been generated, there are no filters within the system to reject that spur, since the filters are tuned only to the designated frequencies.

In other words, notch filters are supposed to reject the NB frequency only and all other frequencies including spur from WB port will be passed.

Another disadvantage of using notch filters is that since each module in turn has to conduct the accumulated power of all the previous modules, so each module must be larger than the previous one. This requires high-power handling capacity of notch filters and consequent increase in size of these filters.

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Fig. 5: A band-pass filter.The band-pass filter, a key element
The band-pass filter is the heart of the band-pass filter combiner that I’ll discuss in a moment. The band-pass filter is composed of two or more cavity filters connected together. These filters look like metal tanks with coaxial cables attached. These form a tuned circuit consisting of a cavity with one or more loops, or probes, connected to the coaxial cable. Energy is inductively coupled by input and output coupling loops. Fig. 5 shows a simplified band-pass filter.

The cavity may be circular or rectangular. When the cavity is excited by the desired RF frequency, a tuning rod system or probe is adjusted to tune the cavity to resonance. Maximum energy is transferred at the resonant frequency. By adjusting the length of the rod, resonant frequency of the filter can be adjusted. The mechanism of a cavity resonator can be compared to a resonant inductor-capacitor (LC) circuit using small values of inductive and capacitive reactance with the internal surfaces of the cavity acting as the LC components.

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Fig. 6: Adding cavities can improve frequency response, but at a cost.Fig. 7: As more cavities are added, group delay increases. Normally each filter consists of one or more identical cavities to achieve the target parameters. Adding cavities or resonators improve frequency response, which in turn increases isolation in close spaced frequencies. However, it also increases group delay (that is, the time delay of the signals), physical size, and cost of the complete filter. Fig. 6 shows the effect on frequency response when adding cavities, and Fig. 7 shows the increase in group delay when adding cavities.

The curve of frequency response becomes squarer — flatter across the pass band, with a sharper roll-off approaching towards ideal characteristic as we keep on increasing the cavities. For most applications, three-cavity band-pass filters are the optimum choice. Two-cavity filters can be used where the frequencies of interest are far enough apart to provide sufficient isolation.

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Fig. 8: A band-pass filter balanced combiner.

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Fig. 9: Connecting the modules to make a practical combiner. The performance of a filter is measured in terms of frequency response, insertion loss, return loss (voltage standing wave ratio, VSWR) and group delay, all of which are expressed as a function of frequency. While none of the individual parameter is optimized in and of itself, the overall performance of the filter is measured and optimized.

Band-pass filter balanced combiner
In this type of combiner, the feed from the Narrowband (NB) signal input is split by the input 3 dB hybrid coupler, and both signals pass through the band-pass filters to the output 3 dB coupler, then added in equal phase at output port. Fig. 8 shows the conceptual signal flow. The wideband signal applied at Port 3 is split by the output 3 dB hybrid, reflected back completely by band-pass filters, and combined at the output port.

The narrowband input is isolated from the wideband input by the 3 dB coupler. But additional isolation is provided due to the stop band attenuation of band-pass filters. However the isolation of WB signal from the NB signal is only as good as that of the 3 dB coupler.

Fig. 9 shows how we connect each filter module to make a practical combiner. By feeding the output of each module to the broadband input of next module, we can expand combiner and increase channels. The number of channels possible in a given frequency band is limited by the minimum spacing between the signals and insertion loss, which may pose practical difficulty; insertion loss increases each time the signal passes through a module. Also for consideration is the power rating of the 3 dB couplers’ output.

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Fig. 10: The four-station combiner at Vadodara, India, serves Radio City (91.1), Big FM (92.7), Red FM (93.5) and Radio Mirchi (98.3). Fig. 10 shows a four-station combiner in Vadodara, India; note each transmitter’s output on the right side feeding its respective combiner module, and each module’s output on the left is “daisy-chained” to the successive module. The four stations’ frequencies are 91.1 MHz, 92.7 MHz, 93.5 MHz and 98.3 MHz.

In today’s world, the combiner has become the crucial part of broadcasting chain. It is important that one be aware of its technicalities and complexities. Depending upon the merits and drawbacks of a combiner, it needs to be selected for the specific application by the system designer. A properly installed and correctly tuned combiner will pass your signals to distant listeners, whereas an un-well combiner may lead to reflections, resulting in the ill health of transmitters.

In India, especially with more cities petitioning for new FM stations, the use of a combiner has become inevitable; once our auction for the third phase of FM frequency allocation takes place, the importance of the combiner will increase rapidly.

With the Telecom Regulatory Authority of India recommendations for 400 kHz channel spacing, the number of available FM channels is likely to increase. Accordingly, the required performance specifications for combiners will be more stringent.

The author is grateful to the following for references used for this article: Shively Labs, “FM Combining Systems,” Warren Stone, “FM Constant Impedance Combiners. Theory and Tuning” and Jampro RF Systems Inc., “Bandpass Combiner Principles and Theory of Operation.”