Back	Introductory Information about Transmitter Combiners… Compiled, written and HTML'd by Mike Morris WA6ILQ

Comments, critiques and suggestions for this page (or any page at this web site) are be appreciated.

There is some confusion over the naming and functionality of Diplexers and Combiners (even Wikipedia gets it wrong occasionally). A diplexer (or triplexer or quadplexer) is a multiband device and is covered in a different section on the Antenna Systems page.
The term "combiner" is also used in the consumer world differently than in the radio world… your author has seen it used in the consumer world to describe a situations where two roof-mounted television antennas (pointed in different directions) are "combined" to one feedline that runs into the house and feeds to one (or more) television receiver(s).

This article is concerned with transmitter combiners that allow two (or more) different frequency Land Mobile Radio ("LMR") transmitters into a single transmitting antenna. The LMR combiner is a single band (also referred to as "in-band") device and are usually used in LMR with transmitters in the 5 to 250 Watt area. Each combiner consists of a system of cavities and isolators that allows multiple transmitters to share one transmitting antenna while preventing the output of one transmitter from feeding into the output of another. A combiner provides isolation between the different transmitters while having a minimal insertion loss to the antenna.

The overall configuration of a two-port (two transmitters, one antenna) cavity-based combiner resembles a "T" with the transmitters connected to the two sides and the antenna connected to the center point. Combiners with more than two ports resemble a star where the transmitters feed the legs and the center point feeds the antenna.

Picture a conventional transmitting system: a transmitter feeds an isolator, then a pass cavity, then a feedline then an antenna.
Then circumstances change and now require two (or more) transmitters to share a single transmit antenna. The configuration now consists of multiple transmitters, each with an isolator and a pass cavity. The outputs of the pass cavities are coupled to a common point by precise electrical length cables. This means knowing the V_f of the cabling AND the wavelength (which translates to a cable length). Any error in the ruler length of that cable (V_f or measurements) will result in a mismatch, and that is likely to create poor Return Loss and higher Insertion Loss. You will want to use high quality cable for those combiner cables (please, no RG-8, RG-213 or LMR cable!). Mil-spec RG-214 or similar is very common in combiner cable harnesses. The combiner output connects to the feedline and then to the antenna. Note that the antenna has to be resonant to the entire range of transmtter frequencies.

The difference is the insertion loss of the combiner assembly. On a properly designed and implemented combiner it can be as low as 1.2 dB or as high as 3 dB. But at a crowded site that may be your only choice - having a repeater on the air through a combiner or not having a repeater at all.

Note that most repeater systems that use combiners usually have a separate master receiving antenna that is multicoupled to each receiver. The receive antenna is usually positioned on the top of the tower to maximize the receive range and, by the way, maximise the receiver to transmitter isolation. This receive antenna is connected to its own feedline, and then to a preamplifier / distribution amplifier panel and then to the receivers. But there has to be some filtering involved… On UHF it is common to see a "window filter" that passes a range of frequencies… usually 4 to 5 MHz wide. Some sites have multiple window filters. On VHF things are a little different.

The page author has equipment at seveveral sites and each one has a single UHF receive antenna at the top of the tower, connected to inch-and-five-eights Heliax™, which connects to a high level AngleLinear preamplifier inside the building, then that feeds a splitter and each port feeds an eight cavity window filter. One window is at amateur UHF, another at 455-460 MHz commercial, another at 465-470 MHz commercial, a third at 473-476 MHz commercial, and one more at 508-511 Mhz commercial. The output of each window filter feeds an AngleLinear distribution amplifier and that feeds a number of system receivers. This situation is covered in more detail on the System Engineering page.

Don't forget that the antenna that the combiner is feeding has a finite power limit. By the time you combine (for example) five transmitters, each with 100 watts out, through a combiner that may have as much as 3 dB loss, that's still 250 watts that's going up the feedline to the antenna, and (poof) goes the under-rated antenna. And remember that after the antenna went (poof) the SWR just skyrocketed and the combiner needs dummy loads that can handle the full transmitter power in continuous duty, and appropriate ventilation to get rid of the heat. There are combiners with more ports, and I have seen multiple installations that required an antenna rated at 500 watts. Not many manufacturers make those, and the ones that are available are usually EXPENSIVE and sometimes are a special order and can have extended delivery times (i.e. are made to order).

So why do people use combiners? Because a combiner system just may be the only way to accomplish what is needed. It may be that there is a limited amount of tower space, or maybe the tower is close to maximum on wind / ice load. If a combiner is the only way to do the job then you engineer the system so that the overall system RF power budget can handle the insertion loss and have the system still be usable. The most common way is to use a community receive antenna that feeds the receivers on multiple systems then anywhere from two to twelve transmitters combine into one transmit antenna.

At one site that I visit semi-regularly the site owner has community receive antennas at the top of the tower… one for 222, one for UHF (406-512 MHz) and one for 800 MHz. The 222 antenna fed 3 systems. The UHF receive antenna on last visit was feeding over 50 systems. The site owner uses multiple UHF transmit antennas rated at 500 watts, and multiple five-port combiners each feeding a separate transmit antenna. The combiner ports are rated at 150 watts each, he conservatively limits transmitter power to 125 watts. Using this design five transmit antennas would support up to 25 transmitters. Eight transmit antennas would support up to forty transmitters.

There is some serious system engineering involved with combiners. Some of the concerns are obvious, some are not. There is a minimum frequency separation amount between transmitters on a single combiner. The amount depends on the design of the combiner, but a useful starting point is 75-100 KHz at VHF and 250-350 KHz at UHF. There is a maximum as well. You need to check the manufacturers data sheets. Plus you have to consider the bandwith of the transmitting antenna.

You don't want the difference between any two two transmit frequencies to be equal to a repeater offset - and repeater offsets are all over the place (especially on commercial VHF).
Yes, two UHF transmitters 600 KHZ apart can nail any 2M repeater at a site… And there are several cities in the USA that have AM stations right on 600 KHz… The same thing will happen.
And I know of another site that is poisoned to 220 MHz repeaters… there is a 1600 KHz AM station about a quarter mile away. The same thing would happen if you had two repeaters 1.6 MHz apart.
And yes, two VHF commercial transmitters 5 MHz apart can nail every UHF repeater at a site. And some areas have commerical repeaters with a 3 MHz offset.

And since the commercial tenants at a site pay the bills the amateur systems (if any) may just have to live with the situation.

A "live with it" story…
Twenty+ years ago the author was told a story about a situation in Texas sometime in the 1960s to the 1980s where two FM broadcast transmitters on the same tower were exactly 5 MHz apart… and this poisoned the tower for anything on UHF commercial LMR. As the story went Motorola went so far as to offer to fully fund a frequency change of either one of the FM stations. Both said, essentially, forget it.
Yes, there are UHF systems (both commercial and amateur) on that tower but they are NOT on 5 Mhz offsets.
(if anyone has any details on that situation please let the page maintainer know).

Another "live with it" story…
Your author has commercial and amateur equipment at a major commercial site where many years ago two commercial paging transmitters (in two different buildings) mixed, and the mix product landed on 147.81 MHz. As a result for over 30 years that particular 2 meter repeater was "upside down"… The owner used the 147.21 MHz (normally the output) frequency as the input.
The rest of the story… It was complicated:
a) The two paging transmitters were tenants in buildings owned by two different leasing companies.
b) The paging transmitters were owned by two different paging companies.
c) The ham system was getting discounted rent in a third building (ham owned) and didn't want to attract attention by making waves. Through other connections the system owner was able to talk to the two paging transmitter techs… They did what they could but the mix was just not solvable.

The final solution? Wait it out. At that time it was obvious that cellphones were killing the paging industry. Finally one of the paging transmitters was shut down and the mix product went away. The owner left things alone because there were still a lot of crystal controlled radios in use. A number of years later the system was later sold, and the new owner flipped it "right side up".

When you get right down to it, a 1.2 to 3 dB loss in the combiner may not be that bad. In many cases the insertion loss of an isolator / circulator, the duplexer and the feedline on a conventional configuration repeater can be equal or greater than that (especially if the feedline is LMR-series cable (or even 1/2 inch Heliax instead of 7/8 inch).

As an example, let's look at one UHF system I am familiar with… it is at a different mountaintop commercial site (ham-owned) with separate master receive antennas for 220 MHz, 440 MHz, 902 MHz and 1200 MHz), each feeding multiple systems.
However we are discussing a UHF system:
1) The UHF receive antenna is a 10 dB UHF Super Stationmaster at the tower top, which is at 120 feet (36.5m) above the ground.
2) The receive feedline is 150-160 feet (45-60m) of inch-and-five-eights Heliax.
3) Due to the high RF levels at the site (over a dozen buildings) there is a AngleLinear High Level Preamp between the Heliax and the ham band window filter.
4) The window filter is 8 cavities in series which make a several MHz wide "window".
5) The output of the window filter feeds an AngleLinear distribution amplifier that feeds over a dozen UHF amateur systems in the building.

The actual UHF repeater system:
1) The system receiver is a MICOR that was rebuilt from 460 MHz to match the factory 440-450 MHz design (this information is on the MICOR page at this web site), and that resulting receiver was peaked for the single frequency of the repeater.
2) The 250 watt transmitter is also a MICOR. There is a backup 100 watt PA deck that can be switched in by a logic output on the repeater controller. That function switches a pair of Dow-Key RF relays plus DC relays that switches the DC power to whichever amplifier is active.
3) The RF relay on the amplifier output(s) feeds a combiner port. The insertion loss of the combiner is less than 2 dB (the combiner port includes an isolator / circulator and a pass cavity).

Another run of inch-and-five-eights Heliax feeds the actual transmit antenna (which is fed by four or five transmitters). That antenna is an 8 or 9 dB fiberglass stick at about 80 feet (25m).

The insertion loss of the combiner is very close to the loss found in the previously used configuration (an isolator plus pass cavity plus duplexer plus about 100 feet of feedline).

Combiners are expensive not only in RF energy but also in money (unless you have a source of used / recycled cavities and isolators). A complete two port turnkey UHF combiner at retail (these prices are from a 2009 quote) will set you back about US$4,600, a four port about US$8,400, an eight port about US$16,000, and that's plus shipping.
See these manufacturer web sites (and don't forget to ask - some manufacturers have amateur radio discounts, some do not):

• Bird Corp's "Combiner and Receiver Multicoupler Design" document   55 KB PDF (local copy)
• db Spectra transmitter combiner products (off page link to the db Spectra web site)
• Sinclair transmitter combiner products (off page link to the Sinclair web site)
• Sinclair trunking combiner products   254 KB PDF (local copy)
  Trunked radio systems frequently have anywhere from two to thirty transmitters close-spaced at one site. This writeup is worth reading for the background information. One of the sites that the author visits semi-regularly has twelve trunked UHF channels (100 watts on each channel) inside a 1.4 MHz chunk of spectrum. Six 2-port combiners allow twelve channels on six transmit antennas. Another site has six trunked UHF channels (90 watts on each channel) in 350 KHz of spectrum and on three 2-port combiners.

Lastly, there are limits to what frequencies can be combined - to some people that bit of engineering is a black art. Combiners have a minimum frequency spacing between ports, some as low as 200 KHz at UHF. The antenna fed by the combiner also has a maximum frequency bandwidth (the point where the SWR becomes intolerable). Plus, you have to really look at the potential for intermodulation and mixes - and don't forget, you have multiple receivers to worry about! At the sites mentioned above, if they were to add one more transmitter to any of the combiners the site manager has to worry about over 50+ receivers at the site (and there are multiple tenants in his building) and just on UHF there are receivers from 406 MHz to 511 MHz. Remember the articles on receiver isolation at this web site? Go re-read them, but now realize that this site manager has to worry about 50+ UHF receivers! Plus the VHF systems! Plus the low band systems (Red Cross on 47 MHz and another building down the road has California Highway Patrol all across 39 to 44 MHz).
And there are a dozen buildings on that mountaintop, and one building is operated by a TLA (three-letter-agency) that is very closed mouthed… And every building owner and site manager on that mountaintop has every other owner / manager in his phone book…

The most unusual transmitter combiner I am aware of is one on Mount Wilson, north of Los Angeles. That site is home to 90% of the FM and TV broadcast transmitters and has a "footprint" that covers 15 million people (about 1/3 of the population of the entire state of California). The combiner was custom designed for the site and combines seven TV transmitters (channels 32, 44, 48, 49, 50, 51 and 56) into one custom designed antenna array that has to handle from 580 to 756 MHz. And that's not the only TV transmitter combiner on Mt. Wilson.

And there are a number of combiner articles on the page here.

Contact Information:

The author, Mike Morris WA6ILQ, can be contacted here.

Text, layout and hand coded HTML © Copyright 1995 and date of last update by Mike Morris WA6ILQ
This web page was split from the main Antenna System page on 12-Nov-2011.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.