Back Combiners...
Compiled by Mike Morris WA6ILQ

There is some confusion over the functionality of Diplexers and Combiners (even wikipedia gets it wrong). A diplexer is multiband and is covered in a different section on the Antenna Systems page. A combiner is single band (or in-band) and consists of a system of cavities and isolators (or cavities and circulators paired with external loads) that allows multiple transmitters to share one transmitting antenna.

The overall configuration of a cavity combiner resembles a star. The transmitters feed the legs and the center point feeds the antenna.

Combiner systems can cost you several dB of your transmitter power. i.e. that 100 watt repeater transmitter that gave you N miles radius coverage on a stand-alone antenna may only have the performace of a 50 watt or 25 watt transmitter once the combiner is in line.

Don't forget that the antenna 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 3 dB loss combiner, 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 combiner needs dummy loads that can handle the full transmitter power in continuous duty, and appropriate ventilation to get rid of the heat.

Note that most repeater systems that use combiners usually have a separate receiving antenna that is multicoupled to each receiver. The receive antenna is usually positioned on the tower to maximize the receiver to transmitter isolation of all of the systems.

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 take the 3 dB loss and have the system still be usable. The most common way is to use a community receive antenna that feeds multiple systems then combine anywhere from two to twelve transmitters into one antenna.

At one site that I visit semi-regularly the site owner has a community receive antenna that initially fed about 20 systems, and at my last visit I saw it was then feeding over 50. He uses transmit antennas rated at 500 watts, and five-port combiners. The combiner ports are rated at 150 watts each, he conservatively limits transmitter power to 125w. Using this design five transmit antennas would support up to 25 transmitters. Eight transmit antennas would support up to 40.

When you get right down to it, a 3 dB combiner loss may not be that bad. In many cases the duplexer insertion loss, the isolator / circulator loss, and the feedline loss may be close to 3 dB by itself.

As an example, lets look at one system I am familiar with...
1) The receive antenna is a 10 dB Super Stationmaster at the tower top, which is 120 feet (36.5m). There is a preamp at the base of the antenna.
2) The receive feedline is 150 feet of 1/2 inch. The tower-top preamp has enough gain that the loss of the 1/2 inch isn't a problem.
3) Due to the high RF levels at the TV site there is a window filter in front of the antenna multicoupler unit. The insertion loss of the 1/2 inch feedline and of the window filter was included in the original signal budget calculations. The multicoupler feeds all but one UHF receiver in the building.
4) The system receiver is a MICOR that was rebuilt from 460 MHz to match the factory 440-450 MHz design.
5) The 250w transmitter is also a MICOR. There is a backup 100w PA deck that can be switched in by a logic output on the repeater controller that switches a pair of RF relays.
6) There is a bandpass cavity after the output RF relay.
7) The output of the pass cavity feeds a combiner port. The insertion loss of the combiner is about 2.8 dB (the combiner port includes a circulator and filtering cavity).
8) The transmit antenna is an 8 dB model at 80 feet (25m), fed by inch-and-five-eights Heliax.

The insertion loss of the combiner alone is pretty close to the loss found in an average duplexer plus isolator.

Combiners are expensive not only in RF energy but also in money. A complete two port turnkey UHF combiner 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 (prices are from a 2009 quote).
See these manufacturer pages (ask - some manufacturers have amateur radio discounts, some do not):
1) http://www.dbspectra.com/categories/Browse-By-Product/Combiners/Cavity-Combiners
2) http://www.sinclairtechnologies.com/catalog/group.aspx?id=14.

Lastly, there are limits to what frequencies can be combined - to some that bit of engineering is a black art. 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 site mentioned above, if the site owner were to add one more transmitter to any of the combiners he'd have to worry about over 50 receivers. Remember the articles on receiver isolation at this web site? Go re-read them, but now realize that this site owner has to worry about 50 receivers, all on different frequencies!
Some of the concerns are obvious - example, you wouldn't want to have any combination of two transmitters whose difference in frequency was equal to the offset of any repeater at the same site (and I know of a couple of sites that are poisoned by two FM broadcast transmitters 5 MHz apart...).

The most unusual combiner I am aware of is one on Mount Wilson, north of Los Angeles. That site is home to 90% of the broadcast transmitters for a county of 10 million people (1/3 of the population of the entire state of California). It combines seven TV transmitters (32, 44, 48, 49, 50, 51 and 56) into one antenna array that has to handle from 580 to 756 MHz. The magazine article is here (unfortunatley less the photos) http://www.tvtechnology.com/prntarticle.aspx?articleid=264719. And that's not the only TV transmitter combiner on Mt. Wilson.

Useful files and off-site links:

  1. http://www.jampro.com/uploads/tech_docs_pdf/combinertheory.pdf
  2. http://radiomagonline.com/transmission/radio_rf_combiners (this is oriented towards broadcast radio)
  3. Bird's "Combiner and Receiver Multicoupler Design" document
The first and last ones are really worth reading.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.



This web page split from the main page 12-Nov-2011.


Text, layout and hand coded HTML © Copyright 1995 and date of last update by Mike Morris WA6ILQ

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.