Recommendation on STL Combiner

Gotta watch those Fresnel zones!
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Recommendation on STL Combiner

Post by bmcglynn » Fri Jun 06, 2014 7:14 am


I'm looking to combine 2 STL transmitters into a common antenna. This is not for a discrete L/R system, but instead for a split site transmission system where the links are 5 MHz apart. I looked at the Marti HRC-10A, although it's specified to work only on 250 kHz spacing differences.

Can anyone recommend a good supplier of STL combiners?

Thanks in advance.

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Re: Recommendation on STL Combiner

Post by awsherrill » Fri Jun 06, 2014 9:15 am

Have a look at Microwave Filter Company:

Insertion loss on these things is pretty steep, so watch your link budget.

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Re: Recommendation on STL Combiner

Post by rftranstv » Fri Jun 06, 2014 11:23 am

thats alot of insertion loss!

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Re: Recommendation on STL Combiner

Post by jammerdave » Sat Jun 07, 2014 10:05 am

the 250 KHz spacing mentioned is most likely a *minimum* spacing. I never liked the Marti combiners, but EMR corp makes stuff that would fit the bill. I've had great luck with their cavities and duplexers. ... _Combiners

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Re: Recommendation on STL Combiner

Post by BigRed » Mon Jul 14, 2014 2:05 pm

If you're still looking for a way to combine the two STL transmitters another "cheap, but not so dirty" option would be to use a 90-degree hybrid, providing that the link budget can take the roughly 3-dB loss to each signal. (I think one of the "fancy" combiners you were looking at had that much insertion loss too . . .) A number of sources (Mini-Circuits, Pasternack, eBay) should have hybrids available in that frequency and power range; and it's got to be a hybrid, a 0-degree device won't cut it for this application. You'll also need a 30-watt load (10-watts would do it, but I like some headroom) for one of the two "output" ports on the hybrid. Just connect the STL transmitters to each of the "input" ports on the hybrid and the antenna to one of the "output" ports, the reject load to the other. The power from each transmitter will be split equally between the two output ports, hence the need for the reject load, and they will be isolated from each other by the isolation value of the hybrid, typically 25 to 30-dB.

That should be all you need unless the transmitters are sensitive to VSWR damage (in case the antenna or the load were to suddenly go away). And if that's the case you can supplement the isolation by using an appropriate circulator (Fairfield Electronics ?) and a 20-watt load (10-watts would do it, but again, I like some headroom). It will add a bit more loss (about 0.5-dB) and triple the cost, but it will add about another 18-dB of isolation and will also insure that each transmitter always has a good load to work into.

Just a thought . . .

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