Feedlines for Repeater Systems
Recently, I had a discussion with someone regarding what feed-line type to use when it comes to repeater use cases. A lot of folks that start building a repeater for Amateur Radio use are trying to be sensitive to the budget, which I can appreciate. But as one wise person told me many years ago...it only hurts once to buy the correct tool/part.
Many folks look at LMR cable as a great option because the loss is very low for VHF/UHF use cases and its affordable. The issue with duplex operation is that the LMR type cable has an aluminum foil shield with a copper braid. Any dissimilar metals is usually a recipe for disaster. In the case with aluminum, aluminum oxide will form which creates tiny diodes over the length of the cable. You will get a reaction with the copper and aluminum - and the result is all the same, you will get noise induced when RF power is applied.
Now I remember when I started building repeaters, I thought that this was all folklore and a marketing scheme to get people to buy more expensive feed-line like Heliax. In fact, many commercial radio installation companies use LMR cable for small repeaters at business locations and such. So in years past, when interacting with Elmers that have been doing hilltop repeater work for years, they were saying No to LMR or any other type of cable with dissimilar shields. Well I thought they were just drinking the marketing Kool-Aid. Until I bought a Service Monitor to conduct tests...then I became aware of what noise they've been talking about the whole time.
When doing a 12 dB SINAD test on a repeater/duplexer, you can easily measure the noise induced from an LMR type cable just by small movements of the cable. As a cable ages, this noise increases. This noise causes desense in the receiver and creates wide-band noise that would impact other repeaters at a commercial facility. That is why most commercial repeater sites will NOT allow any installation with this type of cable (I say 'type' because there are other coax types that have similar construction, such as the Belden 9913). Also, any Nickel or chrome plated connectors would also be out of the question, as these can create intermod anywhere along the RF path since they are ferrous metals.
To demonstrate how much noise is induced by even a short LMR240 cable on the antenna output of a duplexer, I put together a short video comparing LMR240 and RG400 into a dummy load. With a 12dB SINAD test on the receiver, you can see that we go from a 'copy-able' signal, to just RF grudge...with small movements of a relatively new cable that is stored indoors. So imagine the noise that is induced on a longer feed-line that goes up a mast or tower and moves (even slightly) in the wind. You're going to get noise and dense...there is no way around it.
Why say no to LMR type cable for repeaters (video)
If a person does not have a Service Monitor, or a way to test receiver noise, then its pretty hard to quantify this noise. But several years ago, when my radio hobby became more serious (I was building and installing repeaters and building RF PAs) a Service Monitor was a must. Once I put a repeater on the bench to test, then my eyes were fully opened to how the Old-timers doing this for so long were preaching of this very fact of not using LMR cable for this application.
Now for simplex operation LMR cable works great. It has a lot of great characteristics for this use case: its low cost, low loss, can handle relatively high power and easily available for purchase. But for Duplex operation, its imperative to use the right cable, such as Heliax LD4-50a, or whatever corrugated version is appropriate for your power and length.
For more in-depth details on Repeater construction, the repeater-builder.com site has a wealth of information.
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