Wire Matters

There are many things to learn when taking on a project like this.  One of the more surprising for me was that very few types of coax cable are suitable for use on a repeater.  I thought I understood coax pretty well, but I really didn’t.  There is a good chance that some of what YOU think you know isn’t quite right either (check the YouTube video below, I think the gentleman isn’t quite clear on certain coax concepts either!)

Click the image for an unintentionally funny YouTube video about different kinds of coax.

When it comes to 50 ohm cable for two-way radio use, just about all hams are familiar with RG-58 and RG-8.  I got involved in CB radio during the craze of the 1970s, and that’s all there was for coax.  We used RG-58 for mobile installations, and if possible, RG-8 for base stations.  We could afford to generally ignore the quality and construction specifics of our cable because at HF frequencies, coax cable is very forigiving.  Even at 6m, 2m, and on up to 70cm, in many situations you can get generally reasonable results with modest RG-58 and RG-8 coax.

However, certain situations require you to raise your game, and use coax that is designed with special properties.  Repeater service is one such situation.  First of all, a repeater’s antenna is often very far up a tower.  It is very important to preserve as much received signal as possible, and at the same time transfer as much transmitter power as possible to the antenna.  The lower the feedline loss, the better.  Bigger coax generally has lower loss, so any decent repeater installation typically uses 1/2″ or larger Heliax for the main run, and short, more flexible jumpers to complete the run at the ends.

But there is more than loss to consider.  In repeater service, shield coverage is also important.  Lower quality RG-58 and RG-8 has shield braid coverage with lots of holes.  Signal can find its way in and/or out through those little gaps in the braid, and totally ruin the receiver/transmitter isolation that you worked so hard to create with the duplexer.  Heliax has 100% shield coverage (because it is essentially a solid copper pipe), but it is very stiff and isn’t suitable for locations where you need to move the cable now and then, or bend it into tight turns.  So then, what we want in our ideal cable is low loss, 100% shield coverage, and flexibility.  No matter how good the RG-58, it will not have low loss or 100% shield coverage.  RG-8 can have reasonably low loss, but 100% shielding is not possible and flexibility is so-so.  (Believe it or not, plenty of budget-minded hams have installed, and are using repeaters cabled entirely with RG-58!)

Illustration of the basic design of LMR-xxx cable.

One of the more recent arrivals on the scene is Times Microwave LMR-400 cable.  It’s basically the same size as RG-8 and can use most of the same connectors.  The construction uses a foil wrap, covered by a braided shield for 100% coverage.  Loss figures are very low, and there is a “superflex” version with a stranded center that is fairly flexible.  It isn’t super-cheap, but is still within reach for most hams at around $1/ft.  What’s not to like about that?  It might surprise you to learn that experts in the two-way radio field warn STRONGLY against using ANY LMR-xxx cable for repeater service!

Wait, what?  If it has very low loss, 100% shield coverage, is flexible and generally affordable, why can’t we use it on repeaters?  The answer lies in that aluminum foil shield, covered by a tinned copper braid.  Dissimilar metals in contact with each other are bad news.  In the presence of high RF power levels (such as you get when the repeater is transmitting while simultaneously receiving), all those millions of contact points between the braid and foil become millions of little tiny diodes, signal is rectified, and noise is heard on receive.  It might not happen when the cable is new, but eventually it WILL happen – especially if you have it outside, blowing in the breeze.  (An excellent article on repeater duplex noise is available here.  I highly recommend you take time to read it.)

With all the cable types out there, how do you choose the best ones for a repeater?  At the risk of oversimplifying, I think you can break it down this way:

1. 

   Andrews Heliax, 1/4″, shown next to a BNC connector for perspective.

   Use Heliax or other hardline wherever possible.  When that isn’t possible due to flexibility concerns, or when you need a short jumper at the end, then:

2. Use silver-plated, double-shielded coax such as RG-400 (small, flexible, but higher loss) or RG-214 (large, stiff, lower loss).  There may be other types that will work, but stick with these recommendations and you will have the greatest chances of repeater success.

By the way, a few important corollaries:

1. Avoid adapters; use the correct connectors on your cable instead.
2. If adapters can’t be avoided, shop around for silver-plated, Teflon versions from reputable dealers (NOT eBay, probably not at a hamfest).
3. Use genuine, high-quality MIL-spec coax, not cheaper imported versions that are often advertised as RG-214 “type”, for instance.
4. It bears repeating: For duplex (repeater) service, do NOT use LMR-xxx (where xxx is any number).

(For more detailed information on the points above, visit this page at the Repeater Builder web site).

In this sneak peek at the KD2SL repeater duplexer, note the light brown RG-400 jumpers

Sure, these types of cable cost more.  However, this is one area of repeater installation where you can’t afford to compromise performance.  Fortunately for this repeater, I was able to find 1/4″ Heliax jumpers and RG-400 jumpers in the “junk pile” that could easily be shortened to the lengths needed.  With high-quality coax costing nearly $3.00/ft, and proper N connectors costing $10-$20 each, I saved a lot of money.  Also, a local ham donated 50 ft. of 1/2″ Heliax that finished most of the inside run between the duplexer and the power divider.  The connectors on that Heliax were in poor shape, so I found and used two nearly-new N connectors from a short junk pile jumper.

Before I knew better, I bought LMR-400 and prepped the ends with nice N connectors. It serves as a reminder to do your research before you buy!

I’ll conclude with a confession: I bought LMR-400 for use on the repeater, before my research uncovered the reasons NOT to use it.  It still sits on the shelf of my shack, looking pretty with its shiny silver N connectors!  I’m annoyed at the money and time wasted, but I learned a lesson, and I’m thankful that it won’t be used on this repeater!

In our next post, I’ll get back to details about the duplexer, and how it was made from a diplexer and sideband filter from an analog TV transmitter.