So what’s the big deal..??!!

Coax is coax, right..??  The only factor that we should be concerned about is the LOSS or attenuation of the selected cable at the operating frequency. And since most repeater jumper cables are only about 5-6 feet long, we should be able to use virtually anything, including RG-58 which only has loss of about about 0.3db at 100mhz, versus LMR-400 which has attenuation of about 0.07db. Okay, so the difference is 0.23db – but that’s nothing, right..??!!

From the context of the discussion, you would be absolutely right. The loss differential between the two identical length cables wouldn’t make a hair’s bit of difference. And in commercial 2-way service of mobiles or portables, it would be completely acceptable (not preferred, just acceptable). Remember that most 2-way service is SIMPLEX (or half-duplex), with the radio either transmitting, OR receiving at any given moment. This makes it easy, since we don’t have to worry about desensitizing our own receiver when we transmit.

But repeater systems are inherently different – almost all being FULL-DUPLEX in nature (unless it’s a store-and-forward style, Simplex Repeater). In full duplex mode, the receiver is ALWAYS receiving, even when the transmitter is “ON”. Therefore, we have to be very sensitive (sorry for the pun) as to what enters the receiver’s passband. Given that receivers are not (due to cost constraints) extremely selective, there is always the fear that energy from the transmitter might enter the receiver front-end and desense it by making it deaf, or in worst case situations, cause it to create intermod products which affect both your repeater and potentially other radio systems in the immediate area. The technical term for this is:  BAD

Most amateur repeater owners (and many independent commercial operators) do not realize that the quality of their coax jumpers from the transmitter and receiver to the duplexers are CRITICAL for optimum and trouble free operation. In this case, it is NOT about loss characteristics of the coax, but about the shield-coverage of the cable. Consider for a moment, that both RG-58A and RG-8A/U have specified shield coverage of 95-96% – that allows for signals from a transmitter to permeate thru 4-5% of the outer circumference of the cable (the WHOLE length of the cable). In the case of a receiver, that allows a HUGE amount of signal from the transmitter to pass into the front-end of the receiver – again, causing desense or worse.

Take a look at the picture below – in this case, RG-58. The shield (or braid) is quite loose – you can see thru portions of it, since you can see the white dielectric material thru the braid. Great for 2-way simplex since it’s light and flexible, but horrible for repeater work.

Now look at the specs for RG-8. Most folks would say, “hey… BIGGER cable, so it MUST be better” and again, for 2-way simplex, they would be right. But look closely at the specs below..  96% shield coverage, so it’s only marginally better than RG-58 for repeater work (loss differential is negligible at a length of of 4-6 feet).

Now – look at Heliax. It does not matter which size, since all are constructed in the same manner with virtually identical materials. Helix has a SOLID, non-interrupted shield which is extruded copper (Cu) placed over the foam dielectric material, then surrounded by the PVC or Teflon jacket material. The shield has 100% coverage with no gaps – even to the point of the connectors on both sides. In fact, the connectors are engineered specifically for each size of cable and contain materials which resist corrosion between the cable and the connectors, which is known to cause intermod. I’ll bet you didn’t know that sub-standard coax or connectors could cause intermod, right..?? Take a look at the link below and you’ll see just how significant this problem could be.

andrew-braid-over-foil-imd.pdf

So this is Heliax – Solid, 100% shield coverage, a PAIN to work with unless you have the right tools, connectors and training (there is actually an Andrew Heliax Certification that installers must get in order to work on Cellular sites)  - 100% commercial standard and 100% trouble-free if installed correctly. Is it expensive compared to “normal” coax cable – YOU BET..!!   LDF4 (the most common Heliax) runs about $2+ per foot and the connectors are about $25 each. But you gets what you pays for, as they say. Remember our mantra from our original D-Star presentations – “We will install to commercial standards in order to gain reliability and ease of maintenance”. It is no coincidence that ALL major cellular companies use Heliax on their cell sites. It is also no coincidence that WE use Heliax on our systems – it clearly is the best that we can use. Below is a picture of Heliax (LDF4, in this case) – click on the picture to see an expanded view:

Are there other cables which provide 100% shield coverage – absolutely – and we sometimes use them as well. RG-142 comes to mind since it is super flexible, but it has higher loss characteristics and should only be used for short lengths (TxRx Systems uses RG-142 for their duplexer cables between cavities). BTW, RG-142 has a Teflon jacket and is the ONLY cable authorized by the FAA to be used in avionics system on airborne systems.

Well there you have it..
I’ll bet you know more now about Repeater Jumpers that you ever thought you needed to know. But this series of blogs is about EDUCATING you about what we’re doing, not hiding it from any of you. We are about building a cadre of Amateurs who are willing, ready and TRAINED to help support these systems.

Let us know what you think of these articles thru your comments on any of the posts.