A lot of the newer transceivers use modular jacks and plugs for microphone and/or remote cables. While they appear to be RJ45 telephone type jacks and plugs, they aren't! In fact, modular jacks and plugs come in about 50 different configurations. Some are designed for round cables, some for oval cable, some for flat cables, and there are different types for solid and stranded wire. There are keyed and unkeyed ones; ones with external cord grips; and ones for heavy-duty use. Selecting the correct one isn't as easy as it appears.
Adding a little insult, MFJ, and a couple of other manufacturers, use RJ45 telephone-style jacks, plugs, and even silver satin, 8 conductor telephone wire as interconnect cabling. The problem is, they aren't wired like telephone cables, and arbitrarily substituting a telephone cable will garner you an expensive repair!
The tools needed to crimp the various types are also slightly different. One designed for round will not crimp a flat cable correctly, and one for flat cable can actually cut through a round cable. There are some 8 different die sets to accommodate them all.
There are other considerations too. Referring to the Icom microphone jack wiring diagram (roughly 4 times actual size), you can see that the pins are very close together (approximately 1/32"). If you're fat fingered, you may not be able to properly position and hold the wires in the corresponding plug during crimping. The first time I tried it, I went through four plugs before I got it right. And note there are separate ground connections for the microphone and the PTT.
Pin 3 is the audio output on an Icom IC-706MkIIg. On the IC-7000, this pin is used as a microphone "sense" pin which tells the radio an HM-151 (stock mic) is attached. If you plug a Heil headset designed for the IC-706, the radio thinks it's an HM-151, and the transmitted audio sounds distorted. I suspect Heil will have a new model designed especially for the IC-7000 in the near future.
Pin 6, the microphone input, has an 8V DC regulated supply fed to it (this is true of Yaesu and Kenwood as well). Although there is an internal resistor in series with this lead, shorting it to ground can cause both the series resistor and the 8V regulator to over heat causing one or both to eventually fail. Further, shorting pin 1 to ground will cause the 8V regulator to instantly fail! Either way, doing so is an expensive fix as they are both surface-mounted devices, and require special tools to replace.
Pin 2 is a multi purpose input. Shorted to ground during receive, it causes the frequency to go up. Placing a 470 ohm resistor in series to ground causes the frequency to go down. In CW mode, a 3.9k ohm resistor to ground is the dit side of the paddle (or straight key connection), and a 2.2k ohm resistor is the dah side of the paddle (information about the latter can be found on page 34 of the owners manual). There is also an audio out and a squelch switch. All of these possible uses for the jack makes it an inviting target for the uninformed. Incidentally, the maximum current for the 8V line and the sink for the squelch switch is 10 mils! The former is not intended to power ancillary equipment like TNCs.
I encourage everyone to use commercially available interconnect cables when ever possible. For example, Icom makes an adapter to convert the microphone jack from modular to an eight pin standard microphone jack for use with their SM8 and other desk microphones.
Icom is not alone in its use of modular jacks and plugs. Yaesu and Kenwood also use them especially on their mobile transceivers, and similar caveats are in order.
DC cabling doesn't need to be shielded, because its impedance is very low. However, most other amateur radio cabling is shielded, and for very obvious reasons. Unfortunately, some enterprising amateurs think it's okay to use Ethernet CAT5 cable for their interconnections. It isn't. Yes, you can buy shielded CAT5 cable, but the vast majority of it is solid conductor, and should never be used in a mobile environment.
There is another little problem with CAT5, which very few think about, and that is its construction. It isn't 8 separate strands, it's actually 4 pair, with each pair separately twisted together. Depending on the use, and how it is wired (ready-made CAT5 cable assemblies come in several different wiring configurations), crosstalk between the conductors is almost a given.
Just in case you missed the point, using a ready-made CAT5 cable in your mobile just might work. Then again, it might not, and you'll be in for an expensive fix!
In some cases (Icom remote cable for example), the connectors are proprietary, and cannot be purchased, period! Attempting to extend one of these cables by splicing in another piece of wire is a good way to destroy a working cable.
The similar caution should be used with MFJ's interconnect cables, like those supplied with their ALS-500 amplifier remote kit. They are indeed 8 conductor, silver satin telephone cable, but the connectors are installed differently; they're straight, not crossed over. This is one of the very few cases where you can shorten them, and install a new RJ45 plug with a standard telephone crimp tool. That is, as long as your observe the polarity!
I don't suggest making up your own extension cables in the first place as most of them are of proprietary material and configuration. Using pre-made telephone cables and splices isn't the answer either. Remember, most telephone cables flop over. That is to say, pin 1 on one end is pin 8 on the other. Ethernet cables come is straight and crossover configurations, and although their splices look like telephone splices, they're not wired the same. This is one area of amateur radio where you shouldn't scrimp, as the potential damage costs far exceeds the procurement costs of most cables. If you just can't help yourself, and you just have to try, here is a suggestion or two.
Digi-Key sells the proper modular plugs, tools, and dies. They are listed under Modular Connectors and Jacks in their catalog and on-line. They list about 20 different configurations, and there are about 25 more they don't list. They have the proper crimping tools and dies as well. At about $200 each with dies, the tools are not cheap. I caution everyone not to use the cheap Radio Shack crimper. It is designed for flat, stranded telephone cable, yet they'll tell you it is okay for Ethernet cable. It isn't! And it will not properly crimp any of the connectors when round wire is used. If you squeeze hard enough (as suggested in the instruction sheet) you run the risk of separating the insulation which can cause a short (to say nothing about breaking the plastic handles). It just isn't worth taking the chance.
Remember to check your connections for continuity and shorts. A pre-wired jack is a good tool for checking continuity. Also, make sure the wire is properly restrained by the strain relief.
There you have it, and I wish anyone making their own modular style cables a well-earned good luck. You'll need it!