Impedance of MPX cable and adapters

Nice answer there JesseG 😉
In laymans terms, when running a simple audio cable from a soundcard to a transmitter, the impedence is of little or no consequence, Just make up the correct lead and plug it in.
Your TX will most likely have a 75ohm BNC connector on it, so one of those (if you cant get a 75, a 50 will do) and your Trace Marian Alpha card will have a 6.25mm jack plug. use some reasonable grade shielded cable inbetween any you are ready to go.

Yorkie.

Impedance matching is only relevant when transferring power from the driving source to a load. In the case of a broadcast transmitter output amplifier (at the connector) it is designed to have a 50 ohm source impedance. The transmitter load is an antenna with a termination value of 50 ohms resistive. In order to minimize the power transfer loss between the two devices, the conduit (aka coax) must exhibit the same impedance as the source and the load.

Coax cable (or wave guide) at high frequencies is essentially a distributed combination of series inductors and parallel capacitors creating a "balanced out" component. Or in other terms, the series inductive reactance gets canceled out by the parallel capacitive reactance leaving only resistance as a loss component. Reactive components only store energy, where as resistive components convert energy into work (heat). So an ideal cable is essentially a lossless conduit for RF energy (think of it like a bucket brigade where no water dribbles out or is spilled). Real world cables do exhibit losses from two areas. The most significant loss is the dielectric absorption properties from the plastic, foam, or air that fills the space between the center conductor and the sheath. The other loss (minor) comes from the actual resistance of the conductor metals (copper).

In this situation where you are connecting a soundcard mpx output to an exciter input, true impedance matching is not much of an issue for a couple of reasons.
1. The source impedance of a sound card varies greatly depending on the brand, the output driver circuit design, and the type of output being used ie. headphone output (8-200 ohms), line output (200 ohms to several 1000’s of ohms or more).
2. The exciter mpx input designs are mostly if not all high impedance inputs. Typically 10k ohms or more.
3. Due to the low frequencies that make up the mpx signal (practically DC compared to 100’s or 1000’s of MHz) the series inductive component of the coax is so small it essentially provides little contribution in balancing out the parallel capacitance of the cable. In this case 50 ohm coax is nothing more than fancy shielded wire where the impedance of the cable has little relevance.

If the sound card output and the exciter input were both 50 ohms, then yes, the cable impedance would become a factor worthy of consideration. But since this is not the case, worrying about 50 ohm connectors and cable impedance matching is a moot point.

What you need to concentrate on most in creating a low loss connection between the soundcard output and the exciter mpx input is to keep the cable lengths as short as possible, and use the highest quality cable that exhibits the lowest capacitance per unit length (i.e. xxx pF per foot/meter).
Using low quality cable with high distributed capacitance essentially creates a low pass filter that rolls off the higher frequency components of the mpx signal. Leif has been clever enough to design in tilt compensation to mitigate this roll-off effect (special output pre-emphasis to boost the higher mpx frequencies), but it has a finite boost range. So it’s up to you to use the best cable available that ensures the mpx signal remains properly equalized (flat) when it arrives at the exciter input.

Note: When making sound card tilt adjustments it’s very important that you measure the mpx signal at the exciter input, not at the sound card output. Doing so allows you to compensate for the cable roll-off effects.

Geez Martin S, I’m glad you asked!

Mogami cables are awesome, that’s all I use now. I buy by the spool, and use Neutrik gold-plated connectors. But they do sell ready-made cables too. Lower capacitance and inductance than most other cables costing waaaaay more per foot.

CFD 200 (LMR 200)