# Transmission line attenuation

by Simon.T
Tags: attenuation, line, transmission
 P: 15 Dear All I am hoping for some insight into a transmission line problem I have been experiencing. I am quite inexperienced and unqualified in this area so if I am not clear please let me know. In wiring loom assemblies my employer offers there is a twisted transmission pair. The conductors are terminated at either end into a soldered electrical insert. The assemblies come in various lengths (75, 50, 15 and 2.7m). The specification of the un-terminated transmission wire (as quoted from the manufacturer data sheet) is <6dB/150m @ 20MHz. “Scaling” of this value seems particularly intuitive for determining testing limits for verification. What we have noticed is that in the 75m loom, the measured attenuation is approximately half of the scaled manufacturer limit. In the shorter sections the measured attenuation is borderline or a failure; as I mentioned we are scaling the limits so that 15m testing limit is 1/5th of the 75m limit for example. When we mocked up a very short loom (approximately 10cm) and terminated them into connectors, the loss through that assembly was MORE than the 2.7m terminated section. It appears that the loss contributed by the connector inserts is not static. In fact, it seems the measured attenuation is a function of the ratio of “connector medium” to “transmission line medium”; the shorter the loom the more of an impact the connector has on the attenuation. Is what we are seeing representative of typical transmission line characteristics? How can this behaviour be expressed/calculated? From my studies I am familiar with the concept of standing waves, reflections and "ringing" due to impedance mismatching and I am aware there are a number of standard transmission line equations but I’m not sure how those apply here (see the first paragraph :)). Absolutely any help would be appreciated. We are attempting to negotiate more lenient tolerances for the shorter looms that keep failing but we would like some basis for our recommendations. Best regards, Simon Thomas
 Sci Advisor P: 4,032 Are you saying the cables are not terminated with their characteristic impedance? If you did that, you would certainly get standing wave effects that would mask any attenuation effects due to losses in the wire. A quarter wave at 20 MHz in free space is about 3.75 Meters. What is the nominal characteristic impedance of this cable? Is it shielded at all or just a twisted pair? What is a "soldered electrical insert"?
P: 15
 Quote by vk6kro Are you saying the cables are not terminated with their characteristic impedance? If you did that, you would certainly get standing wave effects that would mask any attenuation effects due to losses in the wire. A quarter wave at 20 MHz in free space is about 3.75 Meters. What is the nominal characteristic impedance of this cable? Is it shielded at all or just a twisted pair? What is a "soldered electrical insert"?
Hi, thank you for the reply.

The transmission line we are using is this http://www.gore.com/MungoBlobs/26/32..._PLFWI1279.pdf

The wire is not shielded.

During testing the transmission line is connected to a signal generator with a 132R output impedance and terminated into 132R at the other end which should match the transmission line nicely; I believe the loss is not resulting from this per se - when we look at the frequency response from 1 to 20MHz it is quite a smooth plot with little ringing.

The connector I am refering to is a gold plated brass/copper (not sure - I will find out) pinned, much like a Cannon/military style if you have seen them (http://www.connect-solutions.com/itt...nnector-10.jpg), with solder buckets on the reverse which the stripped and tinned transmission line is soldered in to.

I believe the things we are seeing is due to the transition between the transmission line and connector pins and the impedance mismatch between the mediums, perhaps you can confirm my assumptions.

Simon

 Sci Advisor P: 4,032 Transmission line attenuation So, at the far end of this two wire cable you have a multipin plug and also a matching resistor of some kind? At the same time? It seems like one of these is faulty doesn't it? At 20 MHz a resistor can be quite inductive. Do you know what sort of resistor it is? have you tried testing for loss at a lower frequency, say 5 MHz? This would give you an idea of whether the loss is frequency related and if so, how quickly it happens as you change frequency. How are you measuring the output voltage across the load resistor? With an Oscilloscope?