coaxial cable: noob question - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: March 30, 2013, 08:37:13 am
content: Hi,Why in high frequency application is used a 50/75 ohm cable? it's not better to have a less impedance cable whit a better shielding?

author: Simon
author_rank: Unknown
timestamp: March 30, 2013, 08:39:23 am
content: I think it is partly that is the natural impedance of that cable as made and part we decided to standardize on that

author: G7PSK
author_rank: Super Contributor
timestamp: March 30, 2013, 01:57:41 pm
content: It's to do with the impedance of aerials, an end feed is almost infinity impedance and center is around 50 to 80 ohms. A half wave dipole is usually fed with 75 ohms. Most modern transmitters are made to look into either 50 or 75 ohms so co-ax is made to match that. When working with higher frequency it is quite usual to cut the co-ax to a precise number of half or quarter wave lengths.

author: w2aew
author_rank: Super Contributor
timestamp: March 30, 2013, 02:43:34 pm
content: Quote from: Oracle on March 30, 2013, 08:37:13 amNote that the shielding effectiveness really has little to do with the cable impedance. Don't confuse coax impedance with loss either, although lower impedance cables are generally higher loss, but it really depends on the geometry and construction. 50 ohm hard line is much lower loss than something like RG-8U coax.

author: Galenbo
author_rank: Super Contributor
timestamp: March 31, 2013, 12:07:36 am
content: Quote from: Oracle on March 30, 2013, 08:37:13 amYou describe yourself as noob, I therefore assume you have no HF line knowledge.At a coax cable you can measure 2 meaningful resistances:1)The resistance from the centre of one end, to the other end2)The resistance between the centre and the shieldAt DC, 1) is nearly 0 ohm and 2) is nearly infinite ohms. You won't get 50/75 ohms somewhere.The 50/75 is only meaningful in an HF context, where people want to transfer signals from source to destination without reflection.The 50/75 ohms here have nothing to do with attenuation here, but attenuation/losses exist."better shielding" as you say is another aspect. Nothing to do with resistance, losses, reflection, but can be wanted to reduce ingress.

author: PA4TIM
author_rank: Super Contributor
timestamp: March 31, 2013, 12:39:18 am
content: Coax impedance has not much to do with antennas. We make 75 Ohm or 50 Ohm antennas because we have coax in those values. You can make a resonant antenna in every impedance you want.In WW2 they needed some standards so all the stuff could beinter connected, besides that open lines were not handy for mobile use so the RG series became a standard.50 Ohm has some more loss as 75 Ohm ( if you build them from the same materials/quality) but 50 Ohm can handle more power so fpin those days 75 Ohm became a standard for TV and 50 Ohm for Radio. But these are not the only values. There is 93 Ohm, 63 ( or 62, can not remember, somewhere in the 60 Ohm), populair in Germany in the 50-60's) and there are more.Loss and shielding are much more depending on the build-quality as the impdance. A high SWR increases the loss.AC travels outside a conductor. If there is a return conductor nearby the EM fields stay around the conductor, if not it is radiated. The advantage of coax is that the EM wave travels between core and inside of the shield. Loss depends on leakage through the shield an the sort of dielectric inside. Air is best but not practical. Open or twin line has the lowest loss but is also not always practical.The coax needs to be terminated with its own impedance at the load side. This is to get no reflections.This is called load matching. In special occasions you also want source matching. For instance doing VNA meaurements, or high speed measurements.Reflections you can easy see if you measure a fast Tr step without termination at the scope. This shows as ringing on the trace ( a damping wave starting most times with overshoot)Coax core is a long inductor and over the whole lenght it has capacitance towards the schield. Together this distributed LC network forms a characteristic impedance.