If one signal is 3 dB less than another, it's half as strong. Some types of coax are supposed to attenuate signals they carry by up 8 dB per 100 feet. If a cable was rated as attenuating a signal by 3 dB over a given length, and a source put 100watts into one end, does that mean that HALF of the power is getting absorbed by the cable, such that there's only 50watts reaching the load at the other end?
Attenuation of 3dB would indeed mean 50% of the input energy is absorbed (in terms of power, which dB always refers to - that nifty
20log10 V1/V2 is derived from the power equation).One key thing here though, is that normally the cable attenuation is given at some frequency, with load and source impedances. Copper cables are low pass filters, so their attenuation increases with frequency (although we have some tricks to balance that out in a system).
Cheers
PeteS
I guess that means the higher the frequncy the more it pays to use better feedlines. Thanks, that's usefull info.
-- 73 Tom H VA7FAB
If the signal you are transmitting through the cable is of a particular frequency (i.e carrier, narrow band), then you should be able to calculate the exact loss through a particular length of that cable by looking on a nomograph or by using the manufacturers formula. Cable type, length, and frequency will yield a loss in either the nomograph or formula. Note that these losses usually assume the cable is terminated into its characteristic impedance.
If you are trying to transmit wideband information (i.e. DC - 100 MHz) through the cable, and you want a flat frequency response, you need to use a cable equalizer at one end or the other. In my work 2 decades ago, sometimes 100:1 (40dB) equalizers were necessary.
| I guess that means the higher the frequncy the more it pays to use | better feedlines.
Absolutely correct. For HF, especially the low end, it doesn't matter much what you use. But once you get into the higher frequencies, it starts to matter a lot. The effect is very pronounced.
It's also why once the frequencies get really high (like into the microwaves) that they start using waveguides rather than coax.
| Thanks, that's usefull info.
Indeed.
| VA7FAB
AD5RH
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