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Dear electrical engineers! I am trying to understand what stands behind a 75 ohm (unbalanced) antenna connection on my hifi receiver? I understand the "unbalanced" part, however not the "75 ohm" one. I know that an antenna will have resistance and impedance, which will hopefully result in being 75 ohm overall. Would I be right to say that the receiver will appear as a 75 ohm (resistance + impedance) load from the antenna's point of view? I am trying to use 4nec2 software to model a simple Yagi-Uda receiving antenna and I am not sure how to define a "source" which is the receiver. Thank you.

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  • \$\begingroup\$ surely the receiver is a 'load', not a 'source' and will apear as 75 ohms (pure resistance). \$\endgroup\$ Mar 20, 2016 at 21:07
  • \$\begingroup\$ @JImDearden: The issue is that the software asks what the source impedance is, but doesn't have a place to enter the load impedance. Ventus is asking if these two can be considered the same. \$\endgroup\$
    – Pigrew
    Mar 20, 2016 at 21:28
  • \$\begingroup\$ @JImDearden: that's what I was thinking, too. The problem is compounded by me not having a good knowledge of the subject plus trying to learn a new piece of software. From what I read, each 4NEC must have at least one source. Sources can be either voltage or current type. \$\endgroup\$
    – ventus
    Mar 21, 2016 at 7:35

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Yes, you should assume that the receiver is a 75 ohm load, with one conductor grounded (i.e. unbalanced).

Higher frequency signals will create reflections if they try to go through a point where the impedance does not match. A system where the source, load, and cables all have matching impedances will not produce any reflections (and it will also have maximum power transfer).

For sources, the source impedance is similar to a resistor in series with the source. For loads, it's similar to the resistor connecting the two conductors.

For cables, the concept is more complicated. Wikipedia has a good overview, but the rough idea is that the cable's impedance is the ratio of voltage to current in the cable. Common cable impedances are 50, 75, 150, or 300 ohm.

The antenna design software you use is designed for both transmitting and receiving signals, but transmitting is often more important, so they call it the source impedance. But, the antenna being a symmetrical device (from the transmitting/receiving point of view), it is equivalently the load impedance.

Generally, designers will provide purely real source and load impedances, so you don't need to worry about the imaginary parts of the source/load impedance. Note that the impedance of the antenna is a function of frequency, and your antenna will not be able to have a purely real impedance at all frequencies. Usually a Smith char (polar plot of impedance) is used to visualize impedances. If your impedance is wrong, then a transformer (either coils of wire or quarter-wave lines) could be use to match it to the 75 ohm load.

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  • \$\begingroup\$ thank you for your comment. I will try to digest it. Perhaps a silly question: if the antenna, the cabling and the receiver would all have 75 ohm impedance, would it not add up as it is in series? \$\endgroup\$
    – ventus
    Mar 21, 2016 at 7:36
  • \$\begingroup\$ It does not "add up" as one would think. Think of it as radio waves confined inside a cable. The cable impedance is more of a description of how current and voltage transients will be related. The source/load impedances are like antennas. Matching component impedances is required for "maximum power transfer" and also to prevent reflections (see SWR (standing wave ratio)). The cable has another metric named "loss" which describes how much power it loses per length (e.g. 0.15 dB/meter). But, this loss does not effect its impedance... it just reduces the power that gets to the load. \$\endgroup\$
    – Pigrew
    Mar 21, 2016 at 23:19

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