Let's talk impedance

[Blacksheep6966]

Hi everyone. I'm just getting back into monitoring and antenna building. Over the years I have built a lot of antennas and experimented with several different designs. I think I really enjoy building more than listening sometimes. I just recently finished a coaxial dipole for MilAir that I am extremely happy with the end results. I'm going to post more on that later but first I just wanted to get a few opinions on impedance.

How important do you think impedance is to a receive only antenna? I do not own an antenna analyzer. I've never seen the need for one due to the fact that I'm not a ham so I don't transmit and other than to check and see what the antenna wound up being frequency wise in the end I've just never thought I needed one.

I sometimes use RG-6 as the coax on some of my builds as a lot of people do. I've never seemed to have a noticeable problem with it being at 75ohms. So if we avoid full-wave antennas- for an example due to possible high impedance and other cases where high impedance may be a problem, what are the problems we are looking at? Keeping in mind we are not trying to transmit I'd just like to get some thoughts on this from other builders as to why we consider this?

It something I just have never really thought about and would like to learn more. Educate me...

 

[prcguy]

Antenna performance is reciprocal between transmit and receive. If the antenna has problems and degradation on transmit, it will have the same on receive. An impedance mismatch can also create a frequency selective problem. If you are dealing with purely resistive loads then coax to load mismatch and loss is easily calculated. Now make something frequency sensitive like a length of 75 ohm cable in an otherwise 50 ohm system and its very hard to calculate the outcome and it will be different at different frequencies.

 

[Blacksheep6966]

Very interesting. Thanks