Why Yagi antenna has a loop?

[paulears]

dlwtrunked - I suspect, we are not going to convince him on this. Still, we did try. I just don't understand why he doesn't check these things, because there is so much on the net about the subject?

 

[nespokedru]

You have to stop considering impedance as the same as resistance. They use the same term, but in ac circuits you have to consider capacitance and inductance too. Cable does have loss of course. However if you measure loss in a cable to RF the loss increases as frequency rises. A piece of thin coax 30m long might have nothing come out the end at 1GHz but be fine at 10MHz. Your resistance idea just doesn’t work with this stuff.

That's also a kind of resistance. Radio signal through a wire is AC voltage. In case of AC, there is a different kind of resistance called impedance, which is made up of resistance and reactance. Impedance is Z = √(R^2 + X^2), R is resistance and X is reactance of the inductor. XL = 2πfL is inductive reactance of wire. So, when frequency (f) increases, XL also increases. This will result in higher impedance vale. So, when you increase the frequency of AC, it will be lost if the wire is long. debojitacharjee is right, impedance is also a kind of resistance measured in ohms.

 

[paulears]

I suspect that expanding very simple electronics into capacitance, impedance, resistant, inductance and reactance might be a stretch too far. I think we said resistance is different from impedance very early on and understanding of that was er, incomplete. The water was clouded, and now totally polluted!

 

[merlin]

Not really that complicated. A closed loop may be a DC short but at RF frequecies a loop will reasonate.
A Yagi can even have the reflector and directors in a closed loop configuration.

 

[paulears]

Of course - but try to explain that to a keen newcomer who assumes RF behaves logically!

 

[debojitacharjee]

Of course - but try to explain that to a keen newcomer who assumes RF behaves logically!

RF behaves like how water waves does in a swimming pool. Keep a balloon floating on water, tied to a string and the other end of the string tied to a stone at the bottom of the pool. When you hit the water with anything, the water waves would make the balloon to swing. The same thing happens to an antenna when radio waves hit it. But instead of swinging, its electrons move in to-and-fro directions, creating AC voltage. This AC voltage is then fed to a receiver, which converts to audio.

 

[merlin]

RF behaves like how water waves does in a swimming pool. Keep a balloon floating on water, tied to a string and the other end of the string tied to a stone at the bottom of the pool. When you hit the water with anything, the water waves would make the balloon to swing. The same thing happens to an antenna when radio waves hit it. But instead of swinging, its electrons move in to-and-fro directions, creating AC voltage. This AC voltage is then fed to a receiver, which converts to audio.

Ever see a tuning fork make a stringed instrument vibrate a particular string ? A good example of reasonance and propagation.

 

[paulears]

Debonair - the analogy doesn’t work I’m afraid. We know how it works but it is you who has misunderstood what we are all trying to explain. Your basic premise is wrong.

 

[paulears]

Ever see a tuning fork make a stringed instrument vibrate a particular string ? A good example of reasonance and propagation.

I had never thought of that one - that's a really good one to explain it to beginners.