Antennas

I don't understand what you say. The outgoing wave bounces off the end of the antenna and returns back toward the transmission line, with some energy lost to radiation. Right at the end of the antenna, the current of the outgoing and reflected wave must sum to zero, since there is no other currents involved except the two waves. So the returning wave is current inverted.

The thing that is hard to wrap your brain around is that you are dealing with traveling waves, not lumped components (like capacitors and inductors). So to understand how these two waves act at any point on the antenna or transmission line, you have to deal with the time it takes for any part of the wave to get from point A to point B.

If the antenna were electrically resonant (not too long or too short), the outgoing wave would take a quarter of a cycle time to get to the tip of the antenna and a quarter of a cycle time for its reflection to get back to the feed point. But there is also the inversion at the reflection, which adds another 180 degrees or one half cycle to the total effect, so the returning wave meets the outgoing next cycle, in phase.

If the antenna is short, the reflected wave gets back too soon, and the next cycle sees the last one at an earlier phase in its cycle. For instance, if the outgoing cycle is right at zero volts and going positive, it meets the reflection at a negative voltage heading for zero volts. This is a lot like how it would be if the outgoing wave were driving a lumped capacitor. The capacitor voltage would always be lagging the applied wave. Short antenna = capacitive reactance at the feed point.

If the antenna were long, the reflection would have the (travel) time to get ahead of the drive wave, so it would have a voltage that leads the drive in phase, just as an inductive load would have a voltage than leads its current.

John,

Can you please explain how the length of a flattened loop, like a TV antenna, should compare with an ordinary dipole antenna.

R

"Roger Dewhurst"

** They are virtually the same length.

Do a Google search on " folded dipole ".

Beware of the trap of assuming that any Yagi antenna with the main element folded is a 300 ohm antenna.

........ Phil

What is that supposed to mean? Were you trying to say they are the same "electrical" length? Either way, you're wrong. Try again later.

A full wave loop (like the guy asked about) has a feed point impedance of around 100 ohms when it's shaped like a circle compared to the ~75 ohm impedance of a dipole. When you squash it down till it's nearly the length of a 1/2 wave dipole the feed point impedance shoots up to around

300 ohms while the gain of the antenna drops with the shrinking loop area. To further answer the OPs question, a full wave loop has a theoretical gain of 3dB vs. a dipole. I'm sure it's going to be somewhat less than that when you mash it to a narrow oval, but it will still have more gain than a 1/2 wave dipole. It will also be much "quieter" in terms of the amount of noise it receives.

Why don't you do us all a favor and stop trying to answer antenna questions.

element

I'm guessing this is in reference to some other occasion where you put your foot in your mouth.

"Anthony Fremont"

** Freemont is *hell bent* on proving, over and again, just what a criminal, psychotic pile of sub human shit he really is.

"Roger Dewhurst"

** It is a perfectly clear statement.

But Fremont's grossly autistic brain has mangled it - as usual.

** No, I am not wrong.

A regular di-pole and its folded counterpart have essentially the same length.

** No he did NOT - you illiterate, brain f***ed ASSHOLE !!! ** Fremont, why not do the whole planet a favour.

Go shoot yourself in the head.

** Every this Fremont cunt posts is a wild guess.

Then what would you expect from a pig ignorant, septic tank ham radio puke.

....... Phil

Your reference to a full wave loop implies, to me, that the circumference of the loop approximates to the wavelength. Is this correct?

R >

"Phool Allison"

Please don't top post.

TV

Only in the physical sense of the space it takes to erect it, and even that is a twisted viewpoint since it's still a mashed up loop. That means its length is really twice that of a dipole.

You really need help phool. A folded dipole IS A FULL WAVE LOOP squashed down to regular dipole size (more or less). I see you snipped away about 90% of my post. I'll take that to mean you verified it as factual information after you frantically rifled your way thru the google archives.

I don't see how that will answer the OPs question, but then you aren't really interested in doing that are you? You just wish to disseminate a bunch of BS mixed in with a little bit of decent information. A true savant when it comes to audio and an absolute knuckle dragging simian when it comes to everything else.

put

radio

Yet another imbecile jealous of a ham ticket, what an absolutely pitiful sight. I'm sure you could probably get one now phool, since they made it so easy. You don't even have to learn morse code, but you WOULD have to learn something about antennas.

circumference of

Yes, but as phool said, if you mash into the shape of a folded dipole then stand back and squint a little, it will appear to be the size of a regular dipole which is 1/2 wavelength long.

If the loop is flattened enough that the half look more like pieces of transmission line than they do like a loop antenna, then the electrical length of the folded dipole is about the same as a simple dipole. I think, that the propagation speed may be just a bit lower for the folded dipole, so it may look act a bit longer than a dipole of the same physical length, but I an not too sure of that.

"Anthony Fremont"

** Freemont is *hell bent* on proving, over and again, just what a criminal, psychotic pile of sub human shit he really is.

** It is a perfectly clear statement.

But Fremont's tortured, autistic brain has completely mangled it.

** No he did NOT - you illiterate, brain f***ed ASSHOLE !!!

Fremont, do the whole planet a favour.

Go shoot yourself in the head.

....... Phil

"Phool Allison" wrote....nothing technical

I guess I win again. Thanks for playing.

"Anthony Fremont"

** Fremont is *hell bent* on proving, over and again, just what a criminal, psychotic pile of sub human shit he really is.

Fremont, do the whole planet a favour.

Go shoot yourself in the head.

....... Phil