Antennas

I have concuded somewhere regarding Electricaly short antenna.
I just want to mention it to see if my conclusions are correct.
If the antenna is shorter than 0.25 of the wavelength then there will
be a phase angle as a result of disturbances and that is why there is
impendance rather just resistance
Am i saying it correctly?

 

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

 

** 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.

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 fucked ASSHOLE !!!

** Fremont, why not do the whole planet a favour.

Go shoot yourself in the head.

** Every this Fremont **** 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.

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.

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.

 

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 fucked ASSHOLE !!!

Fremont, do the whole planet a favour.

Go shoot yourself in the head.




........ Phil

 

....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