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Author Topic: Antennas & Transmission Lines (feedlines)  (Read 12105 times)

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Buy me a cigar
No. I don't know how to generate and detect such waves.
I can make a Tesla coil with infinite SWR but that's about it.

Dear Verpies and all.

This might help !!   ;)

Cheers Grum.


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Nanny state ? Left at the gate !! :)
   

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Not with earth as a medium, but "the old scientist" used the air with some results:

https://www.youtube.com/watch?v=wjz-5Lqtxow

Regards Itsu

I must assume those boxes are like the ones I've tried. i.e. they are non-ferrous.

When you see such a demonstration keep in-mind that a non-ferrous 'faraday cage -cake box-' will only block the electric side of the transverse wave. As long as you are in the near-field of the transmitter you will receive a signal because the magnetic side still penetrates the box. At that frequency he may be able to receive a signal even further away.

He should try a steel box that is grounded, like a true Faraday Cage. There should be no signal received.

It is possible to use my suggestion and still receive a signal but that would be due to the magnetic portion of the transverse wave inducing a current in the cage. The magnetic field from that  induced current is mirrored inside the box at a much lower level.


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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
Quite, because the quarter wave xformr can be followed by 150' of the target impedance coax.

So, we use a 1/4 wave length of 50 Ohm coax in series with some arbitrary length of 50 Ohm coax between a 50 Ohm source and mismatched load..is that correct?
   

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So, we use a 1/4 wave length of 50 Ohm coax in series with some arbitrary length of 50 Ohm coax between a 50 Ohm source and mismatched load..is that correct?
Almost. The ¼ wavelength of coax needs to have an impedance of 87Ω for your 50Ω into 150Ω case.
If all you have is 50Ω coax, then there are other solutions - see attached.
   

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A lot of 'if's' when matching.

At 800mHz it might be worthwhile considering a tapered TL match (strip-line) when going balanced to unbalanced. In any case, the 'matching device' needs to be between the antenna and the feed line.

Don't know if that is obvious to all.


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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
Almost. The ¼ wavelength of coax needs to have an impedance of 87Ω for your 50Ω into 150Ω case.
If all you have is 50Ω coax, then there are other solutions - see attached.
Yeah, I figured that. Not so sure you can buy a myriad of various impedance coax cables.
   

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Matching the antenna impedance to the line impedance by
means of strategically placed stubs can be done with some
difficulty.

But it might be easier to alter the antenna physically to accomplish
the match.  When an antenna presents an impedance of 150 Ohms
to the 50 Ohm line what condition are we to recognize regarding the
length of the antenna?  Too long or too short?

What could be done to the antenna to effect an impedance match?



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"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   

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It's not as complicated as it may seem...
That's another myth I've seen floating around; that stubs are used to match or tune a mismatched antenna.

The stubs actually only produce an equal and opposite reflection to what the antenna is producing, so the reflection is canceled before making its way back to the transceiver.
   

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I don't think stubs can do it by themselves.  Only with an impedance transformer they can succeed. ... or L or π networks described in my last attachment.
   

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When faced with the need for odd coax impedance it is usually a good choice to instead change the feed point at the antenna. This has the effect of changing the antenna impedance.

Should this antenna be a center fed 150 Ohm dipole you can feed off-center to obtain an antenna impedance of 100 ohm. Using a 1/4 wave 75 Ohm coax between the antenna and the 50 Ohm feed line will be a good match.

The problem with my suggestion is that the antenna must not be a single dipole or center feed would be 300 Ohms.


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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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Quote from: poynt99
...so the reflection is canceled before making its way back to the transceiver.

Which sounds like it might be an 'impedance transformation' which
produces a match by limiting the resonant condition to the antenna
end of the line.  No?
 


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"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   

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It's not as complicated as it may seem...
It has no effect on the antenna at all; the antenna still produces a reflection.
   

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True, it doesn't physically alter the mismatch at the antenna
but it does alter the standing wave impedance characteristics
within the region of resonance which should result in more
radiated energy.  Reflections must go somewhere.


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"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   

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It's not as complicated as it may seem...
It shouldn't make a significant difference in the radiated power actually. Without a "tuning" stub, the reflected power gets re-reflected from the transmitter so all the energy eventually goes out the antenna anyway.

The stub prevents large VSWR at the transmitter and may also eliminate the long trip the reflection must take to the transmitter and back again. So in that sense, it may facilitate slightly more power pushed out the antenna by reducing line loss.
   
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