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What happens to reflected power?

 
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VK2AAF
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PostPosted: Fri Oct 10, 2014 7:57 pm    Post subject: What happens to reflected power? Reply with quote

Hi antenna-theorists!

I've browsed the forum for an answer to this query, and while there are some fairly close answers, I'm so far not finding a definitive answer supported by evidence. Please forgive me if there's already an answer and kindly direct me to that thread.

Where does power reflected from a mismatched antenna end up?

Is reflected power re-reflected from the transmitter's output stage (or from the antenna tuner, as applicable) then again reflected from the mismatched antenna, bouncing back and forth until it is dissipated as heat in feedline resistance/impedance?

Is reflected power eventually radiated? If it is radiated, given reflection of the incident signal from the mismatched antenna, how does radiation occur? If radiated, would the phase of reflected power lag somewhat behind that of the incident signal due to the time required to bounce off the mismatched antenna, and if so, what effect does this have upon the amount of power radiated from the antenna system?

It occurs to me that non-resonant antenna systems (employing matching networks/antenna tuners) do radiate (some) signal. Are there any resources which describe how non-resonant antenna systems accomplish radiation?

Thanks much and apologies if this topic has been discussed in depth elsewhere.

73 de Brian VK2AAF
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la7no
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Joined: 05 Jul 2014
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Location: Bergen, Norway

PostPosted: Sun Oct 12, 2014 5:18 pm    Post subject: Reply with quote

Hi Brian,

Power reflected from a mismatched load and being returned to the source, will in most cases be re-reflected towards the load. I guess the only exception to this will be when the output of the transmitter or tuner matches the feeder impedance perfectly - something that rarely will be the case.

73es,

Per-Tore
DL/LA7NO
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VK2AAF
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PostPosted: Sun Oct 12, 2014 6:46 pm    Post subject: Reply with quote

Hi Per-Tore,

Thanks very much for your response.

If power reflected from a mismatched antenna travels back down the feedline and then is again reflected off the xmtr output circuit or matching network, it stands to reason that the energy will eventually be dissipated as heat in the resistance/impedance of the feedline.

As such, I wonder how non-resonant antenna systems radiate any signal at all.

I wish I had the mathematical ability to delve into the theory a bit more, but I'm afraid I'm 'maths impaired!'

73 de Brian VK2AAF
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la7no
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PostPosted: Sun Oct 12, 2014 7:13 pm    Post subject: Reply with quote

Hi again,

The re-reflected energy will be partly radiated from the antenna (according to the impedance matchind between antenna and feeder). The rest will again be reflected towards the source. For each 'trip' up/down the feeder, energy will be dissipated.

For a reasonable good coax (f.ex. RG-213), with reasonable length (say, below 50m) and at relatively low frequencies (say, below 30 MHz), the coax loss will not be significant. Most of the energy will finally be radiated from the antenna.

P-T / LA7NO
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la7no
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PostPosted: Mon Oct 13, 2014 7:34 am    Post subject: Reply with quote

Some thoughts about resonant vs non-resonant antennas:

To put it short, a non-resonant antenna will radiate as much power as a resonant antenna provided there is an impedance match to the feeder.

However, a non-resonant antenna will have a different radiation pattern than a resonant one. But for practical reasons we will in most cases not be able to put a resonant antenna up high enough to get its theoretical radiation pattern.

A longer wire antenna will capture more of the wave front of the incoming signal. This will help on the receive side.

But the challenge is to get a non-resonat antenna properly matched. Many matching devices introduce significant losses

P-T / LA7NO
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EA1DDO
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PostPosted: Thu Nov 06, 2014 10:51 am    Post subject: Reply with quote

Hi,

I think the term "non-resonant" is not properlly used.
For example,a 5/8 vertical antena is non-resonant, but matches perfectly to a 50 OHm coax (because there is an impedance adapter at antenna base).

One more example, a simple 1/2 dipole is resonant but shows around 75 OHms so it is not matching 50 OHm coax well.

Finally, the key is not to be resonant or not, the key point is to adapt his impedance to the transmission line properlly or not.

73, Maximo
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VK2AAF
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PostPosted: Thu Nov 06, 2014 10:55 am    Post subject: Reply with quote

Very good points about the impedance match from Maximo & Per-Tore!

Thanks much. Smile

73 de Brian VK2AAF
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la7no
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PostPosted: Thu Nov 06, 2014 11:34 am    Post subject: Reply with quote

EA1DDO wrote:
Hi,

I think the term "non-resonant" is not properlly used.
...
Finally, the key is not to be resonant or not, the key point is to adapt his impedance to the transmission line properlly or not.

73, Maximo


Hi Maximo,

Isn't this what I just said?
In what way is the term being improperly used?

Per-Tore
LA7NO
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EA1DDO
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PostPosted: Thu Nov 06, 2014 3:34 pm    Post subject: Reply with quote

Hi Per-Tore,

Sorry, I did not talk about you.
When I said "not properlly used" was generic, not thinking in you.

Sorry

73, Maximo
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la7no
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PostPosted: Thu Nov 06, 2014 3:46 pm    Post subject: Reply with quote

OK, no problem. Smile

P-T
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