REFLECTED POWER

[Swamp Yankee]

Hello All,
Is it safe to say that "Reflected Power" is the amount of Watts being reflected (pushed) back to your radio? Thank You...

 

[Shadetree Mechanic]

Some how the SWR is an indication of reflected power too. Guys with big amplifiers will have two watt meters in line. One for forward power and the other one hooked up backwards for reflected power. Also I have heard that reflected power is not wasted, it reflects off of the transmitter and goes back out again.

 

[brandon7861]

I think the re-reflection is a tuner-coax thing and not a radio-coax thing. Consider the case with the tuner using 50Ω coax and an antenna with a feed point impedance of 200Ω. If you have 44 electrical degrees of coax, the 200Ω antenna impedance is transformed by the coax to 25-j44. The tuner is adjusted so the output port is set to transform the radio’s 50Ω to the conjugate of that, 25+j44. At that point, the reactance cancels out and the resistances match. Since the tuner port is not the same impedance of the coax, there is a partial re-reflection of the signal back to the antenna. Some is absorbed into the radio and some gets bounced back for another round trip.

In the case of the radio and coax, that same initial reflection from the 200Ω antenna is going to see the radio as more 50Ω coax and absorb it all, since the matching network inside the radio appears as 50Ω too, not the conjugate of some odd value.

Edit: maybe I am misunderstanding that, but that's how it was explained to me.

SWR = (1+sqrt(reflected power/forward power)) / (1-sqrt(reflected power/forward power)) have I mentioned this forum really needs latex editing for maths? Formulas could be typed in real fancy like and its not that difficult to learn to use

 

[loosecannon]

well a lot of this is above my pay grade, but here is more than you'll ever want to know about this subject:

https://www.fars.k6ya.org/docs/Stearns_K6OIK-Conjugate_Match_Myths.pdf

it gets interesting around page 28 where they start stating that Maxwell was wrong.
LC

 

[fourstringburn]

Here's an simple analogy,

Think of your coax as a garden hose, your radio is the faucet, and your antenna is the end of the garden hose.

A high SWR would be like putting your thumb over the water flow creating a backflow and the excess water spraying out all directions yet still much of the water will still shoot out straight.

The higher the SWR would be like putting your thumb tighter over the water flow creating more resistance making more pressure to force the current flow of water out.

Yet despite all this, all the water will flow out until the water is shut off just like all the RF current will shut off when you unkey the radio.

A high SWR will allow the flow of RF current out but a lot of the reflective RF will radiate out the antenna as unproductive heat just like the wasted excess water spray like a hose with your thumb on it.

This can cause a radio to push harder to compensate generating more heat in the final transistors which can lead to failure.

If RF current truly backflowed INTO the finals, then that would be catastrophic.

So no, reflected watts of RF power doesn't go back to a radio. An SWR meter can measure the amount a relective loss which can determine relative power out.

 

[fourstringburn]

Some how the SWR is an indication of reflected power too. Guys with big amplifiers will have two watt meters in line. One for forward power and the other one hooked up backwards for reflected power. Also I have heard that reflected power is not wasted, it reflects off of the transmitter and goes back out again.

Interesting concept however, I wonder if they know a cross-needle type SWR meter gives the same information in one glance.

 

[Swamp Yankee]

Thank You Gentlemen. Another question, is it ok to coil excessive coax and in my case, just hang it up on a beam in the basement? Or do I cut off the excess? Thanks in advance.

 

[Swamp Yankee]

Here's an simple analogy,

Think of your coax as a garden hose, your radio is the faucet, and your antenna is the end of the garden hose.

A high SWR would be like putting your thumb over the water flow creating a backflow and the excess water spraying out all directions yet still much of the water will still shoot out straight.

The higher the SWR would be like putting your thumb tighter over the water flow creating more resistance making more pressure to force the current flow of water out.

Yet despite all this, all the water will flow out until the water is shut off just like all the RF current will shut off when you unkey the radio.

A high SWR will allow the flow of RF current out but a lot of the reflective RF will radiate out the antenna as unproductive heat just like the wasted excess water spray like a hose with your thumb on it.

This can cause a radio to push harder to compensate generating more heat in the final transistors which can lead to failure.

If RF current truly backflowed INTO the finals, then that would be catastrophic.

So no, reflected watts of RF power doesn't go back to a radio. An SWR meter can measure the amount a relective loss which can determine relative power out.

So, if you are putting out 100 watts (per watt meter) and 4 are reflected (per watt meter) you are really putting out 96 watts? Thanks....Oh Big Day Today, my antenna went up...

 

[sp5it]

So, if you are putting out 100 watts (per watt meter) and 4 are reflected (per watt meter) you are really putting out 96 watts? Thanks....Oh Big Day Today, my antenna went up...

Maybe it is only 1 W? Depends on your antenna system

 

[Eldorado828]

So, if you are putting out 100 watts (per watt meter) and 4 are reflected (per watt meter) you are really putting out 96 watts? Thanks....Oh Big Day Today, my antenna went up...

Close but there are other factors as well. Coax losses depending on the coax type and you'll have to factor the length of the coax run as well.

I know that's not on topic with reflect but it's part of the answers to this question.

 

[fourstringburn]

Thank You Gentlemen. Another question, is it ok to coil excessive coax and in my case, just hang it up on a beam in the basement? Or do I cut off the excess? Thanks in advance.

Better to use the excess coil to wind a RF choke to stave off common mode currents.

This is typically done with 5 coax turns around a 2 liter coke bottle for diameter size and mounted close to the antenna feed point as possible.

A dipole or multi dipole beam antenna is a balanced antenna and your feeding it with an unbalanced feed line which creates common mode return currents on the feed line. This can lead to RFI at your station and other issues.

I prefer using a Choke Balun box with internal ferrite rings but many people have sucess with coax wound "ugly baluns".

Search ugly baluns in this forum and there should be many threads on them.

 

[Swamp Yankee]

Thank You Everyone!!!