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POWER LOSS AT VARIOUS SWR READINGS AND RESIDUAL POWER

U

Unit 194

Well-Known Member
Feb 25, 2006
1,233
379
93
New Jersey
ERP = Percentage of Effective Radiated Power

SWR READING/ % OF LOSS/ ERP/ WATTS AVAILABLE

1.0:1------0.0%------100.0%------4.00

1.1:1------0.3%------99.7%------3.99

1.2:1------0.8%------99.2%------3.97

1.3:1------1.7%------98.3%------3.93

1.4:1------2.7%------97.3%------3.89

1.5:1------3.0%------97.0%------3.88

1.6:1------5.0%------95.0%------3.80

1.7:1------6.0%------94.0%------3.76

1.8:1------8.0%------92.0%------3.68

2.0:1------11.0%------89.0%------3.56

2.2:1------14.0%------86.0%------3.44

2.4:1------17.0%------83.0%------3.32

2.6:1------20.0%------80.0%------3.20

3.0:1------25.0%------75.0%------3.00

4.0:1------38.0%------62.0%------2.48

5.0:1------48.0%------52.0%------2.08

6.0:1------55.0%------45.0%------1.80

10.0:1------ 70.0%------ 30.0%----1.20

SWR READING/ % OF LOSS/ ERP/ WATTS AVAILABLE
 

Let's say you are using 100 feet of old RG-8 coax and that it has a line loss of 3 db for that 100 feet at your transmit frequency. Place your SWR meter at the antenna end of the coax. Your transmitter is putting out 100 Watts, into a perfectly matched (1:1) antenna. Your reflected power is 0%. your radiated power is 50 Watts, due to the line loss of 3 db.

Let's increase your SWR to 2:1. That means that about 10% of the power that reaches your antenna is reflected and 90% radiated. OK, 45 Watts radiated and 5 Watts reflected. By the time that 5 Watts gets back to the transmitter, it is down to 2.5 Watts and is reflected back to the antenna. By the time that it gets back to the antenna, it is down to 1.25 Watts. 90% of this is radiated, or 1.12 Watts. Add this to the 45 Watts and you have 46.12 Watts. This process continues until no more measurable power reaches the antenna. In this case, you might get another .03 Watts to radiate.

What does this mean? That of the 50 Watts that reached the antenna the first time, 46.15 Watts was radiated. Or, put another way, 46.5/50=93% of the power radiated. Wait a minute! didn't 194's chart say that we lost 10% (Actually it says 11%, I just rounded it to 10% for simplicity)?

Not all of the reflected power is lost. But, the silver lining of this cloud is, that it shows what a demon that line loss can be. We only have to use better feed line and our radiated power will jump dramatically, even with poor SWR. Also, it shows that if you place the SWR meter at the transmitter, you will get an erroneous reading. In this example your meter would read that 2:1 SWR as 1.6:1. All because of line loss.

Change that old RG-8 to a good quality RG-213 and your radiated power will jump to about 80 Watts. Go to 600 ohm open wire line and your power radiated will be an astounding 97 Watts. All with the same 10% reflected power, due to the 2:1 SWR. Even with a 10:1 SWR, the 600 ohm line will still get over 89 Watts out of the antenna. This is why good tuners are such a good deal.

Try this coax loss calculator and see what I mean.

Line Loss Calculator

Rich
 
Ok, Understand. Please entertain one more:

Why do transistors in radios and amplifiers get hotter when connected to an antenna & feedline with high SWR?

Where does this energy in the form of heat come from?
 
Think of it this way: The transistors are having to work harder to push their rated power against the reflected power. This requires more current (amps). More current---More heat. This is why most good PA's have a fold-back circuit that reduces their power output, if the SWR gets too high.

Rich
 
So a transmitter will draw more current with higher SWR? I'm going to test this some time next week, and post the results. I've got to see this with my own two eyes.
 
kind of makes sence , whats the worse thing you can do to finals ? Usually that amounts to bad SWR and or shorts somewhere in the line , let me see if I can take a guess at this ? the lower the SWR the more the RF can get out ,the higher the swr the harder it is for it to get out so I would guess to say it backs up to a degree , in return this would cause the excessive heat bulid up on the transistor (almost like a choking effect). Over the years I have found that transistors in radios can take some pretty good abuse before they go completly bye bye ,So they will slowly seem to get weaker in time , this is usually caused by high SWR , I've come to believe that CB radios can handle high SWR's up to say 2.5 for a very long time , not the best ,but will still work fairly well , I think that once you are over a 3 it will only be a matter of time before the final will weaken and or pop , the fastest way would be a total short in the line. Finals seem like pretty tough cookies to me over the years and can handle higher swr's then other radios (IM guessing here) , I'd guess to say dual final or exports might be easier abused ? Only a guess on my part. I have seen hundreds of different types of antenna's sold over the years , somewhere good and some were not so good and some were just down right bad )-: meaning that you couldn't get better then a 2 or more , but the botton line with those antenna's they would still work and folks could sell them. When it comes to running power we all tend to know that the lowest match is are best friends when it comes to amps , even though I have run them high in my time , but how high is truly high ? I've ran them at 1.7 and just under 2.0 in my time for months at a time with no complactions that I knew of after the fact , that was a long time ago , with antenna's like the Wilson 1K or 5K , it's just kind of hard to miss with those type of antennas (-: when it comes to SWR , My way of thinking says ,finals have to have room to breathe , the lower the SWR the better they have at breathing (-: and I would think that would apply to all transistors concerning there way out of any antenna.
 
"Most efficient"... :?:

Go to the antenna topic and click on the coax comparison chart that's in Hamin' X's link. Enter the required information for a given variety of coax and see the dynamics for yourself. Try several different varieties, keeping the power and frequency constant. Also, enter "1.5" for the SWR for a real-world figure. No antenna other than a dummy load is going to have a 1:1 SWR at more than a very narrow range of frequencies.

The antenna topic's "stickys" should be required reading for everybody who's into radio.
 
Well,if no antenna is going to have a perfect match besides a dummy load,then how are people running these comp amp's that require an SWR of no more than 1.3?I know where your coming from,its hard to get the SWR down that low on my M103c.1.5 seems to be as good as it will get.Is my watt meter playing tricks on me or what at a higher SWR?I mean i seem to lose close to 100w on my Dosey meter from an SWR of 1.5 down to an SWR of 1.1.Is it better to run it at 1.5 and see 100w more on my meter or run it at 1.1 and see 100w less?
 
I've said it before: you can either sweat bullets trying to get your SWR down from 1.2:1 to 1.0000000:1, OR you can get on the air and have fun. An amplifier that's unable to cope with anything greater than a 1.3:1 SWR is a poor investment, in my opinion.
 
Beetle said:
I've said it before: you can either sweat bullets trying to get your SWR down from 1.2:1 to 1.0000000:1, OR you can get on the air and have fun. An amplifier that's unable to cope with anything greater than a 1.3:1 SWR is a poor investment, in my opinion.[/quote

You said it all bro!
Click to expand...
 
Hillbilly360 said:
Well,if no antenna is going to have a perfect match besides a dummy load,then how are people running these comp amp's that require an SWR of no more than 1.3?I know where your coming from,its hard to get the SWR down that low on my M103c.1.5 seems to be as good as it will get.Is my watt meter playing tricks on me or what at a higher SWR?I mean i seem to lose close to 100w on my Dosey meter from an SWR of 1.5 down to an SWR of 1.1.Is it better to run it at 1.5 and see 100w more on my meter or run it at 1.1 and see 100w less?
Click to expand...

Cheap meters will show different over-all readings with different SWR levels. Thats why with a Bird (and other similar meters) you take the forward reading, and subtract from that any reflected power, to get your total power delivered to the antenna. You also want to keep in mind that the length of the coax will affect what the meter thinks it sees as the SWR. If you went to the trouble of calculating the exact half-wave length for your particular coax, and kept your run to an exact multiple of the half-wave figure, then you can be reasonably sure the SWR is at least roughly accurate. As for the power reading fluctuating with SWR shifts, as I said earlier, the cheaper meters will tend to do that. Depending on the phase of the reflected power when its going throgh the meter, it might make it look higher, lower, or not effect it at all. I'd be willing to bet that if you did some experimenting, that you'd see different levels of fluctuation at different frequencies, for the same reasons.
 

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