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Old 04-10-2015, 12:19 PM   #18
NN5I
Carl, nn5i
 
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Join Date: Sep 2011
Location: Tallahassee, FL
Posts: 1,441
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Quote:
Originally Posted by wa8yxm View Post
Let us assume a 2/1 SWR and a line loss of 10% (Makes math easy)

Transmitter out 100watts
Power delivrd to antenna 90 watts
Power returned 81 watts
Power re-reflcteback to theantenna (Very good tuner) 80 watts
power delivered to antenna 2nd time72 watts
Power reflectd 36 We have now radiated 86 watts

As you can see every time we make the trip we loose line loss. TWICE.

My tuner is o.00 inches or mM from the antenna.. Thus the line loss between tuner and antenna is 0.00 watts.. All my losss are in the wire (Antenna) and tuner... This is because the tuner is the antenna feed point. Long wire antenna. End fed.
Correct in principle, but not in detail. With an SWR of 2.0 at the antenna, the antenna reflects 1/9, not 9/10, of the power coming up the feedline. So (corrected) we have:

Transmitter out 100 watts
Power lost in feedline 10 watts
Power delivered to antenna 90 watts
Power radiated by antenna 80 watts
Power reflected by antenna 10 watts
Power lost in feedline coming down from antenna 1 watt
Power returned to (perfect) tuner and reflected back toward antenna 9 watts
Power delivered to antenna 2nd time 8.1 watts
Power radiated by antenna 2nd time 7.2 watts

Thus, after the first round trip, the antenna is radiating 87.2% of our original 100 watts. An SWR of 2.0 isn't so terribly bad.

Repeating with SWR about 5.8, the antenna reflecting half the power, we have:

*** Note: my math was wrong below, too. I've corrected it now ***

Transmitter out 100 watts
Power lost in feedline 10 watts
Power delivered to antenna 90 watts
Power radiated by antenna 45 watts
Power reflected by antenna 45 watts
Power lost in feedline coming down from antenna 4.5 watts
Power returned to (perfect) tuner and reflected back toward antenna 40.5 watts
Power making its second trip up the feedline and reaching antenna 36.45 watts
Power radiated by antenna 2nd time 18.225 watts

Thus, after the first round trip with SWR=5.8, the antenna is radiating less than 64% of our original 100 watts. Remember that all of the loss is in the feedline. It wasn't so bad at SWR=2, but at SWR=5.8 it's sickening.
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