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SWR/power questions

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  • 2 Post By Captain Kilowatt
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Old 02-08-2012, 08:47 PM
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Default SWR/power questions


I have a Galaxy 949 that's been peaked and tuned. I also have a Cobra 19 thrown under the seat with a cig lighter soldered to it to power it. I tuned the 949 SWRs in the past for a 5' Firestik and a Lil Wilson. I checked it again today to see what's up with the power on the radio's meter as to why it has appeared to have decreased over time and this is what I got.

The radio and truck stop meter with a 3' jumper to an 18' coax to a lil wilson, did the same with the 5' Firestik also with almost identical results. The SWR with the meter inline is 1.5:1-2.5:1, this reads the same on the meter and the radio. With the meter and jumper out, the radio reads .5 lower each end and flat lines with the firestik. Now the power on the meter with the 949 reads 35 spiking and holding 30ish, yet the radio meter reads only about 18 spike and holding 15. The 19 only got 4-5 watts. I think the meter is a RoadPro.

Do these readings sound correct? Do I need a shorter or longer jumper? Is the radio's power meter failing?


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Old 02-09-2012, 03:27 AM
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Are those SWR readings at the ends of the band (1 and 40)? If so, then the antenna is probably about as 'tuned' as it'll get.
As for the difference in the power readings, I'd say the meter in the radio is reading Pep and the external meter is reading Avg.
It all sounds about normal to me. Of course that assumes several things such as the antennas being properly tuned, and the SWR meter being calibrated for the amount of power you are using to check things.
Would changing the length of the feed lines with different length jumpers make any difference? It can certainly change the readings on that meter if the antenna isn't tuned properly, and if the output of the amplifier isn't really close to what it should be. But that won't change any mismatching in the antenna system, it only makes the meter read differently. If you expect either of those antennas to have an SWR that's near 'perfect' across the whole band I'm afraid you are going to be disappointed, they aren't capable of that.
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Old 02-09-2012, 04:25 AM
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The SWR readings are 1 and 40. So the SWR deviation from having the jumper and meter inline and then removed is also to be expected. I have a 7' FireStik I had back in the '90s tucked away. I put it on and it doesn't budge the SWR needle on the radio. Thanks Doc.

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Old 02-09-2012, 05:00 AM
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Short and simple answer is that the antenna is not tuned to provide a perfect 50 ohm match at the base of the antenna. When this occurs changing the length of feedline, like adding a meter and jumper inline, will affect the meter reading at show the SWR value at the meter. Since the inline meter is a few feet from the radio it will read a different SWR value. This is why you will hear people talking abouthow long of a cable do they need between the radio and antenna or between the radio and amplifier for it to make a good match. bottom line is if the SWR changes at the radio end of the cable when you change the length of feedline then the antenna needs to be tuned better.
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Old 02-09-2012, 06:27 AM
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I had a 3' FireStik originally, but due to the mounting location it wouldn't tune. The 5' is better, but the 7' gets over the fiberglass camper of my truck and is tuned well. I know the back corner of the bed is not the best location functionally, but it's where I want/like it. The 7' is just too long for me to ride around civilization with and have it whack everything.


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Old 02-09-2012, 02:40 PM
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Wilson 1000 Mag mount right on top of the roof and you'll see a big boost in overall performance and it will be way easier to tune for better SWR as well.

Looks wise it will actually look less obnoxious than your white fiberglass antennas and will suit the truck better.
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Old 02-09-2012, 03:20 PM
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I run this black 5'. I had the white one years ago and got it out to see how it fared against the camper shell. The 5' does good. The Lil Wilson is for my car. I may get a 1000 before long.


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Old 02-13-2012, 03:11 PM
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Quote:
Originally Posted by Brake Weight View Post
I run this black 5'. I had the white one years ago and got it out to see how it fared against the camper shell. The 5' does good. The Lil Wilson is for my car. I may get a 1000 before long.


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