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Message |
    
Cornbrown
Member
Username: Cornbrown
Post Number: 77
Registered: 7-2005
| | Posted on Wednesday, November 23, 2005 - 6:55 pm: | 
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Can someone tell me what's happening when I dead key on channel 1. My SWR is 1.4:1 when I modulate the SWR goes down to 1.2:1. Can someone explain whats happening.
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Bruce
Senior Member
Username: Bruce
Post Number: 3250
Registered: 9-2003
| | Posted on Wednesday, November 23, 2005 - 7:21 pm: |
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Sounds to me that the diode's in the VSWR bridge are reacting to the audio. I've never seen VSWR measured with audio and dont see any point to doing that. |
Cornbrown
Member
Username: Cornbrown
Post Number: 78
Registered: 7-2005
| | Posted on Wednesday, November 23, 2005 - 8:04 pm: |
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My SWR meter ia always inline.So I'm able to keep trak of my SWR. I just notice that the sses drops when I modulate. |
Moonraker
Member
Username: Moonraker
Post Number: 84
Registered: 3-2005
| | Posted on Wednesday, November 23, 2005 - 8:06 pm: |
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If your deadkey is set too high, your modulation will swing backward on the average power, but still swing forward on pep. Your power goes down and your swr "appears" to drop. I noticed this on one of my radios and that is what I found. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 497
Registered: 11-2001
| | Posted on Thursday, November 24, 2005 - 12:52 am: |
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I agree with those that suggest the dead key is set too high relative to the max output of the transistor. There is just no swing available. |
Cornbrown
Member
Username: Cornbrown
Post Number: 79
Registered: 7-2005
| | Posted on Thursday, November 24, 2005 - 2:42 am: |
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The only thing that drops when I modulate is the my swrs like I said before. Peak Power swings forward with modulation. Average power is steady no back swinging.. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 499
Registered: 11-2001
| | Posted on Thursday, November 24, 2005 - 9:23 am: |
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Ok Cornbrown, I think it has something to do with the length of line used, maybe a longer line will make this more obvious.
When there is a mismatch the reflected is added to the output of the radio, like 4 watts out and 1 watt reflected makes for 5 watts out at the meter. This is why we often hear guys claim they are seeing more watts out than they should, they just have done something to increase the mismatch somehow.
Well there is more to it. Each time the reflected returns down the line the power is diminished some by natural line resistance so the notion above is not 100% gain in output. I think that what you are seeing has something to do with all of this business. I can see it in my mind, but I don't seem able to get it down on the paper at this time.
Marconi |
Therealporkchop
Intermediate Member
Username: Therealporkchop
Post Number: 475
Registered: 11-2002
| | Posted on Friday, November 25, 2005 - 12:18 am: |
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That's normal to see that. You actually want to see the needle move backwards, not forward.
Checking modulated SWR is a good idea in my opinion, cause at key you might have a good SWR, but during power increase (swing) it could go up.
Too high or too low (if there is such a thing) dead key has nothing to do with this. Too high a dead key will cause backward modulation, but not falling SWR readings.
I would follow someone else's advice here about checking SWR. If you have an amp, the SWR meter should be before it or any other accessory you have inline. Meaning it should come right after the radio. Calibrate the meter in the middle of the band (20). Then check it on 20. Write it down. Then turn the radio to channel 1 and check it there and write that down. Then turn it to 40 and check/write that down. DO NOT calibrate the meter when you change to channel 1 or 40. There will be a lot of arguement over this, but Lou Franklin suggest doing it this way, as well as Wilson, Firestik and a few others.
If your SWR is higher on 40 (higher freq) the antenna is too long. The higher the freq, the shorter the antenna. If it's lower on 1, then it's too short. Which ever channel it may(might) flat line on, that is the freq that the antenna is resonant on. It might not flat line anywhere in the band.
Hope this helps... |
Marconi
Advanced Member
Username: Marconi
Post Number: 502
Registered: 11-2001
| | Posted on Friday, November 25, 2005 - 11:11 am: |
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Moving a meter around in the line to any antenna that is not resonant and truly resistive is likely to show you different results on an SWR meter.
I agree to this with Sterling that it is a good idea to know what the SWR condition is between the amp and the radio, but to be more precise right at the radio is better. If you have a long line between the amp and the radio you could still find a reactive point at which to place the meter, and then the meter would likely show yet another reading if the load is not well matched to the line. Again moving the meter around at different spots in a line can appear to be a problem with SWR if the load is not right.
At this point I tend to agree with Sterling when he says "Too high a dead key will cause backward modulation, but not falling SWR readings." I did suggest the opposite above and I think after checking further, I was wrong in that. However I would change his statement to say "..., but maybe not falling SWR readings."
Now, to get back to Cornbrown's problem. On this end I can duplicate fairly close just the conditions he describes, but I cannot duplicate his results, unless I change frequencies. I think this condition may change with frequency.
Cornbrown try this and let us know if going higher in frequency makes the responses you are seeing change, get better or worse, and in the process do you see a point where the response goes away. You may need to go beyond the CB band in order for this to work, if I'm even close here.
If it makes a difference then I suspect what Cornbrown sees here has to do with the reactive part of his load impedance in the antenna, but I can't be sure of that at this point.
It could be his meter, like Bruce suggested too. Cornbrown, have you tried changing the meter?
Marconi |
Cornbrown
Member
Username: Cornbrown
Post Number: 80
Registered: 7-2005
| | Posted on Friday, November 25, 2005 - 4:21 pm: |
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Marconi... the higher I go up the band the less my SWRs drop with modulation. but I am not going to worry about it because my swrs are low and I think I prefer them going down with my modulation and not going up. I think this is happening because of all the equipment I have inline. AMP,radio, two low pass filters, and jumpers. Note I don't have this problem barefoot. |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 882
Registered: 9-2004
| | Posted on Friday, November 25, 2005 - 11:40 pm: |
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the SWR meter swinging backwards is normal. it is just a characteristic of how the circuit works.
it means nothing when setting your SWR, as the meter is not calibrated at an audio rate. this is why modulation meters dont give a true indication of whats really going on. the circuitry required to calibrate such a meter makes it too expensive for the hobbiest market.
if you want to see what your SWR is while you are modulating, you have to calibrate the meter while saying, "ahhhh" into the mic. then, say ahhh again while checking the SWR. itsnot the most accurate method, but then again ive never seen a reason to check my SWR with the mic gain turned up, unless i was checking to see if the mic was working or not.
bottom line. your SWR meter is working correctly.
turn your mic gain all the way down when you are checking SWR.
matt |
Therealporkchop
Intermediate Member
Username: Therealporkchop
Post Number: 479
Registered: 11-2002
| | Posted on Saturday, November 26, 2005 - 12:46 pm: |
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Well put Marconi. And I agree.
However...according to Lou Franklin and someone else I won't name, certain lengths of coax act as an impedance repeater. And if I comprehend the paragraph correctly, any mismatch would be repeated and increased on a meter. Showing a high SWR when in fact, one didn't exsist. OR showing low when actually a high SWR exsisted. That was what I actually meant in conclusion to what I've said.
Franklin also states that a power or swr meter should be inserted into the line before any accessory like an amp or filter, not directly after the radio. Although other than the length of coax being electrically too long or short (repeater) I don't see where it matters as to whether you put it directly after the radio or directly before the amp.
I guess if you could join the radio/meter/amp with couplings instead of coax, then you could get a better reading from amp to antenna, but then you'd change that once you started with jumpers.
And even though I trust Lou and what he says, he also states "don't take my word" and to "read any and everything you can find". I guess he's trying to say that he feels he is right, but there is always the chance he could be wrong.
Moving a meter around in coax would give different reading if it was a impedance repeater. Also moving coax physically can help tell whether or not it's bad, because of the center conductor moving around inside the insulator.
And like Kid said...yeah your meter is working correctly.
Although I will say most meters are inaccurate and the best way to test power and swr would be with a directional meter (Bird or Coaxial). Read the reflected power and compute that into SWR. If you want to read modulation, get a scope. But that's my 2 cents and nobody ask for all that. |
