| Author |
Message |
    
Kennjen
Junior Member
Username: Kennjen
Post Number: 27
Registered: 6-2005
| | Posted on Tuesday, July 12, 2005 - 12:08 am: | 
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Now that I have a swr/power meter, I'm confused about these 2 terms.
On my power meter on AM, the power meter don't swing when I talk. I THOUGHT that that is what "Swing" meant..... Could some one enlighten me on how this swing is measured ?
While I'm at it, how do I measure modulation ? And what the heck is it ?
Now that I've gotten moderately in to this "CB" thing, many things are clear to me, but I still need to be "Elmer'ed".....
Thanks a bunch for any answers.
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Brewdirect
Intermediate Member
Username: Brewdirect
Post Number: 135
Registered: 7-2004
| | Posted on Tuesday, July 12, 2005 - 1:05 am: |
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Swing is measured in PEP mode.
Basically you can measure RMS which is the "average" signal or PEP which will show the peaks and lows of your signal.
If you meter is only a RMS meter often you won't really see any swing.
Swing would be noticed when...you deadkey (aren't talking) and then you talk and the needle swings forward. |
Treetopper
New member
Username: Treetopper
Post Number: 3
Registered: 7-2005
| | Posted on Tuesday, July 12, 2005 - 3:13 am: |
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To be more of a help you need to tell us what unit of swr/power meter you have for some one to explane it to you.
AM= Amplitude Modulation
FM= Frequency Modulation
SSB= Single Side Band = Upper Side Band / Lower Side Band
CW= Constant Wave ---…---…---
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Hotwire
Intermediate Member
Username: Hotwire
Post Number: 432
Registered: 1-2005
| | Posted on Tuesday, July 12, 2005 - 10:26 am: |
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On AM you should see a little bit of needle swing but not much with a stock radio and most of the time the modulation will always be 100% Its when your ded key is low and you run an amp when you see big swing. Try going to SSB and then you should see the needle swing forward when you talk. Just make sure the needle does nothing on SSB when you are keying but being silent. |
Kennjen
Junior Member
Username: Kennjen
Post Number: 28
Registered: 6-2005
| | Posted on Tuesday, July 12, 2005 - 10:32 am: |
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Thanks both,
I was talking mainly about AM. And I have a cheapo Pyramid(SWR-14) swr/watt meter.
I know the concept of AM, FM, SSB, and CW. I just don't know how to measure these concepts. I was especially surprised to find out that my Power meter hardly moves, never mind "SWING", when I talked...
I guess my meter is RMS. I take it that is why it's not swingin'. Funny thing is, it does swing when I'm on SSB. So, many questions.
I'll search a bit more for faq page dealing with these issues. If any one has a link for this type of info, I'd appreciate that.
Thanks. |
Tech833
Moderator
Username: Tech833
Post Number: 1024
Registered: 8-2002
| | Posted on Tuesday, July 12, 2005 - 11:27 am: |
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First, Treetopper: CW stands for Carrier Wave. Carrier on or carrier off gives you the side tone in the receiver.
Second, AM is made by varying the power output of a carrier. When you talk, the power output of your radio goes 2x over carrier power (+100% positive peak) to zero power (-100% , or 100% negative peak modulation). Do this at an audio rate, and you get Amplitude Modulation.
What the CBers call 'swing' is the positive peak power level. You need a peak reading meter to measure it. Copper does sell some peak reading meters.
However, it IS possible to run more than 100% positive peak modulation, which adds loudness in the receiver, but also adds distortion. BTW, it is impossible to run more than 100% negative peak! Off if off. On a 100% negative peak, the transmitter is putting out absolutely zero power to the antenna. That is why it is common for a person 'overmodulated' (running more than 100% negative peak) to be 'walked on' by someone who is not overmodulated. While their transmitter is off so much, your signal comes right through. Clearly, this happens regardless of the power level of the overmodulated station. A 5 watt radio can walk on a 1,000 watt radio if the big one is running overmodulated.
Back on point, running more than 100% positive peak means you will see an RMS meter advance when the transmitter is modulated. That is because the average power rises over carrier alone. |
Brewdirect
Intermediate Member
Username: Brewdirect
Post Number: 136
Registered: 7-2004
| | Posted on Tuesday, July 12, 2005 - 11:57 am: |
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SSB is ALL swing ....sound like you need to read up on the diff between how AM and SSB signals are produced  |
Kennjen
Junior Member
Username: Kennjen
Post Number: 29
Registered: 6-2005
| | Posted on Tuesday, July 12, 2005 - 1:12 pm: |
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HUH...I thought SSB was basically, one sided AM... If AM don't swing on RMS meter, why would SSB ? Hmm.
Yeah, I'll RTFM a bit more, as it were.
I'll be back with more questions when I hit a road block again.
SO much to learn, so little time. I got to study for my tech tix too....
KEN
Thanks all, |
Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 540
Registered: 6-2004
| | Posted on Tuesday, July 12, 2005 - 2:45 pm: |
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AM doesn't swing much if it's done properly with a full power carrier and a functioning limiter. A 100% modulated AM signal will show 50% more AVERAGE power than just the carrier. So if you have a 4 watt carrier and you modulate that carrier 100% you will see 6 watts on an AVERAGE reading power meter. HOWEVER, because of the dynamic range of voice you will usually see less than 100% average modulation - typically about 30%-40%. So instead of seeing a 50% increase of the carrier power you'll see about 30% of that 50% or about 15% more AVERAGE power over the carrier. That's not much swing.
I deal with average reading power meters because PEP meters are basically worthless unless they have a powered movement (I've never seen a CB meter that does) and even then you are asking an analog movement to follow speech which varies at a rate of up to 3,000Hz. That's too fast for any analog movement to follow. The only device that is capable of the speed necessary to show the modulation envelope is an oscilloscope. It's also the only device where you'll actually see the 4:1 ratio of PEP to carrier. |
Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 541
Registered: 6-2004
| | Posted on Tuesday, July 12, 2005 - 2:49 pm: |
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An AM signal is comprised of a carrier, an upper sideband, and a lower sideband. A SSB transmission filters out the carrier and the other sideband so only one sideband is transmitted. Because of this there is no power output unless there is modulation. That's why you need to use the clarifier to receive a SSB transmission. It reinserts the carrier so the receiver can demodulate it. |
Bigbob
Senior Member
Username: Bigbob
Post Number: 2040
Registered: 12-2001
| | Posted on Tuesday, July 12, 2005 - 8:09 pm: |
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I know what most people here think of radio shack,I can already feel my ears blister,but I have one of their digital avg/peak swr/power meters for ranges on power it has auto,20,200,2000,for swr it has 0 to infinity.I was curious and hooked a 4 watt am cb to a coat hanger to check swr,49.9 to 1,I wonder what percentage of loss that is.But anyhow into a dummy load my 2517 shows 24.7 watts on ssb with an ahhhhh,normal voice,with a loud voice it jumps to 35.6 then settles to 24.7.Now as an aside is this all the radio is capable of on this meter,it drives my boomer 400 nicely 430 avg.560 peak on ssb with no major heating(warm to touch).Now is this meter feeding me junk or does this compare to what others get with other meters using dual final radios,I know the S-9 blows me out of the water,but am more interested in the meters comparison,I paid a 100$ for this new so I expect it to be better than average,but I know you all call it radio ****,99$ for the name and 1$ for the meter.Bigbob |
Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 545
Registered: 6-2004
| | Posted on Wednesday, July 13, 2005 - 6:35 am: |
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I own two of those Bigbob, and I bought them both on clearance. I think I paid $30 for one at a store and only $10 for the other at a hamfest because it was missing the manual and the screws to mount the bracket. I love going through the boxes at the Radio Shack stand at my local hamfest. There's always something good and it's usually stuff that they're about to throw away so they're willing to deal.
It's okay for what it is, but I wouldn't trust the PEP measurements anymore than other PEP meters. I've used it mobile and base and overall I like it. The instant VSWR reading is handy, and it's sorta cool to have an LCD meter with a remote head. |
