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Kwhubby
Junior Member
Username: Kwhubby
Post Number: 19
Registered: 12-2004
| | Posted on Friday, November 18, 2005 - 5:50 pm: | 
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Hello, I am trying to figure out something here and have difficulty as it seems that nobody can agree with the same answer. The signal from my "system" has been cranky sometimes being off frequency or the amp can sound really bad. Lately it's been fine and I assume the old problems were due to the faults: loose case ground on radio (for frequency), and a mere class C amplifier on SSB getting- worse when as the battery gets low (distortion).
But I have been told that SWR and/or coax length can be significant in the sound quality of the transmission. Ham information I have read tells me that this shouldn't be the case, specially because my SWR is generally below 2:1. Supposedly the higher SWR will not cause higher 'IMD' distortion. But then again hams are using class AB amplifiers for their 'phone' operation (ssb, am). I read somewhere else that class C amplification needs its system well tuned to perform properly.
The information I heard on the radio from 2 people was that if the swr isn't below 1.7:1 (or even 1.3:1) with full power then it wouldn’t sound good. These two people are A: currently licensed ham and B: former ham. My question is, is this true? and if it is true or somewhat true would lowering the swr increase on power caused by the harmonics using a low-pass filter reduce the distortion?
Also, according to them the coax needs to be 25' long, well mine is 18' and its the stock length that came with my Wilson 1000 trunk lip mounted.
Otherwise I assume that I need to just go get an AB class amplifier (probably a Texas star).
Thanks!
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Patzerozero
Senior Member
Username: Patzerozero
Post Number: 1907
Registered: 7-2004
| | Posted on Friday, November 18, 2005 - 6:59 pm: |
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changing coax length from amplifier to antenna does NOT change SWR's, HOWEVER, at certain lengths, false readings MAY be obtained. you are stuck with the length that comes on MOST mounts, but you can deal with it. jumper lengths between radio & amp can cause problems with swr's when amp is turned on, for that reason, lengths of 6'-12' are usually recommended there. jumpers between amp & swr meter should be in the 6' range as well.
before installing & testing any amp, get the antenna & its swr's set. make sure all connectors are soldered properly & not in danger of falling off. if you want to run an amp, get the swr's under 1.5:1. even magnet mounts can be matched to near perfect, so keep trying.
once swr's are well under 1.5:1, take a ride over some bumpy roads, at higher speeds & if you see the swr's fluctuate, your wilson has a problem with poor connections somewhere, or, it is flying in the breeze & nearing or coming into contact with the vehicle's body. once you're satisfied the antenna is set properly, add that 6'-12' jumper & amp in line. your swr's will probably go up a bit, but as long as they stay under 2:1 at high power, you're good. changing jumper length will probably lower them even more.
class 'C' on SSB CAN work just fine. 
you need to reduce the 'LOSS' of your system as much as possible. each problem with antenna setup compounds itself til your 200 watt texas star is not sending much power out your antenna. poor/cheap coax,connectors, high swr's, high reflect from jumpers or antenna location, etc.
your 'mobile' antenna is far less efficient then a base antenna(where you should still be trying to obtain optimum efficiency)& you need to see that as much of your watts as possible makes it to the antenna & is not lost on its way there. what good is having that 200 watt amp, if more then 1/2 the power never gets to the antenna? |
Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 913
Registered: 6-2004
| | Posted on Friday, November 18, 2005 - 9:00 pm: |
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"class 'C' on SSB CAN work just fine."
Only if your definition of "fine" is that there is output regardless of the quality and amount of harmonics and splatter.
Both AM and SSB need LINEAR amplification because the envelope changes with modulation. Class C amplification isn't linear, so it's not capable of doing that. Class C should only be used with FM, CW or another mode where the envelope doesn't change.
That's why most amateur amplifiers are AB1. They're specifically made to work with SSB and AM. Because they are linear, they can also amplify CW and FM but with less efficiency than class C amplifiers. Most UHF/VHF "bricks" are class C for FM use. |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 1910
Registered: 7-2004
| | Posted on Saturday, November 19, 2005 - 5:16 pm: |
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"class 'C' on SSB CAN work just fine" .
after rewriting my post 3 times, i stand by my guns, in any language. .
it's a several hundred dollar CB, not WNBC...it'll work just fine |
Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 916
Registered: 6-2004
| | Posted on Saturday, November 19, 2005 - 6:14 pm: |
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I didn't write it to convince you otherwise Pat, because I know there is no point in trying to convince you that what you're doing is technically poor - I know you don't care.
I wrote it so someone reading it might think before just blasting away with a poorly made amplifier that is biased improperly for clean operation.
BTW - My $100 CB is probably cleaner in operation than WNBC because it doesn't overmodulate. AM Broadcast stations are infamous for going over 100% positive modulation in an effort to try and sound louder than the next guy. They only back it down when they receive a signal report from a listenener complaining about interference or from the FCC. They're just like you in that they know it's wrong, but they do it anyway. |
Bruce
Senior Member
Username: Bruce
Post Number: 3231
Registered: 9-2003
| | Posted on Saturday, November 19, 2005 - 10:47 pm: |
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CLASS C CANNOT WORK!
That said to get class C you must drive the amp well into flattopping.
Most CB amps DISTORT but on SSB will be linear enough to work ...... even if not up to the best standards.
NOTE on 2 meters my 160 watt amp is not that great either ..... |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 1918
Registered: 7-2004
| | Posted on Sunday, November 20, 2005 - 1:18 pm: |
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i'll sell my gear. all reports of my mobile's signal quality are false. don't know how you could fib to me, 808.
WNBC's signal reports of overmodulation come from the competition, not casual listeners. the casual listener only knows he's listening to the programs' content. he has know clue as to overmodulation, etc. |
Hatchet
Member
Username: Hatchet
Post Number: 56
Registered: 7-2004
| | Posted on Sunday, November 20, 2005 - 9:15 pm: |
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Class C amps do not work fine on SSB. |
Bruce
Senior Member
Username: Bruce
Post Number: 3235
Registered: 9-2003
| | Posted on Sunday, November 20, 2005 - 10:53 pm: |
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PAT a TRUE CLASS C AMP will not pass audio. |
Bigbob
Senior Member
Username: Bigbob
Post Number: 2264
Registered: 12-2001
| | Posted on Monday, November 21, 2005 - 5:13 am: |
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Just like the numbers on the front of amps,class c is a selling gimmick,class c will work but you need a tuned L-C circuit or "tank-circuit" to produce the flywheel effect on the out put,I have an old amp that goes into class c regularly if overdriven,improperly biased,the signal sounds like so much celophane being scrunched,class c without a tuned output sounds just like that,look at some of the old multi-mode radios,they were biased class c for am/fm/cw and ab1 or 2 for ssb,you can get away with it on am because it has a carrier,but you still need a tank circuit to clean it up,class c with ssb is marginal at best with a tank circuit just pure junk without one,so if you're going to a shoot out class c,that's assuming you run am at those,ssb ab-1 or ab-2,oh and by the way a class c amp will use 1/3rd the power of a class ab,if you have 2 amps one says c and the other says ab and they both draw the same amperage and sound good then the c is a ruese,just a selling gimmick.Bigbob |
Bruce
Senior Member
Username: Bruce
Post Number: 3238
Registered: 9-2003
| | Posted on Monday, November 21, 2005 - 7:20 am: |
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Would I run a class "C" amp on SSB ....... not NO ..... HECK NO!
A true CLASS C amp would produce VERY distroted modulation if not cliped almost out compleatly. YES CB amps calling themselfs CLASS "C" will work but only because they are not runing TRUE class C. Back in the good old days of tubes we would bias a tube off 80% of the time or even more and only the peak carrer would turn it on ...... leaving no room tank coil or not for modulation.
NOW I built a class "C" amp for 10 meter FM but even it if you drive it lightly will work SOMEWHAT on SSB ....... sounds like **** but would I use it for that mode ? NO ......... |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 1928
Registered: 7-2004
| | Posted on Monday, November 21, 2005 - 2:09 pm: |
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bigbob, your address isn't valid, sent you a PM.
bruce & dave(who the heck is dave??) & all else-
frank is NOT the builder OR designer of the amp in question, & has NO connection or desire to be associated with it whatsoever. a respected 'big gun' in the amateur world, he agreed that the amp in question IS indeed class C, MORE OR LESS. if MORE OR LESS is the key, then so be it. HE would STILL consider it class C.
his EXACT words as to its use on SSB were '•••• in •••• out'. BUT if the 'in' was good, it COULD work SSB. he DID knock the design of ALL 3 CB amps i showed him schematics for though...and questioned their TRUE class designation compared to what is implied.
what you said SOUNDS like what he was saying, i think, but i couldn't re-read him like i could you, bruce! he DID mention the class 'B' part as you did, bruce, but there still was a 'BUT' as to why considering it class 'C' was allowable.
he DID say it qualified to be called 'C' & it DOES work on SSB, so i'll qualify this with a- CLASS 'C' CB AMP CAN BE USED ON SSB.
best i can do |
Therealporkchop
Intermediate Member
Username: Therealporkchop
Post Number: 467
Registered: 11-2002
| | Posted on Monday, November 21, 2005 - 11:09 pm: |
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Boy howdy....
Well all I can say is I don't think it matters about coax quality, because loss in dB isn't that much under 100 foot. So unless you are talking runs of 100 foot or more, then the crappy cable that comes on your mag mount for example should work fine.
Now if you are having interferrence problems, as I have lately, then yes spring for the best coax you can afford.
NOTHING you do to the coax length will affect SWR. I think Mr. Franklin stated that in his book.
I will agree that the length between the radio and amp, MIGHT, have something to do with it.
But I will also quote someone as saying "MOST cb SWR/Power meters sold on the market are grossly inaccurate".
So if it matters this much to you, get a very good meter first. Something like a Bird which will show reflect. Then use their chart to compute your SWR. Buy the very best coax you can afford and that will work the easiest with your setup. Don't do like me and go buy some super stiff stuff that won't hardly bend just because it is good stuff. If you want to get out that far, set up a base station with a big beam. |
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