why do class C amps need certian jumper lengths ??

[BOOTY MONSTER]

i never read about hams needing certian coax lengths , but class c folks usually have to use a certian length of coax to get things to tune and operate right . why is that ? are class C amps not typically tuned for 50 ohms input and output and is that the culprit or is it something else ??

 

[Captain Kilowatt]

It's not just the class C amps,it's pretty much all CB type amps.The reason it seems to be just class C is because the vast majority of CB amps out there are class C. The reason is that they are built cheaply and most often do not have a tuned input or output.They do not present 50 ohms to the driver radio nor do they have a nominal 50 ohms output impedance.This causes an SWR of more than 1:1 and using a jumper is a cheap way to transform that impedance to 50 ohms. That is the reason so many people use a diferant length because of the differant impedance from amp to amp. Hams for the most part use amps that have a tuned input on the amp.Some of the older amps had no tuned input but that was back in the day of tube finals in the radios and they would tune into almost anything.

 

[190]

I have a new texas star dx500v and a older vintage palomar 4 pill magnum and both of them have perfect input and output swr readings. truthfully i was really surprised since ive seen other texas stars that the swr in between the radio and amp were really bad. other cb amps that i have had that have great input and output swr is the palomar 225 and 350 blue face amps. i have a old sweep tube sos electronics scout 20 and it has a input tuning pot on the back of the amp.

 

[AkBowtie]

It is a strange one Booty. I've always used whatever amount of coax it took to get to where i needed it. Whether it was a C box or a Texas Star or Palomar, I've never had any issues.
Maybe it's grounding issues with the radio and boxes .
Maybe proper coax jumpers being soldered together.
Maybe it's Comp antennas and not having proper height on the shafts per vehicle.
no idea what it is, but I've always went with whatever length it took to get to where i needed it and I've never had a problem.

 

[Drifter]

i never read about hams needing certian coax lengths , but class c folks usually have to use a certian length of coax to get things to tune and operate right . why is that ? are class C amps not typically tuned for 50 ohms input and output and is that the culprit or is it something else ??

I have several Fat Boys, Xforce, a cobra, and a messenger. All are cheap cb amps. I don't have any input SWR problems. The bigger base amps I have all have adjustable input tuning.

 

[W5LZ]

The average run of 'CB' amplifiers tend to have matching problems simply because of their design using 'no tune' input/output circuits. There just aren't any simple/cheap 'one size fits all', 'no tune' circuits that are not a compromise. And since there's always a 'price' to pay for convenience, and since convenience is the only reason for those 'no tune' circuits, which do you think will be the place where that 'price' comes due, in convenience, or performance/capabilities?
Another aspect of it is that 'jockeying' jumper lengths is much easier to do to make that meter read what you want it to, than doing it right. More people/users know how to use a yardstick than correcting the winding of a transformer, or how to correctly 'twiddle' a knob or two.
So it sort of boils down to just how lazy you may be.
- 'Doc

 

[dudmuck]

Class C amps will have input VSWR issues at low RF drive levels.
The reason is that the pills are starting to shut off at reduced RF input drive, which makes the input appear to be de-tuning.
With class C amps, its the "inertia" of the input/output transformers that fill in the gaps when both pills (in a push-pull) are turned off. The trouble is that as you reduce drive the pills are turned off for longer and there is not enough inertia to hold the entire cycle.

The texas star is different. The bias keeps the pills turned on at no RF or low RF input.

 

[W5LZ]

Just how low of a drive level are you talking about? If there's enough input to drive the thing at all, that's typically enough to drive it to full efficiency. Unless you are talking about levels in the milliwatt range, I haven't found that 'not enough drive' thingy to be true. I don't think I've ever heard of anyone driving with only milliwatts.
- 'Doc

 

[9C1Driver]

I'm not buying into that. Texas Star can be some of the worst as far as impedance mismatch. I have owned about 6-8 Texas Star amplifiers over the years, half were pretty good and half were way off. As opposed to sending it in to be re-tuned to 50 ohms I played with jumper lengths to make it work.

On another note, with hand built amplifiers like X-Force, Fatboy, and Joker I have never had a bad match. They all tuned down to a nice low SWR. I usually run a 3 or 6 foot jumper on all my amplifiers.

 

[dudmuck]

Just how low of a drive level are you talking about? If there's enough input to drive the thing at all, that's typically enough to drive it to full efficiency. Unless you are talking about levels in the milliwatt range, I haven't found that 'not enough drive' thingy to be true. I don't think I've ever heard of anyone driving with only milliwatts.
- 'Doc

not down to milliwatts. perhaps below a watt though not enough drive to keep class C pair of 2879 turned on sufficiently.

Consider the basic fact that if less than 0.6v on the base a transistor is off. That means with a push-pull pair less than +/-0.6v on the output if the input transformer would not turn on the pills at all. Now consider that a 1watt signal at 50ohms is 20volts peak-to-peak, and the input transformer converts 50 ohms down to a lower impedance (meaning more current/less voltage), and it should be easy to imagine that less of the signal will be above the 0.6v threshold.

Also, you can see good input VSWR on a class C amplifier if it has an input attenuator.
For example, if you have a 3dB attenuator for 50 ohms: if you have a short or open on the other end you will only see a 6dB return loss on it which is a vswr of 3. In fact I would recommend an input attenuator for these very reason for a radio that could over-drive an amplifier.

I am not speaking on the quality of texas star amps, only upon the concept of why a biased amp should give you a better input vswr at low drives. If the amp aint tuned right or something else is designed poorly, then its all moot.

 

[Chainsawgang]

Try a cheepie BOOMER or fake Palomar you'll see impedance matching issues that will drive you nuts ! I have spent too many hours working with guys trying to get these splatter boxes installed !

 

[frankenstein 320]

the onely way to phase,to get true readout

i never read about hams needing certian coax lengths , but class c folks usually have to use a certian length of coax to get things to tune and operate right . why is that ? are class C amps not typically tuned for 50 ohms input and output and is that the culprit or is it something else ??

take 468 devide by your operating freqency this will give you a physicale half wave at your freqency.then take your velosity factor of your coax like .66 depends on typ of coax type that will give you a half wave elecricly throue a conductor. this is the lengt and the onely length to run between raidio and amp. this will phase them together,same thing from amp to antana use multipls of the electricle length.remember the antana and coax is a circete and needs phased. this onely aplise for one freqency as soon as you chang the chanele it will be of a litle bit but why not be perfict on the chanle you talk on. remember that every half wave the impeadents is 0 that means 0 at your radio 0 at your amp and 0 at the antana.this is the onely way to get a true readout at every end. your swr might go up a little but that is a true reflection now you can tune up antana or amp.

 

[frankenstein 320]

bump down

 

[frankenstein 320]

try a palomar 900v

Try a cheepie BOOMER or fake Palomar you'll see impedance matching issues that will drive you nuts ! I have spent too many hours working with guys trying to get these splatter boxes installed !

a 2290 and a 1446 and a 2879 are all the same pills in all the same boxes you got to remember the rule of thume when it comes to mass production and that is buld it as cheep as posible.you can run 1000 watts with the right circetry and know how.check this out rg8 coax yes the thick black stuff might handle a 1000 watts or whatever its rated at but onely has a 40DB shilding factor on it.that means anything over 10 watts is not shealded.wonder why every one bleeds. 1000 watts is about 60DB so find a cable that has a 60 to 90 db shilding on it and then you can drop the mall all day long without bothering anyone.make shure every thing is phased right to remember the antana and coax is part of the raidio circet.

 

[Moleculo]

take 468 devide by your operating freqency this will give you a physicale half wave at your freqency.then take your velosity factor of your coax like .66 depends on typ of coax type that will give you a half wave elecricly throue a conductor. this is the lengt and the onely length to run between raidio and amp. this will phase them together,same thing from amp to antana use multipls of the electricle length.remember the antana and coax is a circete and needs phased. this onely aplise for one freqency as soon as you chang the chanele it will be of a litle bit but why not be perfict on the chanle you talk on. remember that every half wave the impeadents is 0 that means 0 at your radio 0 at your amp and 0 at the antana.this is the onely way to get a true readout at every end. your swr might go up a little but that is a true reflection now you can tune up antana or amp.

If the input impedance of the amp matches the output impedance of the transceiver, then it doesn't matter what length the coax jumper is. If there is a mismatch between the two impedances, then the coax can act as a matching transformer and then length of it will affect the SWR reading at the transceiver. An electrical 1/2 wave jumper will allow you to see what the input impedance of the amplifier/antenna actually is, but it will not provide any matching transformer characteristics. Don't get me wrong...using the coax as a matching transformer is a compromise.

The biggest issue with using coax to affect SWR readings is that as soon as you change bands, the coax length you were originally using is no longer valid. If I have an amp that I use on 5 bands, I need another way of matching the input impedance than messing with the coax. You can either make the input impedance broad-banded enough to handle the various bands, or you can use some sort of band switching mechanism that deals with it. CB amps only have to deal with one band so if the cheap builders can't get the input impedance right, it's easy enough for them to just tell people to fool with the coax.