Amp coax jumper length

[Captain Kilowatt]

CK , so using a 1/4 wave length of coax ( physical length or electrical length ?) 50 ohm coax the amp would need a 200 ohm input to the amp to to show 50 ohms to the radio ?

Sorry Booty, electrical length. My bad on that. I NEVER talk in physical lengths as it means absolutely nothing unless you plan on using it as a clothesline or to tell someone how much cable I need to reach from point A to point B. A 200 ohm load on the end of 1/4 electrical wavelength of cable will present an impedance of only 12.5 ohms at the radio end. It's a simple ratio as referenced to 50 ohms. 200 is four times 50 so the other end will have 50/4 or 12.5 ohms. Now remember, that is with a 1/4 wavelength of cable which kind of has a special relationship just like a 1/2 wave does. Other ratios are possible with different lengths. If the amp did have a 200 ohm input impedance it could be matched to 50 ohms by some odd ball length of cable hence the trial and error method most often used.

 

[ROC1]

Because he asks questions like other newbies do, please have a little patience, we all run it to people that are....how do I say it....a little slower to learn than others.
He can be a pain in the ass at times, but we put up with it. ( he really is not that bad,,,just difficult at times)
Sometimes we have to draw pictures, but after a while he learns and so do others who are too afraid to ask what some may consider Dumb questions.
Captain posted true Dat.
There are a lot of crappy CB style amp`s out there that have shit for input tuning, many guys do not understand that but that do understand that using a different length jumper solves the problem....it is good that those of us that understand WHY try to help those that do not understand WHY to help increase there knowledge of what is going on..... The proper thing is to FIX the input tuning, but some guys do not get what is going on.......
I do understand it is frustrating at times.
There are almost infinite matching conditions with different lengths of coax, many things affect this, velocity factor input impedance, length of coax, frequency being used, load impedance etc, as a simple rule, most CB guys just say use a XXX length jumper and it will be OK.

73
Jeff

well put... I'll be good now.

 

[BOOTY MONSTER]

If the amp is presenting a 50 ohm load at the input you can run any length of coax and it will present 50 ohms to the radio. If it's not you're better off cutting the coax at 1/2 wave-length. Or, if it's not and you have the means to model the transmission line you can cut it specifically to present a 50 ohm load to the radio - assuming the load at the input isn't waaaay out there, which it shouldn't be.

Regardless you won't see much difference between between a six foot jumper and a 12 foot jumper. Line losses don't really impact unless the line is really long or you're running gobs of power.

thank you CK .

does the 1/2 wl roc recommended also act as a transformer ? im of the impression a 1/2 wl or multiple of it has the least effect on a signal ...... other than db loss for longer runs compared to a shorter run .

 

[W5LZ]

It acts like an 1:1 transformer. What it sees at one end it presents at the other. Or, it doesn't do any 'transforming' at all. Take your pick on how to describe it. Then take two aspirin, a donut, little lemonade and take a nap... dang, that sounds nice, think I will.
- 'Doc

 

[BOOTY MONSTER]

do i dare wonder why he recommended that length if it doesn't have the effect hes been discussing ?

 

[Captain Kilowatt]

do i dare wonder why he recommended that length if it doesn't have the effect hes been discussing ?

What Doc said is correct,a 1/2 wave simply repeats what it sees at the far end. there is no impedance transformation. As for why 1/2 wave was suggested,IDK. Maybe a typo? If not perhaps ROC will tell us why he suggested a 1/2 wave length. He was right about things when he said

" Or, if it's not and you have the means to model the transmission line you can cut it specifically to present a 50 ohm load to the radio - assuming the load at the input isn't waaaay out there, which it shouldn't be."

 

[BOOTY MONSTER]

i guess i focused too much on the 18 ft part . im also of the impression coax has to be coiled to form a balun to change impedance . for years ive been on this forum and the big brains have said over and over that coax length doesn't matter (other than loss when comparing a longer run to a shorter run) and that changing jumper lengths with a amp to get a better vswr doesn't really change anything . this is the first time i've herd of a jumper being a transmission line/changing impedance for a amp with poor input tuning . i've read about it with some weird (for a cb'er) antennas way outside of the cb band and figured it was one of them frequency dependent things..........

i still think ROC was wrong about the 18 ft comment , and that's what my initial post disagreeing with him was commenting on . he got pissed and flamed me and i got pissed and rode his azz about it . it seems at this point that i owe him an apology since his "Or, if it's not and you have the means to model the transmission line you can cut it specifically to present a 50 ohm load to the radio - assuming the load at the input isn't waaaay out there, which it shouldn't be." comment has merit . i think i do well when folks explain why im wrong (when i am) without flaming me , but when i get flamed it kinda pisses me off .

ROC , i apologize .

 

[Beetle]

Booty - no, if you're inserting a quarter-wave section of coax to act as an impedance transformer, it doesn't need to be coiled at all. I use FIVE such matching sections of RG-11 to feed my 5-band quad. I coiled the excess lengths rather than just leaving them hang. For 10 and 12 meters, I used 3/4 wavelength sections to give enough slack in the feedline for the antenna to rotate freely.

From the shack, I have ONE RG-213 feedline which runs to a remote coax switch. From the switch, the individual driven elements are fed with the RG-11 sections. From about 100 ohm feedpoint impedances to very close to 50 ohms.

On 10 and 12 meters, I had to use 3/4 wave sections in order for the antenna to be able to rotate freely. 1/4 wave would have been too short, and would have been disastrous. Any ODD number of quarter-wavelengths has the same impedance-transforming characteristic.

 

[Captain Kilowatt]

Booty, I think you are getting confused about the coiling of the cable because people talk about coiling the cable up at the feedline to make a choke balun. The problem is that the proper name for it is simply "choke" leave out the "balun" part. It is in no way a balun because it does not convert from BALanced to UNbalanced. It simply chokes off the RF that appears on the feedline as a result of feeding a balanced antenna (dipole etc.) with an unbalanced feedline (coax).

 

[5150]

simply put ...

if the out put on the transmitter (radio) is 50 ohms
ant the antenna measures 50 ohms
and the amp is set at 50 ohms
then use whatever length coax as long as its good coax


but if your radio is putting out 50 ohm and your antenna is set at 36 ohms
and you attempt to adjust it out by different lengths of coax

the only thing you will accomplish is "meterbaiting " your ego! as the meter can not see the wave in relation to where its measured to the relationship to the position of the wave!

coax is just a feed line , from point "A" to point "B" a transmission line between the two

 

[Robb]

simply put ...

if the out put on the transmitter (radio) is 50 ohms
ant the antenna measures 50 ohms
and the amp is set at 50 ohms
then use whatever length coax as long as its good coax


but if your radio is putting out 50 ohm and your antenna is set at 36 ohms
and you attempt to adjust it out by different lengths of coax

the only thing you will accomplish is "meterbaiting " your ego! as the meter can not see the wave in relation to where its measured to the relationship to the position of the wave!

coax is just a feed line , from point "A" to point "B" a transmission line between the two

Hey Fitty:

Sometimes the input of an amp doesn't have a 50 ohm INPUT impedance. It can sometimes have a problem and isn't at 50 ohms; sometimes more or less. That is what they are talking about here. Otherwise, if everything else was correct and equal, you would be right.

Be talking to ya on the band . . .

"Two Shoes"

 

[eagle1911]

Seems to me that if only more builders included a single strategically placed variable capacitor (cost ~$2.00 in the worst case) in the input network of their amplifiers this would become one of the easiest issues to correct.. just give that cap a twist until you read the lowest input SWR. Then jumper length should be irrelevant..

 

[AudioShockwav]

coax is just a feed line , from point "A" to point "B" a transmission line between the two

I agree, as long as the Radio, Amp, and antenna all present 50 ohm loads.

But we (they) often see Crappy CB style Amps that have screwed up input.

Just as in a full 1/4 wave whip install, because of how it might be mounted, will not show a 50 ohm load.

The thing to do would be fix the problem at the feedpoint, but the generally accepted practice is to "trim" the Coax to allow the Radio/Amp to see a 50 ohm load by using the coax to act as an impedance transformer.
this is where there is often confusion when a newbe ask`s:

How much Coax do I need to put on my XYZ antenna to make it tune right.
A dozen guy`s say:

Just enough to get from the radio to the antenna, and that is the correct answer, as long as the Radio, (Amp,) and antenna all present 50 ohm loads.

73
Jeff

 

[Robb]

Seems to me that if only more builders included a single strategically placed variable capacitor (cost ~$2.00 in the worst case) in the input network of their amplifiers this would become one of the easiest issues to correct.. just give that cap a twist until you read the lowest input SWR. Then jumper length should be irrelevant..

Well said, and I agree.

Bought a cheap Palomar 100w amp awhile back, and it has one in there.

 

[W5LZ]

And then you get to the point where if there's a problem, fix that problem where it exists, not by changing something else to compensate or hide it. If a tunable input on an amplifier is a good idea, then a tunable output on that amplifier is an even better idea. There goes that "one size fit's all", 'no tune' amplifier! mainly because there is no "one size that fit's all", and there are no antennas that have a normally occurring 50 ohm input impedance without an impedance matching device being used in/on that antenna. Not a 1/4 wave, nor a 1/2 wave, a 5/8 wave, or pick any 'wave' you want, none of them are 50 ohms without help. Or better yet, say '50ohm +/- 0j' instead of just 50 ohms. It does make more sense.
Chew on that for a while...
- 'Doc