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Adjusting SWR with amp

You still with us T-Bone ? :) After a bout with Doc and Beetle you could sometimes be worse off then when you first got started :p It has a way of getting deeper then one antisapated :? It's all good ! Really sounds like your on your way here dude. That harmonic one had me gasping for air :oops: Good defination Beetle
 
rf harmonics are a common cause of errors in SWR measurements. these harmonics are generally excessive only with a coaxial signal output since this type of output often passes harmonics more efficiently than the fundamental. with coaxial systems it is common for the harmonic standing wave pattern to be equal to or greater than the fundamental standing wave. unlike garden variety swr meters which are not frequency selective, the slotted line probe may be erroneously tuned to a second harmonic. even if a probe is tuned properly, the presence of harmonics may still couple sufficient energy into the probe to give wrong readings. to minimize the possible effects from harmonics, a signal generator used as the source should be rated low in harmonic content. suitable low pass filters may be inserted between the signal source and the other equipment to reduce harmonic effects.

as to the antenna under test, a 1/4 wave @ 27.185 Mhz. will radiate IMD3 harmonics @ 81.555 Mhz. rather efficiently since a 1/4 wave @ 27Mhz. is 3/4 wavelength @ 81Mhz.. the antenna is resonant at both frequencies and input impedances are practically identical while radiation at the even harmonics is greatly attenuated due to the much higher impedance (mismatch) presented.

the most important consideration with respect to all of this is that the source signal (transmitter, exciter, generator, ie.) be as free of harmonic energy as is possible. this is more likely to be the case if the source is operated at as low a power level as possible, using the minimum amount of power required to obtain a set (calibration) reading before measurements are made.
 
What I want to know, is why my swr changes when I change jumper length between the radio and amp?
I have to run it like this...
radio--18 ft--amp--any length--meter--antenna
With everything in the same order, but change the radio to amp jumper to a shorter one, swr goes higher the shorter it gets. It does'nt matter if amp is on or off...it's still high, but higher with the amp on.
With the 18ft jumper in there, the swr stays the same with amp on or off, and is the same as what the MFJ analyzer says, when just hooked to the feedline.
Should I just run 18 ft, where I really only need 3 or 6?
 
you asked for it....

the impedance in a standing wave pattern is greatest at a voltage maximum. a shunt admittance such as the line section in an swr meter lowers these impedances, causing the measured SWR to be lower than the true SWR. The maxima and minima are shifted from their natural position when the swr meter is inserted into the line and this effect is more pronounced at a voltage maximum than at a voltage minimum.

this effect is loosely simulated when the jumper lengths are varied.

"........ why my swr changes when I change jumper length between the radio and amp?"

because either the output impedance of the transmitter or the input impedance of the amplifier (or both) are something other than R=50, X=0.
 
SWR increases when power is added.

The antenna is not the problem. In this case, it is the amplifier. You have already established that the antenna is properly tuned and in good working order with low SWR, except when power is applied. Assume a ham operator is on 10 meters using a solid state amplifier. With the radio only, the SWR is 1.1:1; when the amplifier is turned on, the SWR jumps to 2.0:1. The amplifier is not only transmitting at 28 MHz, but is also transmitting on a second frequency of 56MHz. This is known as a ''second harmonic'' (2X the fundamental frequency of 28 MHz, transmitting at 56 MHz). Thus the SWR meter is reading both the reflected signal of the normal frequency and the rejected second harmonic signal. The antenna will not accept energy transmitted at 56 MHz, and returns it all back to the radio, which shows up on the meter as high SWR because the meter can not tell the difference between 28 MHz and 56 MHz. In fact, as much as 30% of the power is at 56 MHz. This is generally due to an amplifier that is not adequately filtered. Adding a Low-pass filter at the amplifier output is the only solution. For best results, connect the low pass filter directly to the amplifier using a barrel connector.

Now this is to reply to the question of coax length. How many of you change the length of your coax to tune your antenna? One of my good friends said to me, "I think changing the length of the coax is the same as moving the gamma rod adjustment on my Moonraker 4". Sorry to say, this is not true. As most people will find, varying the length of coax to the antenna will vary the SWR that the SWR meter is reporting. Actually, SWR should remain relatively constant no matter how long the coax is or where it is placed on the line (if its 5 feet down the coax from the radio or 50 feet down the coax from the radio). In most cases, the cause of inconsistent SWR meter readings is from poor SWR meter design or component aging / failure. For the SWR meter to read consistent SWR readings on the coax, the meter has to have an impedance itself of exactly 50 Ohms. Any deviation of the SWR meter's self impedance (from 50 Ohms) from poor design or component aging / error / failure will cause slightly inconsistent SWR readings when the SWR meters position on the coax or length of the coax is varied. In practice, generally you will find varying the coax length seemingly effects the SWR reading. Most SWR meters (built into radio and external type meters) and impedance "humps" in coax lines and connectors will cause minor variations in SWR as jumpers and coax length are varied. In reality, the mismatch at the antenna's feed point / coax junction is unchanged. Therefore - the actual SWR is unchanged.

Another reason SWR could vary is from the situation where the coax is acting as part of the antenna. Not a favorable or normal situation. The signal is traveling back down the outside of the of the coax braid (note power should only be traveling on the inside on the coax braid). Therefore, the coax is part of antenna system and changing the coax length will change the SWR. This situation is more likely to occur in mobile installations. You can try to eliminate this situation (called "Common mode currents") by winding an "RF Choke". Wind about 6ft of RG-213 or RG-8 into a coil (6 to 8 turns). For RG-58 use 4ft with 6 to 8 turns. Wind the coax up, placing each turn right next to one another. Use electrical tape to secure turns together. You should place these as close to the antenna as possible. Right at the antenna coax connection point being optimum. Most times, you can verify that you have common mode currents flowing back down the coax by grabbing hold of the coax while transmitting and moving the coax around. You can watch the SWR waver by moving the coax while transmitting (don't speak into mic!). You have to do this with all the doors closed from inside the vehicle. SWR should waver, if you notice that SWR jumps rapidly between two values, you might have a intermittent (bad) connection in the connectors (PL-259s) on the coax. In most cases of "common mode currents", just grabbing the coax will cause the SWR to change.

The "RF choke" described above stops the signal from traveling back down the outside of the coax. The signal inside the coax is * u n a f f e c t e d * by the choke (contrary to what you may have heard about coiling up excess coax). Common mode current kills antenna efficiency. You could have a decent SWR and not realize half your signal is being broadcast into you car (result very poor antenna performance). If your linear amplifier causes serious problems with your car's computer, lights, etc....you may have common mode currents. If moving the coax around the vehicle results in SWR change, this is a good indicator you have common mode currents flowing back down the coax line.

This doesn't happen often with base station antennas. Most base antennas have some type of device that will decouple the antenna from the feed line (gamma match, balun, etc.). Make sure you run your feedline (coax) straight down from the antenna, taking care not to run close to antenna to prevent "common mode" currents which could still occur if coax is oriented in a way to pick up strong antenna signal.

Hope all of this helps.
 
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LOTS of information here !!

This goes hand in hand with the same trouble I have with a BOOMER 600. I guess they are cheaply made & don't have enough filtering.
So,......I'm going to use some of these solutions here & try a TVI filter between the amp & antenna , re-calibrate & check SWR with the filter in line !!
 
"SWR increases when power is added."

SWR does not increase when all measured power is produced at the fundamental operating frequency.

"....Assume a ham operator is on 10 meters using a solid state amplifier. With the radio only, the SWR is 1.1:1; when the amplifier is turned on, the SWR jumps to 2.0:1. The amplifier is not only transmitting at 28 MHz, but is also transmitting on a second frequency of 56MHz. This is known as a ''second harmonic'' (2X the fundamental frequency of 28 MHz, transmitting at 56 MHz)."

http://forum.worldwidedx.com/viewtopic.php?p=112254&highlight=pushpull#112254

"Thus the SWR meter is reading both the reflected signal of the normal frequency and the rejected second harmonic signal. The antenna will not accept energy transmitted at 56 MHz, and returns it all back to the radio, which shows up on the meter as high SWR because the meter can not tell the difference between 28 MHz and 56 MHz. In fact, as much as 30% of the power is at 56 MHz."

for 30% of the power developed to be at a frequency of 56Mhz. the devices in the amplifier would have to be producing IMD2 levels worse than -4db. to -6db. when referenced to the the rated output of a single device. that's a switch, not an amplifier.

"This is generally due to an amplifier that is not adequately filtered."

no, IMD2 levels running -4db. - -6db. would be due to excessive drive and excessive operating voltages.

"Adding a Low-pass filter at the amplifier output is the only solution. For best results, connect the low pass filter directly to the amplifier using a barrel connector."

LMAO....

"Now this is to reply to the question of coax length. How many of you change the length of your coax to tune your antenna?"

well, he's not asking about that. he was referring to the length of feedline between the output of the transmitter and the input of the amplifier.

"As most people will find, varying the length of coax to the antenna will vary the SWR that the SWR meter is reporting."

"Actually, SWR should remain relatively constant no matter how long the coax is or where it is placed on the line (if its 5 feet down the coax from the radio or 50 feet down the coax from the radio)."

wrong. longer runs of feedline with their increased matched attenuation will show lower levels of standing waves than comparitively shorter runs.

"In most cases, the cause of inconsistent SWR meter readings is from poor SWR meter design or component aging / failure. For the SWR meter to read consistent SWR readings on the coax, the meter has to have an impedance itself of exactly 50 Ohms. Any deviation of the SWR meter's self impedance (from 50 Ohms) from poor design or component aging / error / failure will cause slightly inconsistent SWR readings when the SWR meters position on the coax or length of the coax is varied."

wrong. perfectly working measuring devices including expensive network analyzer probes will show different readings as they are placed at different sections in any line where the load and/or the line is anything but R=50, X=0. show me a half a dozen brand new swr meters and a source with a flat line and load and they'll all read the same anywhere in the line. show me the same six and a 50 ohm line with a mismatched load and i'll show you a laundry list of "swr readings" from several spots along a line that no longer exhibits its characteristic impedance of 50 ohms.

"In practice, generally you will find varying the coax length seemingly effects the SWR reading."

that's because the changing lengths shift the voltage minimum and voltage maximum points up and down the line. since swr meters measure minimum and maximum voltages from which the standing wave ratio is derived it consequently follows that for an accurate swr measurement of any line where R does not = 50 and X does not = 0 at the antenna input then the point in the line where the measurement is taken is critical and as i mentioned earlier for the least amount of line loading and most accurate indication the measurement must be taken at a point on the line where voltage is at absolute minimum.

SWR = EMax / Emin....if you're measuring swr at a section in the line where EMax and EMin is not occurring then you're not measuring swr. that's why the readings change when the length of any interconnecting cable is altered. only in a flat line where the characteristic impedance of the feedline is exactly equal to the feedponint impedance at the load will you find EMax and EMin virtually anywhere along the length of the line.

similarly, if a transmitter source is connected to the input of an amplifier you have the same scenario. only when both the source output and the amplifier input present R=50 / X=0 will the length of the 50 ohm line in between them become unimportant as the ratio of EMax to EMin is the same all along the line.
 
and that has exactly what to do with the original posters question? HAM SHMAM. just because someone passed a test involving nothing more demanding than simple rote memorization doesn't mean he knows anything and before you get all indignant i'm one of them and i'm not in any hurry to renew before my grace period runs out.

here, take a look at another thread written along these same lines almost a year ago and then ask me if i care. if "HAMS" wrote some of the stuff you passed along in that last post, (just so you know) that's the type of misleading information that i've learned to expect from MOST "HAMS" over the course of my last 34 years. they're no different than cb'er hobbyists in that respect.

http://forum.worldwidedx.com/viewtopic.php?p=91108&highlight=cebik#91108
you don't get the nod from guys like bob85 and Toll_Free because you don't know what you're talking about.

every length of feedline is a matching transformer....Cebik.
 
The place where SWR should be measured is directly at the feedpoint of the antenna, with only a few inches of coax used to connect the bridge to the antenna. Knowing what the SWR is at THAT point is crucial to coming up with a good installation.

Then, realize that SWR meters are almost invariably normalized for 50 ohms. If your SWR is not very close to 1:1 at the feedpoint, and you're using 50-ohm cable, you'll see perceptible SWR/reflected power. The amount of reflected power will of course increase as the forward power increases. The RATIO between forward and reflected power will NOT change significantly. One watt reflected at 100 watts forward is the same ratio as ten watts reflected at 1KW forward.
 
I would suggest that when quoting Cebik that you quote all of the statement and in it's context. Sure, any length of coax can be an impedance transformer. But you have to admit that impedance transforming is not it's primary 'job'. In most cases it isn't the best, or most efficient method of doing that transforming since it can lead to it's own destruction.
What's the use? This dead horse has been beat to much already and I'm tired of it.
- 'Doc
 
"........and I'm tired of it."
you can leave any time you like.

Engineering is the process of making
workable compromises in design goals where
theories guiding different aspects of the design are
in conflict, making it impossible to optimize all the
goals. Good engineering is simply recognizing the
correct choices in the compromises and relaxing
the right goals. We amateurs spend many hours
budding and pruning antenna systems. Wouldn't it
be worthwhile spending some of that time learning
how to engineer the design in order to make correct
trade-off decisions among related factors instead of
letting King VSWR dictate the design?
FIRST, we need to improve our knowledge
of reflection mechanics and transmission-line
propagation to understand…

1) why reflected power by itself is an
unimportant factor in determining how efficiently
power is being delivered to the antenna.
2) the effect of line attenuation (to discover
why it is the KEY factor which will tell us when
and how much to be concerned with reflected
power and when to ignore it).
3) why all power fed into the line, minus
the amount lost in line attenuation, is absorbed in
the load regardless of the mismatch at the antenna
terminals.
4) why reflection loss (mismatch loss) is
canceled at the line input by reflection gain
5) why a low SWR reading by itself is no
more a guarantee that power is being radiated
efficiently than a high SWR reading guarantees it is
being wasted.
6) why SWR is not the culprit in transmitter
loading problems - why the real culprit is the
change in line-input impedance resulting from the
SWR, and why we have complete control over the
impedance without necessarily being concerned
with the SWR.
7) the importance of thinking in terms of
resistive and reactive components of impedance
instead of SWR alone, and why SWR by itself is
ambiguous, especially from the viewpoint of the
selection and adjustment of coupling and matching
circuitry.
SECOND, we need to become aware that
with moderate lengths of low-loss coax, such as we
commonly use for feed lines, loss of power because
of reflected power in the hf bands can be
insignificant, no matter how high the SWR. For
example, if the line SWR is 3, 4, or even 5 to 1 and
the attenuation is low enough to ignore the
reflected power, reducing the SWR will yield no
significant improvement in radiated power because
all the power being fed into the line is already
being absorbed in the load. This point has especial
significance for center-loaded mobile whips,
because of the extremely low attenuation of the
short feed line.
Many operators, cb'ers and hams alike, unknowingly
select loading coils of low Q because "they produce
a lower SWR than any other types of coils." They do,
indeed, produce a lower SWR, because of their higher
loss resistance.
THIRD, we should become more familiar
with the universally known, predictable behavior
off-resonance antenna-terminal impedance and its
correlation with SWR. This knowledge provides
scientific basis for evaluating SWR-indicator
readings in determining whether the behavior of
our system is normal or abnormal, instead of
blindly accepting low SWR as good, or rejecting
high SWR as bad.
 
Hey T-bone !! You still with us ? !! :p How much more deeper you think they can get ? They can feed you a smorgasbord of information , How perfect does it have to be for anybody ? IM just glad it wasn't required to know all of this to get across the street with my cb radio / given coax / amp and my back pocket SWR meter . I feel after 20 sum years the basics have gotten me by and I personally didn't have to go to college to get deeper then I personally needed to be. Wow ! It's looking more and more I have lived a antenna coax SWR lie my whole radio life :roll: all because I kept it simple and it always seemed to work so well for me ?. It must have been the RF God's watching over me ? No burnt up radios / amps / antenna's or feedlines !! Trust me here , IM not knocking the knowledge , IM just saying I would have been gone decades ago if I personally would have had to gotten so deep into it.(it would have more then likely taken the fun out of it for me) But I'd be one of the first ones to admit , that it just don't have to be perfect to work and to work well. It just doesn't have to get so deep for the average Joe's of the CB world . How hard does it have to be ? It don't !! Buy a decent well made talked about antenna and follow the freak'in instructions ! drill the hole in the roof , run the line and chances are , with your little back pocket SWR meter , you will see 1.1 / 1.2 / 1.3 / 1.4 / 1.5 or maybe even a little more , now follow the instructions on how to tune it lower if possible , slide the stinger up and down from channnels 1/20/40 Dah Da !! your lower then 1.5 around the band on both ends !! You the man !! Now get that amp in line and check things out , 9 times out of 10 your false reading of maybe going from 1.1 to 1.3 will be totally Bob Bitch'in !! you are good to go Joe !! Now enjoy your radio and power and let them know who's the boss,but most of all , HAVE FUN ! ;)
 
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