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high swr when more power is added
- Thread starter troyota
- Start date
Ideally the scope should have a MHz rating higher then the 27 MHz signal you are looking at. Typically that would mean having to buy a more expensive 40 MHz scope. The reality is ones rated as low as 20 MHz are perfectly useful for monitoring the envelope of a CB signal.
I think even a 20 MHz scope is a good investment for anyone interested in learning how to adjust a station for optimum operating conditions. Especially useful in setting carrier levels and modulation levels. Also great for anyone interested in learning more about how electronics works. For hobby use you need not spend more then $75 to $100 on a used scope.
Okay. So for right now I would like to have the equipment to tune and align my 148 gtl and other radios properly. Besides a frequency generator, a dummy load, and a scope, what else do I need? Are there brands to look for and ones to stay away from? Is a 40 mhz scope essential once you get into alignments? I would rather learn how to do it myself and have the equipment than have to send it to someone.
B
BOOTY MONSTER
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Well, I've only been here for less then a year
well your comments remind me of freecells comments about harmonics/splatter caused by over driving a amp give false power measurements on meters .
heres 2 paragraphs from the link that get to the meat of the matter in his
http://www.firecommunications.com/notreal.txt
"the largest factor contributing to the highly erroneous readings produced by the majority of hf
wattmeters is created when energy begins to appear at frequencies 3, 5, 7 (and higher) times
the fundamental operating frequency. the more the drive and operating voltage is increased the
more pronounced the effect becomes. in addition to the sum addition of the energy present at
these multiples of the operating frequency the problem is compounded by the wattmeter design
itself. the line section is designed to sample a specific amount of energy at the fundamental
operating frequency in a direct relationship to the physical length of the line section as that
length correlates to a given fraction of a wavelength at the intended operating frequency. when
energy begins appearing at the odd harmonics not only is the power added by the line section but
it is also amplified because as frequency is increased the line section becomes longer with
regard to the fractional wavelength represented by the line section initially at 27 mhz.. there
fore the line section is "oversampling" the energy levels occurring at the frequencies previously
mentioned. once these harmonics begin to occur the environment is ripe for a sort of controlled
"oscillation", the harmonic energy present acting as the trigger.
now in addition to the energy present at 27 mhz. you now have substantial levels of vhf and in
some cases even uhf energy directly affecting the accuracy of the measuring device."
..................................................
i know were talking about two different measurements , vswr and power output . but the theme seems to be harmonics making our meters accuracy more or less compleetly unreliable .
...............................if im getting the ghist of whats being said ?????
so IF...... if im remotely on the right track ......... why does changing coax lengths seem to "fix" the higher vswr issue with the amp on for many folks ? or even if im way off .... why does the longer coax trick seem to work ? i know its fooling the meter , but how does it fool the meter ?
As long as you have a good power supply and decent watt meter about the only other essential thing I would add would be digital volt, ohm, amp meter with at least a diode check scale. Better ones have transistor checkers built in but you can test a transistor with the diode check scale. Unless you're working with VHF equipment the higher frequency scopes aren't needed for most work. The exceptions would be when working with more complex HF rigs that often use mixer frequencies well into the VHF range.
loosecannon i dont have a low pass filter. im in the process of building one. i bought some doorknob caps last week and i have a coil turned but have not had time to put it together. it may not work but i like to experiment.
anyway i shockwave i would love to go ahead and fix this one. i can let you know it is a palomar deluxe 35a with 4 of the sd1446. i really dont need to use it but it makes a nice driver however i want a clean signal for sure. do i need to use disc caps?
thanks again
anyway i shockwave i would love to go ahead and fix this one. i can let you know it is a palomar deluxe 35a with 4 of the sd1446. i really dont need to use it but it makes a nice driver
however i want a clean signal for sure. do i need to use disc caps?thanks again
While I agree with most of FreeCell's comments we are talking about two different conditions. Extreme harmonics can cause the watt meter to exaggerate forward and reflected readings. When the SWR goes from 1.1:1 bafefoot to 2:1 when you turn a solid state amp on, most of the time that's oscillation. In this case turning the drive up often lowers the SWR because the fundamental frequency drowns out the oscillation.
Changing coax lengths can appear to fix SWR problems in the oscillating amplifier for the reasons I described in the above post relating to any slight change effecting the operating characteristics of an unstable amp. The second way coax length effects SWR in the absence of an oscillation is due to a mismatch between the coax and the antenna.
This creates standing waves on the coax. Depending on the length of the coax you can find a spot where the standing waves on the coax will represent a 50 ohm match to the meter. Due to the phase of the current and voltage on the line. This does not compensate for the lost power in the mismatch between antenna and 50 ohm coax.
loosecannon i dont have a low pass filter. im in the process of building one. i bought some doorknob caps last week and i have a coil turned but have not had time to put it together. it may not work but i like to experiment.
anyway i shockwave i would love to go ahead and fix this one. i can let you know it is a palomar deluxe 35a with 4 of the sd1446. i really dont need to use it but it makes a nice driver
thanks again
Ceramic disk caps are what is often used here.
Just make sure you understand how to connect the cap and resistor to each transistor to form the negative feedback circuit. I believe someplace in this forum negative feedback is covered with pictures and schematics.
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You may open this amp and find the negative feedback is already there since this is not the only cause of this type of oscillation. You'll have to look and see. Poor RF layout and PC boards with inadequate ground surfaces can also be the source of this problem. It is essential that the emitter terminal of the transistors be at absolute RF and DC ground potential.
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