| Author |
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Bman02
| | Posted on Tuesday, March 11, 2003 - 7:54 pm: | 
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When i use my crappy rg58 jumper and the amp is off swrs are about 1.4 or so.With the amp on they stay about the same or lower.When i use my quality rg8x jumper the swrs are flat with amp off and when i turn it on they shoot sky high.Im just using the meter on the radio(tried 2 different radios-same thing)and the rg58 jumper is 2 feet long and the rg8x jumper is 3 feet long.Any ideas why its doing this??? |
Bill Clintone
| | Posted on Tuesday, March 11, 2003 - 9:51 pm: |
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Try another jumper...that one may be soldered incorrectly. |
Wyatt_Earp
| | Posted on Tuesday, March 11, 2003 - 10:14 pm: |
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I always heard that reading swr's when using an amp is ridiculous as you do not get the true swr reading. You should always check them while the amp is off. |
Kc0gxz
| | Posted on Tuesday, March 11, 2003 - 10:19 pm: |
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Try experimenting with different lengths of the rg8x and see what you come up with. And don't depend on the radios' SWR meter. Use something better if possible unless you know that the one in the radio is good. Most of them just give you a ballpark figure.
Jeff, kc0gxz. |
bruce
| | Posted on Wednesday, March 12, 2003 - 12:33 am: |
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also remember some amps put out high amounts of out of band power and your SWR bridge MAY be seeing the reflected 2. 3 rd harmonic and white noise. |
Kiwikid
| | Posted on Wednesday, March 12, 2003 - 4:47 am: |
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Hello Bman02,
You ain't the only one with this type of SWR problem.I have had good flat swr readings on low power and with each gradual increase of applied rf power I read a gradual increase in swr.And with each step up or down in power I have recalibrated the swr meter.I have tried this with different meters ,different coax and different radio/amplifier combinations with the same screwed up readings.With a dummy load hanging off the end of the coax the readings are OK.I agree with BRUCE there must be harmonics floating around but how does one filter these out,with a good TVI filter hanging off the amp?
73 KIWI KID |
Cbnut
| | Posted on Friday, April 18, 2003 - 1:46 am: |
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Ok, here's the deal..... MOST if not ALL CB "linear" amplifiers have nothing linear about them. Linear meaning that you put a sine wave into them, you get a sine-wave out. Most CB amplifiers are CLASS-C, or somewhere between classes, often of poor design, and no filtering. The reason they do this is simply because on a cheap "peak reading" CB wattmeter, such as a DOSY, Para-Dynamics, Workman, etc, it makes the meter peg over.... stop for a second, and concider why.
First off, let's discuss what happens when a "CB" type amplifier is overdriven, and non-linear, as is such the case when driven with a CB type radio that has been "peaked and tuned". Usually, this means that the audio limiter has been clipped, and the radio itself is exceeding 100% modulation on negative peaks. This means that the radio is creating spurs, and harmonics.
Now... concider feeding a poorly designed amplifier that is non-linear, or class C.... you have a real messy output signal when looked at for spectral purity. Harmonics are being created, which are even, or odd multiples of the main frequency. For example.... the 2nd harmonic or 27Mhz is 54, and so on.
Take your average CB wattmeter. It will read accurately on 27 MHZ, but will read inaccuratly at 54Mhz. In fact, it will give you rediculously generous readings at 54Mhz. You could feed about 2 watts in to a typical DOSY wattmeter at 54Mhz, and it would read perhaps 200 watts. This is why some CB amplifier manufacturers get away with their rediculous claims of output power.
Now, back to the question. Keep in mind that the amplifier is generating harmonics. What frequency is your antenna tuned for? Right around 27 Mhz right? Well, it's NOT resonant, or in other words tuned for all of the other junk the amplifier is throwing out.
We all know what an SWR reading is right? Power reflected back down the feedline, or coax that the antenna is not radiating. Therefore, because the antenna is not tuned for this garbage the amplifier is creating, it's reflected, and your SWR meter "sees" this, and tells you you have a high amount of reflected power, or in other words, a high SWR.
I once had a guy that refused to use a low-pass filter because he swore that when he hooked it up that it cut his "power" in half. The truth was, that it filtered out alot of the junk, or harmonics, that made his wattmeter give rediculous readings.
Hope this helps. |
Sparkomatic
| | Posted on Friday, April 18, 2003 - 6:51 pm: |
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On all the XFORCE and Davemade mobile amps I have run, jumper lengths have solved all the high SWR problems. With my Texas Ranger 296 and XFORCE XT400 a 6 foot jumper yielded 3-1 SWRs but a 9 ft yielded a flat match. I don't know and don't care why this happens but trying different jumpers has worked for me and some close friends who run comp amps. My Palomar 775 reads the same SWRs regardless of jumper length. There aren't too many mobiles that can give me a run for the money in a keydown contest so I have to assume I am doing the right things. |
frasiercrane
| | Posted on Thursday, July 17, 2003 - 1:04 am: |
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the reason you get different swr readings with different cable lengths is simple... draw a graph of a sine wave, a straight zero line with the wave going above and below it (just like in math class in school)
The zero line is your coax, the sine wave is the swr voltage. The higher the swr, the higher the peaks of the wave. But, at the points where the wave crosses the zero line, you will measure no voltage no matter how high the swr. So what happens is, if you cut your coax to the right length, your meter will end up on one of these zero points and will be fooled into thinking the swr is flat. In theory, you should be able to take 2 jumpers, one say 3 feet long, and another say 6 feet. put the meter in the middle of them and hook them up, short one to the radio, long one to the amp. note the swr. now swap them.. you'll still have the exact same jumper length, but the meter will be in a different spot and the reading should change, showing it's not the length of the cable that matters, but the meter placement. |
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