| Author |
Message |
    
Fdirsh
| | Posted on Friday, May 09, 2003 - 9:11 pm: | 
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I have a palomar 900V. I have an antron 99. I have a set of beams.
A-99 SWR 1.1
Beam SWR... vert. 1.1 flat 1.2
900v has SWR of 1.3 on antron while on operate
900v has swr of 3.5 on beams while on operate and will not foward modulate.
All base boxes I have run were fine. I also tried a palomar 775V which would run on beams except for high SWR while in operate.
Why is this? |
Samuel Skolfield
| | Posted on Saturday, May 10, 2003 - 10:22 pm: |
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Hey, I was wondering how to calibrate a CB antenna to my cobra WX ST 18. SHould I do it myself, or have a shop calibrate it for me. If so, do you know of a shop that I could sent the antenna, and the cB away to be callibrated?
Thanks alot. |
2600
| | Posted on Sunday, May 11, 2003 - 1:49 am: |
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Uhhh, gee, you have what the engineer types call a "conditionally stable" amplifier. That means that small differences in the load you hook it to can cause large differences in how stable it is. Clearly, there is a subtle difference between the antennas that the SWR meter is too simple-minded to let you see. The amplifier IS seeing this difference, and is turning itself into an oscillator when transmitting into the beam. Once it starts to oscillate, the harmonic frequencies that get created in the amplifier take a big upward jump in power. The high SWR reading you see is actually the extra frequency or two or four that the amplifier delivers when it oscillates. Since those frequencies are never near the channel you are on, nearly all of THAT part of the total power comes back down the coax, since the antenna isn't tuned for those frequencies. This high SWR reading doesn't mean the antenna is acting wrong. It isn't meant to absorb 54 MHz, 81 MHz, 108 MHz and the rest of the harmonic frequencies that will come leaking out of an unstable amplifier. The sum total of all that stuff just bounces back and drives the reflected reading far higher than it shows barefoot. You want to know what the antenna itself is doing? Check it with the radio barefoot.
Believe it or not, changing the length of the INPUT side coax jumper has been known to stop this. It's not an elegant solution, but it's cheap to try if you have a variety of different-length coax jumpers handy.
Since it seems that only a small change of antenna SWR triggers the instability, changing the length of the coax to the beam, adding jumpers and barrel connectors in line, might also affect it. Nearly every other potential solution costs more to try.
If it offers no improvement, inserting a TVI low-pass filter between the amplifier and the antenna has been known to stop this, too. Who can say that the extra coax jumper needed to patch it in line wasn't what fixed the problem for folks who report that it works for them? Might help, might not. Can't tell unless you try it.
One other thing to consider is the coax cable on the beam. It's a too-common practice to just "fold over" the coax shield braid inside the PL-259 connector without soldering it. Everybody solders the center pin. It's out where God and everybody can see it. Unscrew the shell back over the plug body so you can see the holes around the center circumference of the PL-259. If you see little copper strands, but no solder, this can really aggravate the problem you are having. A barefoot radio will appear to behave, but an amplifier will heat that "friction-only" ground connection inside the plug, and cause the copper to oxidise, ruining the metal-to-metal connction that carries your amplifier power. Wiggle the cable around, and the braid strands squirm around a little inside the plug. This scrapes away enough oxide from the small shield strands that the ground is restored, for a while. A barefoot radio won't create that kind of heat, but an amplifier will. The solder isn't there as "glue", but to create a true metal-to-metal current path that oxygen can't penetrate and degrade.
If the jumper on the input side of the amplifer has this problem, it can make the thing unstable, or make it marginally stable, where just changing antennas makes it go insane.
73 |
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