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high swr when amp on. flat swr when amp off

I

iceman78

Active Member
Feb 15, 2009
135
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I have a galaxy 959 going to a k40 magnamount on an 05 Titan. no probs, sounds good SWR 1.1:1. When I put my little Texas Star Mod V 100 watter, swr's shoot up to 2.5. Patch cord is good.

Galaxy 959 --> T.S. 100 w. --> k40 swr's are perfect even with the amp inline just as long as it's off. the second I flip the switch swr's bug out. amp is run directly to both battery terminals. was told i should ground the radio & amp together and also ground the amp to the vehicle. would hate to ruin the radio or amp so any help would be appreciated!

us

getting this reading from swr meter in radio. meter is accurate because i checked the reading (before amp was installed) against paradynamics meter and reading was the same. also something interesting--when i check the swr using radio meter, i dead key and get swr of 2.5 but when i whistle, it swings down to an acceptable swr. this is puzzling : (
 
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iceman78 said:
getting this reading from swr meter in radio. meter is accurate because i checked the reading (before amp was installed) against paradynamics meter and reading was the same. also something interesting--when i check the swr using radio meter, i dead key and get swr of 2.5 but when i whistle, it swings down to an acceptable swr. this is puzzling : (
Click to expand...

So the only meter you're using is the one built into the radio? That helps diagnose things: What you're seeing is not a problem at all with the antenna. Your amplifier's input impedance is nowhere near a match for your radio's output and the coax jumper's characteristic impedance. Get the amplifier fixed.

Also, get a decent external meter. Your radio's meter won't tell you anything about your antenna system if there's an amplifier operating between radio and antenna.
 
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I assume you have your swr meter at the antenna connector in both cases?

The amp your using is a single 2sc2290. The drive must be low, like around 2 watts deadkey.
Also make sure your DC power wires to the amp are beefy enough for 15 amps.

Beetle is right, measuring swr at the radio with the amp on only tells you what the amp is reflecting back into the radio. Which btw should be good also, when your dead key level is proper.
 
just grounded everything well. interestingly enough when i use the rf power knob to lower the dead key to 2w the swr's are high 2.5-3. when i up the power into the amp (dead key around 3-5w the swr's drop to arond 1.5 going to get a wilson 5000 tomorrow maybe that will help some, if not it will at least be a nice toy to have.
 
I'm not clear yet...are you measuring the SWR between the radio and amp or between the amp and the antenna?
 
Although the meter in the radio is something to go on , but far from accurate , plus your using a "mag-mount" (not a solid ground to begin with.) You said you had a PD meter ? put that in line with the amp and antenna , set the radio at about 1 watt and turn the amp on , calobrate the PD meter on SWR and see what you get from there.

Most Texas Star amps will read a little high but not much more then 1.5 at the most. Inside the radio type meters in a 949 are not the best way to go in this matter. Try another meter before you do anything else. Mag-mounts can be tricky concerning RF from time to time as well. Best of luck to you.
 
moleculo said:
I'm not clear yet...are you measuring the SWR between the radio and amp or between the amp and the antenna?
Click to expand...

Neither, he's using the radios meter to assume what's going on and as beetle stated his radio is seeing a higher than normal input swr from the amplifier, 2.5 high and he needs to get the amplifier's input tuned closer to what the radio would like to see.

This handy little calculator has helped on many occasions, just install a power meter inline betweed the radio and the amplifier using a half wave of coax in and out of the meter into a 50'ohm dummy load and key up and enter the forward and reverse power readings from the meter and it will show you your input swr from the amplifier.
 
just swapped the 2' jumper with a 6' and it flattened out the swr. double checked it with radio swr meter and paradynamics...both check out fine. i'd like to thank you gentlemen for the great input. now the only issue is getting some more power!
 
iceman78 said:
just swapped the 2' jumper with a 6' and it flattened out the swr. double checked it with radio swr meter and paradynamics...both check out fine. i'd like to thank you gentlemen for the great input. now the only issue is getting some more power!
Click to expand...
Buy a larger amplifer and drive it with the mod v.
 
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.
 
The only thing that's been established is that the input impedance of the amplifier isn't what it should/could be. Changing jumper length and the SWR going down only confirms that.
That amplifier may be producing harmonic energy but you can't prove it just by changing jumper lengths. Those harmonics would be on the output, not the input anyway.
Check the simple things first then try for the 'exotic' things, just makes sense. The most common reason for an impedance mismatch between radio and amplifier is a mis-tuned input circuit in the amplifier. Especially since that amplifier uses a "no-tune", -compromise-, input/output circuit, which are very seldom, if ever, even close to being 'right'. ("One size fit's all" is only true in one case, everything other case is always a mis-fit.) The 'tip-off' is changing the jumper's length making a difference. The only thing that can possibly do is change the resulting impedance of the amplifier's input that the radio 'sees'. That means that the input impedance wasn't right to start with. It works, obviously, but the problem is still there, it's just being masked by that jumper's contribution.
Why is that so hard to understand?
- 'Doc
 
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StahlMaster said:
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.
Click to expand...
^can not stand it^^can not stand it^^can not stand it^^can not stand it^
 
iceman78 said:
just swapped the 2' jumper with a 6' and it flattened out the swr. double checked it with radio swr meter and paradynamics...both check out fine. i'd like to thank you gentlemen for the great input. now the only issue is getting some more power!
Click to expand...

It may have "flattened out the swr", but you still have a mismatch between the 50 ohm coax and whatever the input impedance of the amplifier happens to be (apparently quite different from 50 ohms). All you've done by changing the feedline length between radio and amplifier is put a band-aid on a serious wound. You're still losing power between the radio and amplifier. Fix the problem, not the symptoms.
 
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