couple of questions about swr

[nat19]

Ok so I have been thinking and I have two questions. I have been researching coax jumpers and how sometimes with certain amps etc.. You can change the length of the jumper and get your swr down, but people claim that your swr isn't changing it's just fooling your meter. So here is my theory... Lets say you theoretically you have a system with a three foot jumper that shows a swr of 1.5, with the amp off and you turn the amp on and get a swr of 2.1 so therfore it's too high. You switch to a nine foot jumper and you swr drops to 1.5 with the amp on. People would say your just fooling your meter, but imagine you needed the nine foot jumper to reach your radio to begin with and you would have a swr of 1.5, so you would think nothing of it. See where I am going?? So my question is if you use a longer jumper to lower swr in some cases is it acceptable. I know coax length shouldn't affect swr but we know that for various reasons in some cases it does.

My second question is say you check your swr with a 4 watt dead key and you have a 1.5:1. You then modulate and it goes up to a 1.8:1. (I know that power shouldn't affect swr but it does in some cases) So is your swr two high or does swr with modulation not mean squat? Should swr go up when you modulate?

 

[wavrider]

Length of jumpers have to do with velocity factor of the type of coax you are using.

In theory the amplifier should have a tuned input to present a 50 ohm impedance to the transceiver. In reality unless it is a well designed amplifier input vswr can be off the charts.

So if you are using a 6' jumper and get a 2:1 vswr input to the amp, changing it to a 9' jumper makes the input vswr 1.5:1, then you know you either need to use the 9' jumper (band aid) or rework the input tune on the amplifier.

When you use a 4 watt dead key to "calibrate" the vswr meter it is thus semi accurate only at 4 watts.
Once the radio is modulated and the power output increases with PEP the meter is no longer "calibrated" so the VSWR reading will be in-accurate.

A dual type meter, one that shows power output and uses a separate circuit/needle movement to simultaneously display power out and vswr is the better way to go as far as meters are concerned IMO.

 

[mackmobile43]

I have none of these issues with my amp on or off, modulated signal or resting carrier.

If I chose to check swr with my amp turned on I calibrate the meter to the corresponding amount of power applied.

No real big secret.

 

[Robb]

I've often wondered what causes an amp to have improper impedance. Never read a thread on it. Perhaps an amp guru can answer that, in case I have to deal with this issue in the future. This question has everything to do with this thread.

 

[nat19]

I just want to make it clear that I don't have these problems I am theoretically asking them to get answers. lol Thanks for the replys so far though, I think I understand now. By the way Robb I have often wondered the same thing.

 

[W5LZ]

Why do some amplifiers have a seemingly wrong input impedance (or output impedance)? Most commonly because the input/output circuits are not the right impedance. Think of it like this. If the output impedance of your radio is 50 ohms, the best SWR between it and the antenna (or that amplifier) is when the input of that antenna (or amplifier) is 50 ohms too. It also means that the feed line between the radio and antenna (or amplifier) ought to be 50 ohms too. If those three conditions are met, then the SWR will be low. (I doubt if a really real 1:1 SWR is ever going to happen, all this stuff is man-made so will have some slight differences. If it's at or below about 1.5:1, that's as good as it'll probably get and there's nothing wrong withit at all.) Forget about the amplifier for a minut to make things simpler. If the output of the radio is 50 ohms, the feed line is 50 ohms, but you are seeing a difference in SWR if you change the feed line length (jumpers, whatever), then that antenna is NOT 50 ohms. If that feed line length makes a difference in SWR between the radio and the amplifier, the exact same thing is true, and the most likely source of the mismatch is with the input of the amplifier. That's one of the problems with 'no-tune' input -OR- output circuits, they are never going to be exactly right, they aren't adjustable.
The biggest problem with relying on SWR as an indication of how well an antenna is tuned is that SWR meter can't distinguish between the resistance and reactance that make up impedance. There are gobs of impedances where the resistance isn't 50 ohms and the reactance isn't zero. The definition of resonant is zero reactance. So there's the problem right there, you're trying to measure tire air pressure with a tablespoon instead of an air gauge. It just don't work that way, sorry.
When you change the length of the feed line you are only fooling the meter, putting it into a portion of the feed line where the SWR is low. That SWR is still there, it's just being 'hidden' from the meter and the meter is too dumb to know it.
There's no easy way of explaining about impedance unless you know what it is to start with. So find out from a reputable source.
- 'Doc

 

[mackmobile43]

Seems that lengthening or shortening the radiating portion of an antenna is the same as adding or taking away a jumper since it's appearance to earth is off.

If this is done correctly then you should not see this issue with coax unless as stated the amp's input or output impedance is not what the radio needs to see.

 

[Robb]

OK; so let's say that the radio's impedance is 50, and the antenna is 50. So what is going on in the amp itself ('parts/circuit') that is wrong? Just the jumper itself?!?

I haven't worked on any amps too much as yet. What parts are causing the mismatch?

Who is the amp guru here?

Guess I didn't clarify enough the first time . . .

 

[wavrider]

I've often wondered what causes an amp to have improper impedance. Never read a thread on it. Perhaps an amp guru can answer that, in case I have to deal with this issue in the future. This question has everything to do with this thread.

ROBB good question and it would take a lot of typing to answer it thoroughly.

Designing Ampiifiers, Part 4

Rich Measures has explained the input and output networks in the above link.

 

[hookedon6]

OK; so let's say that the radio's impedance is 50, and the antenna is 50. So what is going on in the amp itself ('parts/circuit') that is wrong? Just the jumper itself?!? ... .

the EZ answer is "phase shift" in the amp,....... think about voltage & current loops in the actual coaxop:

 

[W5LZ]

Follow this along...
You have the radio, a jumper to the SWR meter, and then the feed line to the antenna connected to that meter. When you check SWR you find that it measures 1.1:1 for instance. You want to add an amplifier to the set up so, disconnect that jumper from the radio but leave it connected to the SWR meter and feed line to the antenna. Connect that jumper you just removed from the radio to the output of that amplifier. Use another jumper between the radio and the amplifier. When you check SWR again with the amplifier 'off', the SWR jumps to some ridiculously high value. What are the possible places for there to be a problem? There are two possible places for a problem to exist, the new jumper between the radio and the amplifier, or the by-pass circuit inside that amplifier. The easiest way to check for jumper problems is to disconnect the SWR meter jumper and the radio jumper and connect them together with a 'double-female'. If there's no ridiculously high SWR anymore, it's back reasonably close to what it was to start with, then where does that tell you that there's a problem?
Now, if there was that ridiculously high SWR when you connected those two jumpers together, then you have definite reason to think it's that new jumper, right?
All that's with that amplifier 'off'. Now, try the same thing with it turned 'on', and recalibrate that SWR meter if needed. The same things apply, and the same reasoning will hold true.
If you happen to have the appropriate sized dummy load, checking those jumpers is even easier. And while it'll be more difficult, replacing that antenna with that dummy load will also check the feed line. So what if the jumpers and feed line check out okay when using that dummy load instead of the antenna? It should tell you that the @#$ antenna isn't resonant and is not a 50 ohm load.
Do you see the reasoning in the above example? Now apply that reasoning to the situation in question. Next case!
- 'Doc