[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Preamp / SWR issue



Kevin Schuchmann wrote:
> Hi Nate,
>   either I'm misunderstanding what your trying to say or I think your statement is wrong.
>
> If you add a "large amp" your SWR will not change unless the impedance of the amp
> is different than your rig, amps are not bi-directional and do not pass reflected
> power from the antenna back to the rig itself through the amp.
>
>  If you have your  rig--coax--amp--coax--antenna, then you will have a swr ratio
> between the rig and the amp and a different swr between the amp and the antenna.
>
> I guess if you had a really huge mismatch then depending on the design of the amp
> it might throw  off the input impedance and change your swr  but if its that bad you
> have a bigger problem to fix   :-)
>
> Please explain where I'm going wrong here...
>
> 73
> Kevin WA6FWF
> Amsat-UK #6505
>
>   

You're right Kevin, I typed that thinking about changes in frequency, 
not impedance and SWR.

The poster that told the gentleman to put a nice solid known-50-ohm load 
at the end of the cable and retest was on the right track.

What I was off rambling about, which is probably obvious to those 
reading along, was that if you dump, let's say 1W into a system that has 
a 2:1 SWR you don't lose as much power as say dumping 100W into that 
same system.  The overall system losses eat you alive as you go up in power.

You can demo what I'm meaning by this by modeling a 1W transmitter into 
some known lossy cable and then a badly matched antenna, and then doing 
the same numbers at 100W.  You can see how the curve gets real ugly, fast.

Then, in that same vein, I was also mixing in that it's nice to know the 
difference between your antenna reflections and issues, and whatever's 
happening in the cable itself. 

If your cable is perfect, and all your reflections and power loss are at 
your antenna (perfect world?), putting that 50 ohm load on the end of 
the cable, you should see a 1:1 (or very close to it) SWR.   Put the 
antenna back on and your SWR changes to whatever the antenna is doing.

But... nothing is ever that perfect in the real world.  The measurement 
with the 50 ohm load at the antenna end of the cable will show you what 
your cable is doing. 

You do need a very good known 50 ohm termination to do that test, that 
can handle whatever power you're throwing at it, however.

I never did hear back from the original poster, as to whether he was 
really talking about an amp or a pre-amp.  The comment about the 
poorly-designed/implemented directional couplers was a good one for his 
problem.  The new pre-amp may be messing with the overall impedance of 
the system which will change the measured SWR.

The only thing that should change SWR measured at the same place in the 
system when adding a device is a change in impedance caused by adding 
the new device and connecting cables. 

Lots of things out there aren't nice pretty 50 ohm devices...

Hopefully that's better... usually I type up such silliness at much 
later hours of the night, but I was pretty wiped out when I was reading 
that night.  :-)

And of course, I could still be wrong... always willing to admit that 
and learn.  As much as the hobby has a lot of real RF engineers hanging 
around, I'm proud to say I'm a true Amateur at all of this.  ;-)  Always 
willing to learn.

I definitely find that knowing what the cable itself is doing 
independent of the antenna hooked to it, is useful information.  
Especially if a system starts misbehaving after being installed for a 
while... did something happen to the antenna or the cable?  Having a 
baseline is nice in those situations.

Nate WY0X
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home