Topband
[Top] [All Lists]

## Re: Topband: Stub measurement error

 To: "George Cutsogeorge" , Re: Topband: Stub measurement error "Tom Rauch" Tom Rauch Wed, 5 Sep 2007 19:36:33 -0400
 ```> Here are some measured numbers. The transmission measured > frequency of this > RG-213 stub is 28.535 MHz. The reflection measured > frequency is 28.785 MHz. By transmission and reflection, are you referring to S21 and S12? If so, how could it be different with a symmetrical T? The fact it is different, if you are using a symmetrical T, means something is wrong in the test setup. > The reflection measured frequency with the tee is 28.380 > MHz. The stub has > 35.8 dB loss at the null frequency. If the reflection > measurement is used > to cut the stub, the attenuation at the desired frequency > will be about 3 to > 4 dB less. That is not acceptable in my book. That 35 dB wouldn't be true in an antenna system or even into a dummy load with a transmitter and receiver, because the real system does not behave anywhere like a pure 50 ohm source and load like a good generator or S parameter test set looks like. Let's look at an example. Say we have a 14.025 signal and want to null the second harmonic out of a 28.050 receiver. The 14MHz shorted stub would indeed see around 50 ohms at 14 with a good antenna, but at 28 the impedance where the stub is connected could be anything. Since the source of the harmonic isn't 50 j0 or anywhere close at 28 MHz, and since the load isn't 50j 0 or anywhere close, the attenuation would be totally different than we measured in an S parameter test set anyway. So why pick nits? If we really want to squeeze the last dB out a stub has to be tuned in the actual working system. If we do that and move 200kHz on 14 MHz, it's all out of whack anyway because the harmonic moved 400kHz.. The bottom line here is if we need 35 dB attenuation and are depending on a single stub and can't live with 30dB, we'll only get that through pure dumb luck in the working system. It won't repeat what we measure no matter what because the system radically changes. If a degree of line length affects the system's performance....it's time to look at the design. A degree is about one inch on ten meters, or a foot and a half on 160. 73 Tom _______________________________________________ Topband mailing list Topband@contesting.com http://lists.contesting.com/mailman/listinfo/topband ```
 Current Thread Topband: Stub measurement error, George Cutsogeorge Re: Topband: Stub measurement error, Tom Rauch <= Topband: Stub measurement error, George Cutsogeorge Topband: Stub measurement error, George Cutsogeorge Re: Topband: Stub measurement error, Tom Rauch