Here are a few comments about bandswitching stubs from my experience
building several systems of various kinds.
> I presume you are already using bandpass filters,
> which are more effective than stubs.
The statement above is only partly true. Single band filters are quite
good and follow the predicted curves closely. However, the bandswitching
filters available to amateurs do not. There is some degree of stray
coupling between bands and the attenuation in the stop band is not as high
as it could be. Ten meter rejection is the worst and I have measured it as
low as 18 to 20 dB for pass bands of 80 and 40 meters. A single RG213 stub
for 80 or 40 meters can provide 20 to 25 dB of attenuation on 10 meters and
combinations of stub will provide much more. Also, any noise and harmonics
generated in the final amplifier will not be attenuated on transmit with
low power filters.
> I have seen a typical attenuation for RG-213 stubs of 24 ~dB. RG-213
> coils up neatly, has a predictable velocity factor, and is easy to
> cut.
Typical 2nd harmonic attenuation for a single stub of RG213 is 25 to 28 dB.
For two stubs in parallel, one cut for CW and the other for SSB, this
number goes up to 30 to 32 dB. These numbers can be increased to 50 to 65
dB when using a coupling element between the stubs. Of course the
switching becomes more complex for coupled stubs.
Using a network analyzer to cut stubs can provide a null accuracy of a
couple of kHz. The same accuracy can be obtained with an MFJ unit if an
external detector is used, such as an oscilloscope or RF voltmeter.
Coax cutters with half moon blades make the job easy. The RF Connection
sells one for $11. When cutting shorted stubs this tool can cut half way
through for making the measurement. I find that cutting stubs to their
open end frequency and then shorting them is not as accurate.
George Cutsogeorge, W2VJN
> > If making two stubs per band, for higher Q, what coax would
> >you
> >use for the quarter-wave link between the two stubs? More hardline,
> >or a
> >coil of RG-213?
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