Yup, I did specify "good", not "great" isolation for the Top Ten
switches. I'm aware of their limitations. I agree that it's always
desirable to have 70+ dB port-to-port isolation if you can get it (and
afford it), but depending on the application it's certainly not always
necessary. During the phase of my career when I was involved in cellular
base station RF system design, in which performance was always balanced
with cost (and guess which one won if push came to shove), I learned to
do balanced designs that made sure no component was over-specified. This
experience has carried through to my HF station design.
In my particular case, I'm switching TX bandpass filters that supplement
the main BPF at the output of the radio to suppress harmonics and noise
from the amplifier. There's a pair of the Top Ten 2-ways at the input
and output ports of the TX BPF bank that lets me share a single bank of
filters between two radios. The filter banks are switched by a 2X6
switch matrix at each end that has around 60 dB port-to-port isolation.
The bandpass filters themselves have about 80 dB worst case adjacent
band isolation, so assuming the I/O switches each have 60 dB or so of
isolation, that's theoretically -120 dB leakage contributed by the
selector switches. This leakage is thus about 40 dB down from the worst
case BPF rejection, therefore negligible.Even if the total switch
isolation were only 80dB, this would mean the additional power
contributed from switch leakage would increase the total coupling (and
therefore decrease the isolation) by only 3 dB. The 2-way switches are
used to either enable the TX filter bank for the contest bands or bypass
it for the WARC bands. So putting a 2-way switch with mediocre isolation
on each I/O port of the selector switches has no effect on system
performance in the contest bands. Actually, even though the whole
network is wired with LMR-240 the coax shielding is probably the
limiting factor for total station-to-station isolation anyhow. I agree
that there are places where excellent isolation is required, and should
be specified. Like you, I used to use a WX0B SixPak, but many years ago
I replaced with a microHAM Double Ten switch, which has isolation
similar to the 4O3A switch, for this reason.
As for VSWR effects, I've learned long ago that attempting to make
complex RF systems involving filters and multiple switching points over
the almost 4 octaves of the HF to 6 meter range look like 50 + j0 on
every band is an essentially impossible endeavor. Fortunately,
well-designed transmitters these days are not overly sensitive to
moderate VSWR loads; as long as there's not a really bad mismatch
somewhere in the system they should take normal mismatches in stride.
The OP was inquiring about switches that are suitable to switch multiple
radios to one amplifier input, which implies that only one radio at a
time will be active. For that the Top Ten switches are plenty good enough.
73...
Randy, W8FN
On 2/17/2019 8:57 PM, Jim Brown wrote:
On 2/17/2019 6:26 PM, Randy Farmer wrote:
I concur with the comments on the Top Ten 2-way switches. I'm using
about 10 of them at various places in my SO2R contest station antenna
and BPF switching networks. They work well and have good isolation. I
On the basis of my measurements with a Vector Network Analyzer, I am
NOT impressed with these switches, and consider them poorly designed.
The major design flaw is that they depend upon the chassis for signal
return, so the signal path gets increasingly badly mismatched at 15M
and above. I didn't measure crosstalk, but this design error would
certainly impact it. I reduced the mismatch by about half by adding
braid from the coax connectors to the circuit board and following (as
best I could) the signal path to provide a return. Why? Because the
"ground" layer is broken under the top layer, so it can't act as a
return either!
Someone cited 50 dB as "good isolation." When I replaced my WX0B 6x2,
I measured isolation of about 55 dB. The replacement, a 4O3A 6x2,
measured 75 dB worst case. And other one I'm using now, the 4O3A
Antenna Genius, measures better than 95 dB worst case.
If you look inside a well designed power amp, you'll see transmission
line running from input and output for both TX and RX path. My Ten Tec
Titans were built that way, and so is the KPA500. You'll also see
mini-coax jumpers tying sections of the K3 together.
73, Jim K9YC
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