[Elecraft] Looking for a way to switch an antenna input to the KAT500

[Randy Farmer]

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