Strange. I've got loads of them in many places in my station antenna
switching and have never had a problem. Since I recently acquired a new
NanoVNA-F and have three spare Top Ten A/B selectors, I decided to
measure them all. I won't quote all the measurement results, but I will
summarize.
1. Over the range of 1.5 MHz to 54 MHz, the insertion loss of a selected
port went from about -1.6 dB worst case at 1.5 MHz to about -0.35 dB
worst case at 54 MHz. The insertion loss curve (S21) was monotonic over
the range.
2. Isolation between the input port and either unselected output port
and between the two output ports was almost constant across the
frequency sweep, measuring about -83 dB. There were some irregularities
in the isolation across the sweep range, with deeper nulls measuring
around -95 dB at some frequencies.
3. Reflection coefficient (S11) varied from about -45 dB (VSWR = 1.01)
at 1.5 MHz to a worst case -18 dB (VSWR = 1.29) at 54 MHz. Not great for
6 meters, but not all that bad. The VSWR curve was monotonic across the
sweep range. VSWR at 29.7 MHz was below 1.1.
I've learned (the hard way) over the years as an RF systems engineer to
take measurements with numbers like -80 dB with a large grain of salt,
but the NanoVNA is pretty impressive for a small inexpensive instrument.
I also used it to measure a Low Band Systems 20 meter bandpass filter
and a LBS 20-15-10 Triplexer. In every case the measurements obtained
with the NanoVNA were within ±0.1 dB of the results that appeared on the
network analyzer sweeps furnished with the LBS components. I took care
to custom calibrate the unit over the 1.5 MHz - 54 MHz range rather than
relying on the wideband calibration table that ships as a default.
The three Top Ten switches I measured appear to have been manufactured
at different times over the product's life span. One was old enough that
its case was not painted and had holes for uninstalled LED indicators.
The second sample had an unpainted case. The third sample had a painted
case like all current production.
My conclusion: There's nothing wrong with the Top Ten A/B selector that
would disqualify its use for switching duty at HF and probably even at 6
meters. YMMV, but that's my opinion based on a combination of real-world
usage and, now, measurements.
73...
Randy, W8FN
On 1/30/2022 2:52 PM, Jim Brown wrote:
Have you ever measured the SWR through them? It's not pretty. RF
design is terrible, using chassis return as the RF path due to circuit
board layout that breaks the "ground" layer under signal traces. AND
this CAUSES crosstalk on higher bands. I had some back-to-back in my
station, but had to do surgery on the circuit boards to make them
useable on the higher bands. Eventually replaced them with back to
back Tohtsu relays.
73, Jim K9YC
On 1/30/2022 11:33 AM, David Needham wrote:
I really like the Top Ten Devices A/B Station Selectors (Part number
TDS-AB-Selector) as 2x1 coax switches with high isolation between
ports for
setting up a Multi transmitter contest station. Unfortunately, it looks
like they are now out of production. If anyone has any they are
willing to
sell, please contact me off list at AA4VT dot Dave at Gmail dot com.
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