[TowerTalk] WTB Top Ten Devices A/B Station Selectors

[Randy Farmer]

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.