Monday, March 30, 2009

Coaxial Switches

I've been looking for a good coaxial switch for quite some time! I went through the "usual suspects" - MFJ, Daiwa, Diamond and Alpha-Delta. The quality of execution varies from pure junk (MFJ) to very good (Diamond and Alpha-Delta).

Alpha-Delta 2 and Diamond CX-210.

When it comes to internal design and construction, all of these switches share common design concept (especially the A/B (SPDT) type) - a moveable contact "finger" engaged by the lever and a couple of contact points in the same cavity. They are not "true coaxial switches" and in my book they are far from "great"! This design is just a notch better than an "open frame relay" type A/B switch. There are problems with poor port-to-port isolation, insertion loss, SWR, frequency response, reliability and power handling, etc - all common problems for many "ham grade" (read "inexpensive") coax switches.
I was on a quest for a great coax switch and I finally found it - the Transco M1460 series switches.

Transco M1460-30 double pole manual transfer switch.

Transco was a US company, manufacturing high-end RF switches and relays many years ago. Their line of switches was acquired by DowKey and the manual type of the M1460 series was discontinued.

It looks to me that when it comes to RF switches it is the classic case of "they don't make them like they used to"! Truth be told - the Transco switches are not your average "ham grade" type - these are very high quality commercial / military RF switches with the price tag of hundreds of dollars in their time. The good news is that now they can be found sometimes at hamfests or eBay for anywhere from $10 to $50. These unassuming, old switches have incredibly clever yet simple internal mechanics and were manufactured with extreme precision and tight tolerances for the US Army, Navy, aviation, etc. (some of them are labeled as "US Property"). There are a couple of types - manual and motorized type, each in different configurations, port number (and voltages for the electric type). At 10 MHz, typical isolation is >80 dB (the actual is close to 100dB), VSWR 1.05 and Insertion Loss 0.05 dB. Some switches are rated for use at up to 10 GHz with maximum power of 300W CW at 3 GHz! On the HF frequencies, this translates to more than 1.5 kW (in practice it is only limited by the N type connector power ratings).
I managed to acquire a couple of M1460-30 (Double-pole transfer switch) and M1460-4 (4 position switch) for the price of a single Diamond or Alpha-Delta STDP switch!
M1460-4 (or 5-6) is a straight-forward 4(5,6) position switch designed to connect single port to 1-4(6) ports.

M1460-30 transfer switch can be used as an A/B switch as well but it is really handy as a transfer switch - I am using it to connect my two amplifiers to one of the antennas and a dummy load. With this switch I am able to swap the amplifiers to different ports and decide which one is connected to the antenna while the other is connected to the dummy load. It can be used also to swap 2 antennas to 2 transceivers or as by-pass/in-line switching of amplifier, preamp, pre-selector, filter or antenna tuner. It is very versatile type of switch around the ham-shack.

These Transco switches are good up to the GHz range and are using silver-plated N-type female connectors with teflon dielectric.

At the back side (internal, pictured on the right) of the N connectors there are (4) contact points with spring-loaded pins. The internal switching assembly (picture, left) contains two contact "bridges" (true coaxial lines) with 2 silver plated contact points (each) in constant-impedance channels (teflon dielectric). When the switch is rotated, two ball-bearings located at 180 degrees from each other (left, center) are pressing against a special action disc with 4 (at 90 degrees) radial groves (right, center). In the center of this disk there is a sleeve bearing for the shaft. The groves are with beveled edge to facilitate easy sliding in/out of the ball bearings. Normally, the ball bearings are resting in the radial groves, but during rotation of the switch they come out of the groves and press against the raised part of the disc, pushing back the whole switching assembly on its axis and away from the static contacts. In other words, the radial movement is converted into axial movement.

There are 6 springs in the coax bridge assembly creating an opposing force in the axial direction (located on the front side of the bridge assembly around the switch shaft and pressing onto a special graphite covered friction plate). The axial play causes the whole assembly to retract away from the contact points plane and allows for a clean rotation. When the rotation is completed and it comes to a stop at the next switch position, the steel balls are pushed back into the next set radial groves. The "bridge" assembly is pushed on its axis by the springs, flush against the contact surface (back plate) and the contact bridges in the assembly push against the N-connectors contact pins (also spring-loaded) and close the circuits. The back face of the switch assembly is pressed against the back-plate, grounding the assembly. As a result from this design feature, both contact pairs are completely sealed/shielded from each other creating very high port isolation. The SWR is extremely low and there is no impedance bump because of the constant 50 ohm impedance cavity of the contact channels. The actual contact points are silver plated and with large contact areas allowing for high power use. Very good electrical contact is established by the large, polished, flat faces of the contact points and internal springs in the N-connector portion, pressing the pins against the corresponding contacts in the bridge assembly.
There is a loud "clank" when the switch is completed and in the next position, but the rotation is nice and very smooth. The switching knob slightly moves out on its axis (about a 1/8 inch) at the beginning of the switch cycle and retracts back at the end of the cycle. Its all automatic, not like in Bird 74 where the operator must manually pull the lever out, rotate and then push it back at the end of the rotation.
The only "drawback" of this design (if I can even call it "drawback") is the relatively high force need to operate the switch - it feels a bit "heavy" at the very beginning of the rotation. A large (easy to handle) knob or lever is needed to actuate the switch with less effort.
There are 4 screws on the front of the switch used to attach a "face plate" with labels for the positions. Two screws on the side of the switch are used for mounting.

M1460-4 is a more complex design but very similar. Instead of a sleeve bearing in the center of the static contact back-plate with the N-connectors, there is a special silver-plated coaxial two-part contact arrangement. Also, the grooved plate and the steel ball-bearings are reversed in place - the plate is located on the switch rotor and the ball bearings are on the static back-plate. There are more springs (8) and the pressure plate (friction surface) located on the back, around the shaft is larger.

The motorized version is comprised of two parts - RF Head and Actuator. The Actuator is driven by an AC or DC motor, and includes a "gear box" speed reducer and a rotary switch which disconnects the motor once the switch is in the next position. The RF Head could be separated from the actuator and used as a manual switch if needed. The separation is not that difficult but a proper lever needs to be used for manual actuation since the shaft of the RF head is shorter than the standard manual version.

For maintenance, I cleaned and lubricated the contacts with Deoxit D5 solution and treated them with Deoxit ProGold. I removed the old grease and lubricated the pressure plate, both sleeve bearings and the action ball bearings with white lithium grease. I also removed the tarnish from the silver plated N-connectors with a product called Tarn-X.

Transco switches are clearly the best RF switches I've seen so far and IMHO are better than the famous Bird 74 switch. I replaced all of the RF switches in my shack with Transco getting excellent results!