Coaxial Relays

Converting surplus equipment has always been a fascinating eventboth in the convening and die hunt to locate items to convert. What then do we convert?

*elL we convert the things we can find in surplus, scrounging the-scrap dealers and swap meets, looking for candidates upon which to work the fine art of reconstruction. to turn that junk into gold. I am not into collecting old fishing lures or going to sporting events. Maybe I am tweaked towards bias, but professional sports are more big business than sporting events.

A point of view from a test equipment junkie

What gels me up and going, besides my very active grandchildren, is the hunt for a choice piece of microwave surplus materia] that can be modified into something useful. It would he nice if whatever was located in surplus functioned as is. but then thai would take the fun out of the hum and reconstruction, What then can be found in surplus that seems to be overlooked?

The first major thing thai 1 have observed is coaxial relays, especially the 24-voii-ae-tuated SMA postages tamp-si ze micro wave varieiy, Most people shun them for several reasons, with the main one being that they don't operate from 12-volt DC. There have been several articles oil how to disassemble these expensive microwave relays and rewind the coil for 12 vol is DC. I never did thai, as ii seemed to be too much trouble. A solution, but hot for me. What then?

Latching Coaxial Rele
Photo A, Picture of a few varieties of SMA coaxial relays. From left to right; SMA SPOT latching (left and center topi and a 4-porl SMA transfer relay (right top). Standard SMA SPDT24-valt relay center bottom, All relays rated to 18 GHz.

Finding a solution

Creating a 24-volt power source to operate these fine relays seemed a project worth taking oil. Not wishing to use extra power from batteries for portable operation, main operation from 12 volts was desired for all units constructed. A simple DC voltage booster needed to be developed. In scrounging local sources. 1 loeated several DC-to-DC power converters of small physical size thai were fully isolated input to output. Of course, if a 12- to 24-volt converter could be located, problem solved. Other alternatives should be explored using other voltage supplies,

A 12- to 5-Volt isolated power supply (DC -to-DC power block) seems to be common at most surplus scrap dealers, and can be utilized by stacking the 5-volt output on top of the main 12-volt DC input, creating a +1'7-volt line for relay switching. Using 24-volt coaxial relays and operating them on 17 volts seemed reasonable. Of 8 relays tested, 4 relays operated frotn the 17-voIt supply. The remaining 4 relays functioned with 19 volts DC applied.

All relays tested in this batch were standard operating relays. DC voltage was applied to switch from Rx common mode to Tx mode. Another type of relay looking similar in physical appearance to the above relay is what is called ar*latching" relay, I located some at our local swap meet and purchased the batch. These latching relays were cursed with the old stigma of a 28-volt DC coih The swap meet seller wanted $5 each for them because wire ends were still soldered to the relays. Such a deal! I counteroffers three for and he accepted, so 1 took the 6 relays he had.

Photo B. Picture of high-loss BNC relay not usable ai 1296 MHz (center top), better suited for low frequencies like 50 or ¡44 MHz. early SMA version of a better-thati-BNC relay, but still not top performer above I GHz, is at far right,

The relays obtained were Mi crone ties RSM-2DtLJ type. Without a spec sheet and many no hits searching on the Web for details. i powered them up with ray bench supply to test for basic operation and switching contact integrity. They all worked just fine, latching and all. Latching, by the way, means the switch w ill remain in the selected position even when the DC voltage is removed. To transfer back to its original position another pin needs to have DC applied and ii switches hack. Again, the DC voltage can he removed and the switch stays in the new latched position quite a current-saving feature. Non latching relays require voltage to be applied all the time the relay is in operated condition. Then when relay voltage is removed, it returns to normal receive condition with no voltage applied. Latching relays require a pulse of DC voltage to toggle to a set state, and no current flows after [he latch functions. To go to the other state, it needs a pulse of DC voltage on another control pin and the relay latches in the other state and opens current flow in a new state of operation i decided to try the latching relays at a lower voltage switch point and tried 17 volts that I had just used for the other relay tests. Alt relays latched and sw itched just fine — no hang-ups or false operations. 1 set up a simple lamp circuit to watch opening and closure of the SMA relay contacts and finally assured myself that the relays would function not only at 17 volts, but at 12 volts, too.

What a stroke of luck. Every trip foraging through surplus material does not always turn tip a gold purse for the search efforts. Many trips turn up nothing. Not to mention trips that did not pan ouL if you keep your failures secret, many will think you arc a surplus junk man extraordinaire, when in reality what is happening is that a few good trips make up for the many that might have supplied you only with information on new items vet to be received. The early bird gets I he worm scenario works here most of the time. If you snooze, you loose. Check out your Local swap mceis. Lots of dead searches and nothing found, but persistence will pay off. occasionally allowing you to hit the jackpot, I( just requires dedieaiion to the search to locate parts inexpensively.

Why, then, was ] excited when i located SMA postage-stamp-size microwave coaxial relays? Why not get excited aboul some BNC or type connectori/ed coaxial relays? Well, being into microwave operations from 1296 Mil/ to 10 GHz, it is my opinion that the SMA relay rates supreme. They are small, low-profile switches that use a microwave-rated miniature connector. They have been tested to have onlv a few tenths of loss through the

relay contacts and most will handle at 10 GH/. 10 watts of power. It's very important when generating power at microwave frequencies as it is precious and you don't want to give it up to excess loss in components used.

Let's lake the common BNC coaxial relay found in many surplus shops and equipment sLores, and look at a recent conversion I checked out for performance. The unit I tested used four BNC relays constructed in a 1296 MHz transverter. The four BNC relays in this converter design switched a L-wait power amp in and out of the circuit and then switched the receive preamplifier back into the antenna after the transmit relays released. I tested the circuit performance arid found that I could onlv get one mr ^

half of a watt output on the antenna connector and sensitivity was low in receive.

Checking loss through the relay contacts, 1 tou\K\ that each BNC relay and associated coaxial cable and connectors contributed about 1 to 2 dB of loss at 1296 MHz.

i tested the BNC relays at 2 meters (144 Mf It) and found that the loss was a few tenths of a dB. Not wishing to add relays for VHF use to the junk box, I dumped them. When I chcckcd a large <LN" connector-type relay, I found a few tenths of loss at 1296 MHz and good isolation between the switched side and open port pf the relay. However, I did not want to use a relay as large as the "N" connector unit and thought it would be better to shift them from high power applications for HF to low microwave frequency use. The SMA relay still reigns supreme due to its very low loss miniature size and great isolation between ports at all microwave frequencies from DC to 10 GHz. Some premium types are rated to 26 GHz.

After rebuilding the 1296 MHz converter and removing all the BNC relays, I was able

Amateur High Power Microwaves
Photo C\ Picture of large, high-power "N" tonneciorized on right; and SMA version of same relay on the left. Both rated to 2 GHz for reasonably good performance. Just large and high-power.
Coaxial Relay

Photo IX Very high-power coaxial switch constructed with vacuum relay contacts for very high-power handling RF switching capabilities* These types of relays use vacuum switch contacts and solenoid driver coils, Which are totally replaceable. Photo with cover remo \ ed showj lltc vi -h ire ce ra}} i ic body of eacfi six itc'h eletnent for a sit ig Ie po Ie (toi tb le ih n)w (SPOTI coax relay. This coax relay ts getting high-tech and very high-power

Photo IX Very high-power coaxial switch constructed with vacuum relay contacts for very high-power handling RF switching capabilities* These types of relays use vacuum switch contacts and solenoid driver coils, Which are totally replaceable. Photo with cover remo \ ed showj lltc vi -h ire ce ra}} i ic body of eacfi six itc'h eletnent for a sit ig Ie po Ie (toi tb le ih n)w (SPOTI coax relay. This coax relay ts getting high-tech and very high-power to improve the receive sensitivity some 4 dB 011 transmit and now have L2 watts of power output on the system. Removing itie BNC relays and their high switching loss did the trick. By changing the circuit s switching configuration I was able to use only one S PPT SMA coaxial relay to do all the switching. I deemed it was not necessary to switch the preamp and power amp, and let the coax switching take place on preamp in and RF out to the antenna. With the old design using BNC relays, the isolation between preamp and the power RF

amplifier would have been very poor and possibly destroyed the RF preamp if switching were done with a single BNC relay. Isolation at 1296 MHz with the SMA relay measured in excess of 30 dB loss between the switched side of the relay and the nonswiLched side.

The bottom line with RF coaxial relays is to pick Lip w hat you can for your projects but have a handle on which type will better serve your application and frequency of operation specifications-Yes, the SMA connector is the best choice for micro wave frequencies. Power specifications roll off as frequency is increased. I haven't observed any real problems running 10 watts of power at 10 GHz* I would not try to push ratings and go for the maximum power, as I am a Conservative user and believe in not pushing the maximum envelope. Do I use an SMA relay tor sw itching the 1 and 5 watt amplifiers described in last month's 73 Magazine column? You bet! SMA forever.

Some general rules on SMA coaxial switches are; (1) Small size can handle higher power levels, with some manufacturers claiming several hundred watts at YHF and derated in power at 10 GHz to the 20-watt range. (2) Isolation or crosstalk between open and closed side of the switch is rated in dB and can be as great as 50 dB hi some relays. These are maximum ratings for certain types, and 1 recommend not pushing any device to its maximum ratings,

"N" type relays are rated in the 500- to 1,000-waU range at HF/VHF and are reduced in power to the 50- to 100-watt ranges at frequencies of 2 to 3 GHz. Typically. they are large in size and can be ou tit tied with SMA connectors as shown in Photo C, if you look close, i don I try and push ratings, and I reserve this type of relay for HF to 1296 MHz high-power rigs.

Photo E shows die largest coaxial relay in my collection. It weighs in at 7 pounds, and that's without connectors attached. It's made for very-iarge-diameter one-inch helix cable for industrial high-power switching. A centerpiece to be sure of, or a great door stop if you're not into very high-power relays,

BNC and related UHF connectors and associated relays are better left for 2 to 30 MHz for the purist. Yes, I know they are used at much higher frequencies, but compared to "N** and "SMA" types, the BNC and UHF connectors are left in the dust. If you locate a relay and want to determine its ratings, try searching the Web for its original manufacturer. However, ii might be difficult, as most companies have merged or just gone away.

As with all projects, ami especially amateur micro wave-re luted items covering this coaxial relay application, I will be glad to answer questions. Drop me a note on E-mail to [[email protected]].

Well, that's it for this month. Coming up, I will be covering usage of microwave attenuators, detectors, and other coaxial test devices. 73, Chuck WB61GR ®

Photo E. Now, just when you think you have seen ¡everything, here is a coaxial relay> with connectors, that weighs in ot seven pounds* A short section of coax cable is attached with connectors on each endt contributing a half pound Consider this relay in the multi-nudti-kilowatt level Shown with smaller UN'J- and SMA-type relays for size comparison; miniature SMA shown center front is I-inch square.

Photo E. Now, just when you think you have seen ¡everything, here is a coaxial relay> with connectors, that weighs in ot seven pounds* A short section of coax cable is attached with connectors on each endt contributing a half pound Consider this relay in the multi-nudti-kilowatt level Shown with smaller UN'J- and SMA-type relays for size comparison; miniature SMA shown center front is I-inch square.

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