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Author Topic: SB221 qsk upgrade  (Read 2625 times)

Posts: 4

« on: February 11, 2005, 03:39:08 PM »

 Greetings all. Quickly, after a several year hiatus from radio, I'm getting the urge to get back on.

What I have is a SB221 that I'd like to conver to qsk. I started this project about 5 years ago but it was quickly forgotten and shoved way back, waaayyyy back on the list of jobs to do ever since I took the MSF course and got my motorcycle endorsement. 40K miles later and more tires used up than I can recall I want to get back on the low bands with a little power.

Question is this: Vacuum relays or PIN diodes? I'm a builder by nature and trade. PIN diodes are appealing becaue of their speed and absolute silent operation. Vacuum relays are getting a little too pricey, hard to find, and obsolete but for a few applications. Google searches haven't yielded too many homebrew PIN diode projects that I'd like to review before embarking on a full-fledged design. Limited time.

Suggestions before I procede?

Thanks all..


Posts: 21764

« Reply #1 on: February 11, 2005, 04:22:05 PM »

PINs are my choice, and I added diode T-R switching to my old Henry 3K Premier many years ago for that reason.  They worked well and never failed; however, I heard a lot of people say they do fail in lightning-prone areas, from lightning transients.  That never happened to me, but I'm in L.A., where lightning almost never happens.

In my case, the PIN switch was made complex because I built a 2-output power supply to make them really sing: Used high reverse voltage for max isolation, and then a lot of forward current for min attenuation.  The Henry couldn't provide any of that, so my "PIN diode T-R switch project" became a big power supply with diodes as accessories, so to speak.

I bought the RF chokes and coupling/blocking capacitors from Apex Electronics, an electronics surplus house in L.A., but I see that pretty appropriate parts are available today from RF Parts Company

I used two varieties of Unitrode/Microsemi PINs, selected for optimum performance in pass and shunt switch modes.  Probably that was overkill, but the diodes were available.

There hasn't been a lot published on this in the "big" periodicals.  I wrote up my project, with schematics and photos, and submitted it to the League for publication in QST and they declined it, suggesting they put it in QEX (the experimenter's mag) instead.  I declined that, because I wanted it to have better circulation in the public domain.  That may have been a foible, since it never got published at all!

Let me know if you'd like the schematic for what I built, and I can FAX or scan and e-mail it to you.  It goes back several years, but worked reliably.


Posts: 4

« Reply #2 on: February 12, 2005, 03:27:51 PM »

Ahh, just the support I was looking for.

In my case, the diodes are not yet available, but soon will be.  10KW rf switching job at work.

That plus the cost and general unavailability of vacuum relays and the prices that the Ebay robber-barrons are asking for them make them unnatractive.

What sort of scheme did you come up with (if any) for failure detection?  I guess a failure with a amplifier switch wouldn't be as deadly as a T/R failure in a tranceiver, but it still needs consideration.  I've only done some low power 1/4 wave series/shunt low power vhf switches before.  This is my first of a couple of forays into high power rf solid state switching.

Oh, your call sounds familiar.  Have we worked on Field Day maybe??  Just curious.



Posts: 21764

« Reply #3 on: February 14, 2005, 09:33:56 AM »

Don't know about Field Day, maybe.

The 1/4-wave stub stuff doesn't work for multi-band HF work, of course.  Isolation is purely from the diodes, which are DC isolated with RF chokes and capacitors so the switch can cover a broad spectrum; although as long as the carrier lifetime is with PINs, I don't know if my design would work below 1.8 MHz or not (never tried it).

As for fault detection, I don't use any.  Of course, I haven't had a "fault" in 15 years, so I likely saved work and money by simply ignoring this.  The PINs themselves could fail either open or short.  A built-in failsafe for a short would be to use a fuse in the power supply that powers the PINs, and use a rating that's not much above the normal current draw for the operating switch in the TX mode.  If a diode fails short, the fuse will blow and alert you that something happened.  I can't think of any reasonable way to detect an open, but simply using the switch it would be obvious if an open occurred: An open in the RX path would create much reduced sensitivity, and an open in the TX path would create very different PA tuning and little to no output power.

73 & good luck!

Steve WB2WIK/6


Posts: 173

« Reply #4 on: February 15, 2005, 02:18:01 PM »

PIN diodes don't work well at 160 - the carrier lifetime is so short that you'll end up biasing them with nearly as much DC current as the peak RF.  In that case, you're just about as well off by using any Schottky rectifier diodes - parallel them to get enough current rating.

For HF amps I prefer vacuum relays.  With VHF/UHF amps, you can use those quarter wave techniques, narrow-banding the switch, which vastly reduces the energy coupled into the diodes when there's a nearby lightning strike.

Just watch the ham email reflectors and BBSes to see how many people repeatedly replace PIN diodes in HF amps - but rarely in VHF/UHF amps.


Posts: 3160

« Reply #5 on: February 25, 2005, 01:27:47 PM »

Rick Measures, AG6K has an entire article (his original November / December 1990 QST article - with 2001 updates) on circuit improvements for the Heathkit SB-220, including QSK modifications/upgrade.

Rick has the Matsushita RF input reed-relay referenced in the doucmentation and as well as the Kilovac vacuum relay.

MAX-GAIN usually has a supply of surplus vacuum relays (e.g., Kilovac HC-1 and Jennings RJ1A) available at reasonable prices.

Best of luck with the project

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