CWTD June 5, 2012

June 5, 2012

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Retro SWR

Analysis, design and construction of a simple & useful SWR meter from yesteryear!

Overview

Remember those old lovable and simple SWR meters from back in the Heathkit days? Simple tap off the coax, a couple of diodes as the forward and reverse RF probes, and a sensitive meter that can be switched to read in either direction? Well, what's old is new again as we explore the topic in this episode of CWTD, but using a couple of simplifying techniques to produce something as workable as before ... and equally fun in the building process!

73, George N2APB & Joe N2CX

Audio Recording ... (Listen to the MP3 podcast)

Discussion Notes:

<20:01:08> "W2AEW - Alan": The "ugly style" SWR meter is beautiful!
<20:11:35> "Pete - WB2QLL": Original SWR meter derived from a circuit called "Monimatch"
<20:11:43> "WA0ITP Terry": Shades of the old monimatch circuit
<20:11:55> "George - N2APB": Ahhhh yes, I remember the Monimatch!
<20:14:38> "Sam NV1P": can a schottky diode be used to decrease the drop?
<20:14:51> "Sam NV1P": ok
<20:15:37> "W2AEW - Alan": For a 200uA meter movement - what values are the two series limit resistors and the adjust pot?
<20:18:00> "Armand WA1UQO": The gray one IS 100ua
<20:18:25> "George - N2APB": Yup, that's what the meter is on the one sitting in front of me!
<20:18:51> "Mike WA8BXN": what does the 82 ohm resistor do?
<20:19:41> "Pete - WB2QLL": All the old Japanese SWR bridges by Lafayette, Swan, etc. used 100ua meters.
<20:20:07> "Mike WA8BXN": i think i do somewhere
<20:20:09> "Sam NV1P": 2 of them
<20:20:30> "Sam NV1P": not heath, but my father built one of them and my main elmer built the other
<20:20:48> "WA0ITP Terry": Y, homebrew Monimatch from years ago.
<20:21:40> "Joe N2CX": I picked up a CB SWR meter for $2 at a hamfest just to get the cheap analog panel meter movement!
<20:21:57> "Pete - WB2QLL": Dual meter job I bought from Madison in the early 70's - still used it.
<20:22:31> "Rick K3IND": how about vhf?
<20:24:12> "George - N2APB": VHF operation is VHF ... The principles work the same, but need to be very careful of the lead lengths.
<20:27:18> "Pete - WB2QLL": What's the unlabeled resistor?
<20:27:33> "Pete - WB2QLL": The one above the unlabeled cap :)
<20:27:34> "George - N2APB": Filtering of the detected voltage.
<20:27:56> "W2AEW - Alan": The Emtech ZM-2 Z-Match tuner discussed 2 weeks ago, uses a resistive bridge to drive it's visual SWR indicator
<20:27:57> "George - N2APB": On the order of 4.7K with a .01uF going to ground.
<20:28:25> "George - N2APB": Oh yeah, you're right! I forgot about the ZM-2 ATU.
<20:29:43> "Pete - WB2QLL": So does the BLT from Hendricks
<20:35:45> "W2AEW - Alan": Hmmm - if the antenna impedance is <50ohms, will this still work as drawn?
<20:38:31> "George - N2APB": Yeah, I believe so.
<20:43:27> "W2AEW - Alan": FYI - here's the schematic of the ZM-2 - using a bridge for SWR detection, but uses a transformer and LED to indicate imbalance http://www.qsl.net/kj6vu/ZM.html
<20:46:04> "Mike WA8BXN": When do I need to know the swr number value? Can't I just always tune for minimum reflected value?
<20:46:14> "W2AEW - Alan": I like meters! (but the Tayloe detector works well)
<20:47:46> "Pete - WB2QLL": tag strips
<20:50:17> "W2AEW - Alan": Here's a photo of the resistive bridge and Tayloe detector in my ZM-2 ATU http://www.flickr.com/photos/w2aew/2341244975/in/set-72157611590230153/
<20:51:07> "W2AEW - Alan": Beautiful ugly construction on that George (oxymoron)
<20:53:38> "Rick K3IND": Can you give a Mouser or DigiKey part number for those BNCs?
<20:54:57> "W2AEW - Alan": BNCs look like there: http://www.mouser.com/ProductDetail/Amphenol-RF/31-5540/?qs=qhgJLQbFbXniJwRomxXiFq1Y8dDqr0QZ
<20:55:48> "W2AEW - Alan": or these: http://www.mouser.com/ProductDetail/Amphenol-RF/31-5540/?qs=qhgJLQbFbXniJwRomxXiFq1Y8dDqr0QZ
<20:57:01> "George - N2APB": Yep, Those are the ones. I use them in may of our projects: Micro908, Growler, etc.
<20:57:11> "W2AEW - Alan": Those big tip irons are great for PL-259 soldering too.
<20:57:59> "Mike WA8BXN": sop receiver?
<20:58:39> "W2AEW - Alan": If you use brass hardware, you can solder machine nuts into the corners, then you can use machine screws to assemble the top
<20:58:57> "George - N2APB": GREAT point on the brass nuts in the corner!!
<20:59:20> "George - N2APB": And yes, the SOP Receiver used the copper-clad enclosure.
<21:00:39> "Joe N2CX": But you had best use a stainless steel screw to hold the nuts in place so you don't solder the screw at the same time!
<21:04:05> "W2AEW - Alan": I'm going to want one!
<21:04:51> "Armand WA1UQO": Same here Alan!
<21:06:16> "Mike WA8BXN": can i measure power with this project?
<21:08:01> "Jeff W7WWA": Are you talking about putting a tap in front of this design for higher power user?
<21:08:28> "Joe N2CX": Yes it's something like a tpa, but not exactly.
<21:08:28> "Pete - WB2QLL": Was a circuit for the external dummy version in Electric Radio 10-12 years ago.
<21:09:14> "Joe N2CX": Could have been in Electric Radio. I've seen it in various publications since my 60's Novice days
<21:11:29> "Brooks - WA5OZE": How did it become known as a "Retro" SWR meter design?
<21:14:11> "George - N2APB": "Retro" as in simple bridges from yesteryear, but with a simple-yet-improving use of resistors instead of toroid sampling of RF.
<21:14:22> "George - N2APB": Also, a "retro" construction method, as we mentioned.
<21:17:16> "Joe N2CX": Plus using the meter gives it a "retro" look.
<21:20:16> "W2AEW - Alan": I really like the sky-wire technique
<21:20:18> "George - N2APB": Meter? Meter? We (don't) ... DO! ... need a stinkin' meter!!!
<21:20:32> "WA0ITP Terry": Thanks for pulling these sessions together, appreciate it.
<21:20:32> "George - N2APB": Ha! Skywire. I love it!
<21:20:40> "W2AEW - Alan": that's what I always called it
<21:20:58> "W2AEW - Alan": 73

SESSION NOTES

Retro SWR

The old Heathkit SWR bridges ... recall the nostalgia days of building simple kits?!

From Wikipedia ... "The SWR meter or VSWR (voltage standing wave ratio) meter measures the standing wave ratio in a transmission line. The meter can be used to indicate the degree of mismatch between a transmission line and its load (usually a radio antenna), or evaluate the effectiveness of impedance matching efforts."