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Old 26th Jun 2022, 5:38 pm   #1
G6Tanuki
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Default "KW Match" SWR-meter.

Got this today at the Newbury rally; paid £3. It's been 'got-at' by a previous owner, who has replaced the original Belling-Lee type TV-aerial sockets with some strange SO239-types. The inside-the-box wiring being something that would have made Rowley Shears G8KW cry... [coax braid soldered directly to the socket-retaining-nuts using what I suspect was a pickaxe heated in a forge].

The 'sampling loop' is a length of old-style 75-Ohm semi-airspaced coax with a wire threaded down one of the longitudinal air-spaces; given that these days everything is 50-Ohm, I'm minded to replace this with a length of 50-Ohm coax, stripping off the outer sheath, bunching-up the braid to let me introduce a length of wire, then heatshrinking over the result.

BNC sockets to replace the nasty SO239-stuff too.

The diode used is also something I have never seen before... a curious white ceramic-type thing looking a bit like an old 1-Watt ceramic cased resistor. Knowing the issues with old diodes, I feel it should be replaced by something from my stock of nice OA47 gold-bonded ones.

Overall, I like its robust and uncompromising styling, and am sure it can be rendered fully-usable-for-the-21st-century without too much effort.
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Old 26th Jun 2022, 7:19 pm   #2
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Default Re: "KW Match" SWR-meter.

Looks like a good old piece of kit and for £3

Here's a similar thing I made over 40 years ago. It too uses a length of co-ax cable with a sense wire threaded under the braid. I call it a reflectometer rather than an SWR metre, as it's not calibrated. I still use it in my vintage style station.

73

Aub, G4KQL
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Old 27th Jun 2022, 11:49 am   #3
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Default Re: "KW Match" SWR-meter.

The original connectors were Belling-Lee TV aerial sockets but BNC sockets will look nicer!

73

Roger
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Old 27th Jun 2022, 7:36 pm   #4
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Default Re: "KW Match" SWR-meter.

Yes, though instinctively I fit the 4-bolt-mount BNCs from the rear, which would leave the two legacy-holes from the original Belling-Lee sockets visible.

Aesthetics ahead of 'form-follows-function' ? I must be getting soft in my sixties.

Tests with 150W of RF show that the strange diode no longer provides any sensible kind of detection-capability. Somewhere in the attic I have a bunch of glass-bubble 1N34A Germanium diodes, which would work well as a replacement... butn do I want to disturb the bat-roost to try and dig them out?
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Old 27th Jun 2022, 8:15 pm   #5
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Default Re: "KW Match" SWR-meter.

I think that case deserves some better directional coupler than coax and a threaded wire. It will be frequency sensitive.

There is a very nice HF/VHF Directional Coupler used in the Welz power meters. Surprisingly good claims, 1.8-150MHz and up to 1kW. I would love to be able to reverse engineer it. I have one. I took the SO239s off it and fitted N type.
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Old 27th Jun 2022, 11:02 pm   #6
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Default Re: "KW Match" SWR-meter.

Have a look in the G-QRP club SPRAT number 61

David
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Old 28th Jun 2022, 8:36 am   #7
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Default Re: "KW Match" SWR-meter.

Quote:
Originally Posted by Jon_G4MDC View Post
I think that case deserves some better directional coupler than coax and a threaded wire. It will be frequency sensitive.
Yes, it is frequency sensitive as you get more voltage appearing on the sense wire at higher frequencies. Just back off the sensitivity control to compensate. Very simple and works well.

Cheers

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Old 6th Jul 2022, 8:16 pm   #8
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Default Re: "KW Match" SWR-meter.

Coming back to this in the context of replacing the hacked-in SO239 sockets with BNCs:m from what I can see the original design used the kind of plastic-bodied Belling-Lee 75-Ohm coax sockets where both inner and braid were insulated from whatever they were mounted to.

So the braid of the coax sampling-loop was not connected to the case of the meter.

It would seem problematic to me to replace the nasty SO239s with BNC sockets whose outer connected to the case as well as the coax-braid; surely bt providing a 'bypass' path through the case this would compromise the idea that the sensor's sampling-loop in the coax could sensibly see the characteristics of what was going-on in the coax?
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Old 4th Nov 2022, 6:34 pm   #9
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Default Re: "KW Match" SWR-meter.

Coming back to this after a few months, I spent the afternoon replacing the kludged-in-place SO239 sockets with some nice flange-mount BNC ones.

Only required a bit of filing of the mounting holes on he sockets and then everything lined up nicely with no disruption to the front panel apparance.

Time to test it., A whiff of RF on 28MHz.... nothing!

The little diode was open-circuit.

Pop in a new 1N34A, test again.

Nothing.

Turns out the meter is open-circuit! Which probably renders the thing BER because it's a really odd style of meter and the chance of finding something similar is rather low,

Which probably explains why it was in a pile of junk at the rally.

Oh well... some you win some you lose. At least I gained a double-pole-changeover switch, a 50K potentiomete and a couple of knobs for my money.
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Old 4th Nov 2022, 7:02 pm   #10
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Default Re: "KW Match" SWR-meter.

We've all been there!
At times like this it is often time for a
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Old 4th Nov 2022, 7:03 pm   #11
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Default Re: "KW Match" SWR-meter.

Try asking on the KW-Radios list on Groups.io

Someone may just have meter

Fred
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Quote:
Originally Posted by G6Tanuki View Post
Coming back to this after a few months, I spent the afternoon replacing the kludged-in-place SO239 sockets with some nice flange-mount BNC ones.

Only required a bit of filing of the mounting holes on he sockets and then everything lined up nicely with no disruption to the front panel apparance.

Time to test it., A whiff of RF on 28MHz.... nothing!

The little diode was open-circuit.

Pop in a new 1N34A, test again.

Nothing.

Turns out the meter is open-circuit! Which probably renders the thing BER because it's a really odd style of meter and the chance of finding something similar is rather low,

Which probably explains why it was in a pile of junk at the rally.

Oh well... some you win some you lose. At least I gained a double-pole-changeover switch, a 50K potentiomete and a couple of knobs for my money.
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Old 4th Nov 2022, 7:10 pm   #12
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Default Re: "KW Match" SWR-meter.

That's an idea. Alternatively I may just hack out the hole in the front of the case and fit a more easily-acquired 500-microamp meter.

[I'm not an originality-obsessive: just making it work so I can use it with my little AEL3030 transceiver will be good enough for me!]
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Old 4th Nov 2022, 7:36 pm   #13
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Default Re: "KW Match" SWR-meter.

Is it really a four screw fixing meter? Not the usual size round ones that have 3 hole fixing.
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Old 4th Nov 2022, 7:56 pm   #14
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Default Re: "KW Match" SWR-meter.

It's strange: the meter itself [which I believe to be original, it is marked "KW Electronics" on the dial] uses a screw-on retaining ring from the rear; the four screws have washers/nuts/spacers fitted on the rear and serve to clamp the edge of the screw-on retaining ring to the front panel.

Makes me think that there may have been two different types of meter fitted, and the screws which may have been needed to retain one style of meter are fitted in this case just to fill the holes in the panel that would be used for a 4-screw version.
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Old 4th Nov 2022, 9:41 pm   #15
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Default Re: "KW Match" SWR-meter.

Racal used the form of meter with a big threaded retaining ring.

But you could always replace the meter. Something a bit bigger to cover multitude of sins.

The directional coupler bridge I put in Sprat 61 is very flat across frequency.

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Old 5th Nov 2022, 1:02 pm   #16
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Default Re: "KW Match" SWR-meter.

Ah yes, I thought I remembered seeing that style of meter somewhere...

It is marked on the back:

Anders Electronics Ltd.
104 Hampstead Road NW1.
Tel. Euston 1639

Which gives an idea of its age, the named exchange thing having become obsolete by the mid 1960s.

When the rally season resumes next year I will be on the lookout for this type of meter, until then I will Park the job and get on with something more useful.
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Old 5th Nov 2022, 1:26 pm   #17
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Default Re: "KW Match" SWR-meter.

A bit more googling shows that "Mullard Magic" have an Anders 500uA meter with a KW Electronics SWR scale calibration.

See https://mullard.org/collections/comp...ter-kw-valiant

Though their one has a 4-hole-mount square front panel. Which explains the four extraneous holes in the metal box housing my round SWR meter....!

Also marked "Meter Test No 2, ZA13592" which suggests it is a repurposed surplus part.

They want £22 for it!
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Old 11th Jun 2023, 9:40 pm   #18
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Default Re: "KW Match" SWR-meter.

Thread reopened at OP's request.
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Old 11th Jun 2023, 10:14 pm   #19
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Default Re: "KW Match" SWR-meter.

If you're really stuck for a meter I'e got a selection of modern styles in the attic. and can have a rootle round. Oh and that directional coupler in Sprat works fine with a 1mA movement.

But you're missing out on one cool trick... have separate meters for forwards and reverse power then you can forget about VSWR, just null reflected power with your ATU and read your forwards power directly. One clance sees everything and no messing about with switches or cal pots.

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Old 12th Jun 2023, 11:51 am   #20
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Default Re: "KW Match" SWR-meter.

Thanks to the Mods for re-opening this so I can give a 'final' on it.

I acquired a square 100uA 'SIFAM' meter from the West of England rally yesterday; cost me £1. It had a 'Failed QA' sticker on it, but the needle moved freely. It just wasn't zero-able by the front adjuster - which was undoubtedly the reason for the QA failure. Investigation showed that it had been assembled without the adjuster's pin being properly aligned so it wasn't fitting into the hairspring mounting's slot. Easily fixed!

Now you will remember when I removed the old failed original circular meter I noticed four unused holes that were covered by the round meter's flange. These aligned almost perfectly with the four mounting-studs on the rear of the Sifam meter, only needing a quick tweak with a rat's tail to make the fit perfect.

OK, the new meter is square, and doesn't have an SWR meter calibration, but it does look age-appropriate in a way which my other idea - using one of the clear-plastic-faced meters that are available cheaply - wouldn't have been.

I'll draw up a calibration chart using a bunch of 50-Ohm resistors to simulate various mismatches, but apart from that I am happy; this will only ever be a SWR 'indicator' for use in antenna experiments where I won't shed too many tears if I knock it off a ladder or forget it and leave it outside where it gets wet; for anything needing accurate measurement I have a Bird.

Either way, for a total expenditure of less than a fiver, it's nice to see a bit of 50's gear brought back into service.
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