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Old 12th Nov 2018, 1:22 am   #81
Radio1950
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Default Re: RF power meter calibration

Quote:
Originally Posted by Radio Wrangler View Post

When G3OGR was playing with 807s, all he was required to do was measure the HT and the DC anode current. Life was simpler back then!
Memories.

When I was about 15 or 16, I made a HF TX using a Geloso VFO and an 807 RF PA with 2 x 807 as mod.

My "wattmeter" was a 60 or 75 W light bulb as a dummy load, with brightness as an indicator of RF power.
Calibration?
After peaking the TX pi-coupler into the bulb, you had another bulb of the same power next to it on the bench, fed with mains through a rheostat, and adjusted the rheostat for identical brightness, then did the maths.
A couple of layers of grease-proof sandwich paper laid over the two bulbs made visual comparison easier.

Advance ten years, and there I was, alone, on night shift, checking the RF pulse power of a 2.5MW ATC radar.
A "Power and SWR Meter" had an array of 15? neon tubes on the side of a section of wave-guide, each with end metal probe, and the 300mm long (for L band) array of neons was pushed by lever into the long side of the wave-guide, maybe about 1 cm. The length of the neon discharge, indicating power, was read against a calibrated chart on glass. The variation in the length of the neon discharge along the array gave an indication of SWR.

Calibration?
I have absolutely no idea how the factory would have done this, although it was probably with calorimetry and a load.
We did have a dummy load, a tapered graphite termination in WG, about four feet long.

The "Power and SWR" device is that black thing in the photo; apologies for quality.
.
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Old 12th Nov 2018, 10:19 am   #82
Terry_VK5TM
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Default Re: RF power meter calibration

LS120 and LS240 Superdisks, once very popular here in Australia.

I think I may still have one floating around in one of the boxes in the shed.

More info: https://en.wikipedia.org/wiki/SuperDisk
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Old 12th Nov 2018, 8:47 pm   #83
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Default Re: RF power meter calibration

Jeremy; are you trying to obtain LS120 disks; if so, please pm me.

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Old 12th Nov 2018, 10:27 pm   #84
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Default Re: RF power meter calibration

I've managed to buy a couple of them and they are for storing the recovery disk info so a fresh HDD can be loaded with Win98 and the scope SW and drivers by the scope itself from a fresh bootup. Thanks for the offer though

I found a couple of x10 scope probe voltage derating charts and thought they may be of interest. The cheapo Rigol PVP2150 x10 probe seems to have a very aggressive derating curve above a couple of MHz. The other chart is for the Tek P2220 x10 scope probe.

The limit for 71Vrms is a few MHz so this limits the frequency range it could be used for measuring a 100W transmission into a 50R load. However, I'm not sure what will happen if these limits are ignored. The probe will start to absorb some power and turn it into heat as the frequency increases but I don't know if this is the reason for the curve or if there is another weakness in the probe design.

If a randomly selected x10 probe has an Rp of 2500 ohm at 30MHz then at 71V RMS this would mean 2W of power dissipated in the scope probe for example.
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Old 12th Nov 2018, 10:49 pm   #85
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Default Re: RF power meter calibration

OK, somewhere I have an old 120 drive and a few discs to go with it, dating back to the 90's.

Re Power Meters, I ordered a couple of log-power meter modules using AD8310 (440MHz), giving a direct reading to a DMM for just over a £5er each. Of course the 8307 is probably the best know of those chips and that runs to 500MHz, but is slightly more expensive.

It will offer an interesting alternative (and better?) to the use of either a scope or diode probe to measure input to my dummy load.

No flange resistors yet .

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Old 13th Nov 2018, 12:03 am   #86
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Default Re: RF power meter calibration

Wow, a working LS120 drive is much more interesting but I'm not sure I actually need one because the waveform data I get off the scope will usually just be a small file of a few kb. At the moment I'm trying to do this over LAN to a little EeePC. But I am tempted to ask what you would want for the LS120 drive if it is a runner.
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Old 13th Nov 2018, 12:13 am   #87
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Default Re: RF power meter calibration

Jeremy, give me day or three to dig it out and check it and I'll get back to you.
I used to use it for backing up back in the day when 120MB was a significant capacity. Have you decided not to use the PM system on the forum?

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Old 3rd Dec 2018, 5:54 pm   #88
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Default Re: RF power meter calibration

Steve,

looks like you're not too far away from me.
I have a shack equipped with modern accurate RF test gear and could calibrate your stuff for you. PM me if you've not sorted something out already.
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