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Old 4th Mar 2019, 6:33 pm   #1
MrBungle
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Default Hammond box mounting and RF tightness

I have picked up an old project again, the spectrum analyser, this week.

I have designed a 40dB post filter IF amp strip and built it manhattan style. This has been mounted in the lid of a Hammond box with the panel mount SMA for RF ports and FT cap for power sticking up from the top. The hammond box was picked so it was a fraction of the shortest wavelength long (it won't hit more than about 250MHz)

Now the question comes on how am I going to mount this in a chassis? I didn't think about this first typically and decided to wing it, perhaps stupidly ... I figure I can drill holes through the bottom of the "tub" part of the Hammond box and use M3 hardware to bolt it down to the chassis so:

bolt, washer, hammond box base, washer. At this point I'll have a Hammond box with a couple of bolts sticking out of the bottom. Then when it comes to set them in place I can unscrew the bolt, then gently drop it through the 1.6mm aluminium chassis and bolt underneath. I will end up with bolt, washer, hammond box base, washer, chassis sheet, washer, nut. This should give reasonable ground connection as well and allow me to open a module to adjust or fix something without getting a headache.

Can anyone think of any problems with this approach before I go and drill holes in my hammond box?
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Old 4th Mar 2019, 6:42 pm   #2
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Default Re: hammond box mounting and RF tightness

IMHO that will be fine, a screw in each corner for tidiness and a wee bit more grounding perhaps.
 
Old 4th Mar 2019, 7:03 pm   #3
Scimitar
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Default Re: hammond box mounting and RF tightness

Rivnuts in the bottom.
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Old 4th Mar 2019, 7:15 pm   #4
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Default Re: hammond box mounting and RF tightness

Never knew those existed - thanks for the suggestion!
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Old 15th Mar 2019, 10:28 pm   #5
Philips210
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Default Re: Hammond box mounting and RF tightness

Hi.

The Rivnut is a good suggestion by Scimitar. In the past, I used something similar for fixing to aluminium chassis assemblies. I think they were known as Clinchnuts.

Regards,
Symon.
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Old 15th Mar 2019, 10:40 pm   #6
MrBungle
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Default Re: Hammond box mounting and RF tightness

I found some of them on ebay in the end - they are very good!
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Old 15th Mar 2019, 11:27 pm   #7
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Default Re: Hammond box mounting and RF tightness

There could be logic in mounting your Hammond box [assuming it provides all-round shielding] essentially 'free-floating' from the main chassis, using suitable standoffs and Nylon-66 nuts/bolts, then taking the RF/DC ins and outs to the rest of the circuit via coax and feedthroughs.

I did something similar in a piece of kit some time back, where the need was to isolate a lump of fast digital-circuitry [which lived in the shielded box] from the RF-chassis which had a Kilowatt or so of power flying around on transmit but also needed to hear 0.1Microvolt-or-better on receive.

"Floating" the digital stuff in its box was the only way I found to reliably stop it locking-up on transmit or injecting crud into the front-end on receive.
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Old 16th Mar 2019, 12:05 am   #8
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Default Re: Hammond box mounting and RF tightness

Interesting. I am actually having some noise trouble with this. The power sensor is an AD8307 and I'm using a 12-bit MCP3208 ADC to pull the power reading off it. Spent two days trying to work out why I couldn't get a totally linear reading, playing with the ADC and the software and it turns out that the AD8307 is picking up the switching hash off my LED desk lamp! I didn't work that out until Sat when I got some time to work on it in daylight. I only tested the power sensor standalone in daylight too. Power sensor is now going in its own box!
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