Low-band AM Tx: Chassis and finishing

This post closes off the 160 and 80m ‘Low-band’ AM transmitter series, with a description of the chassis construction, fitting the assemblies into the chassis, and front panel finishing.

Chassis

The chassis is custom-built to the dimensions of a 2U rack chassis, using sheet and angle aluminium. Fasteners are M2.5 and M3 countersunk bolts and rivets. Six black anodised M5 hex head bolts push through the front panel to secure the panel to a 3mm angle bracket which is in turn bolted onto the 4mm base. The front panel is as per the 2U standard. The base measures 438mm wide by 400mm deep. The sides are standard 80 x 20 x 3mm aluminium angle stock, cut to length and bolted to the top side of the base. There is no rear panel — rather, the rear space is mostly filled by the angle brackets of two the sub-assemblies.

Front panel

Both display modules mounted.
Rear view of front panel with display modules mounted.

The front panel is 3mm aluminium, cut to 2U size at the supplier. It was then drilled and worked for all controls (by hand), then given a metal primer coat, followed by 5 to 6 thin coats of an enamel spray. The colour is called, appropriately, ‘aluminium’, basically a silver-grey tone. Deca-dry lettering was then applied, followed by 4 to 5 thin coats of clear satin enamel, with light rubbings of 600 grit between the later coats. The panel is held to the 4mm base using a piece of 3mm angle.

Handles

These pieces were found on the RS Components site, and they make a very nice addition to the ‘rack look’ of the unit.

SWR and Power metering

A digital SWR and power meter was built, as a side project, and installed on the front panel, next to the main display. This module is Arduino controlled, uses four-digit LED modules for each of SWR and power, and a Breune Bridge for RF sampling. The power calculation does not use an AD8307 logarithmic amplifier as is usually done, instead, it uses a software approximation to the formula for sensed voltage to RF power mapping, derived from actual values and an online curve-fitting website. The full story is here.

Conclusion

The chassis completes the AM transmitter. Check out the other posts and videos in this project. Thanks for reading and please leave a comment.

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2 thoughts on “Low-band AM Tx: Chassis and finishing

  1. Shaun P says:

    Well done Paul.

    Shaun – VK6BEK

    Like

    • Paul Taylor says:

      Tnx Shaun. On the chassis, you can buy a 2U commercial rack box or DIY with stock aluminum, and simple tools, for about a quarter of the price. But each has its merits.

      Like

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