Menu

Author

488 posts

HP 85046A 3GHz S-Parameter Test Set Teardown

Front Panel

Front Panel

This enormous box is the S-Parameter Test Set from a HP 8753C 3GHz network analyser. This unit contains the required components to automate the testing process for items such as cables, antennas & RF networks.

7mm Test Ports

7mm Test Ports

The main EUT test ports are APC-7mm type connectors – a very expensive genderless RF connector that provides very repeatable coaxial connections.

N-Type Analyser Interconnects

N-Type Analyser Interconnects

The interconnects for the RF input, Reference output back to the analyser, A&B ports are N-Type, and should be connected to the main unit with phase-matched cables.

Lid Removed

Lid Removed

On removing the lid, it’s almost a completely empty box! All the RF magic is done in the first 150mm behind the front pane, apart from a coil of semi-rigid coax on the right, which will be to match the lengths of cables in the unit for phase purposes. There’s not much visible on the top here, just the control board, which takes signalling from the main analyser unit, and only has some glue logic & comparators. There’s a very nice 3-port solid-state RF switch in the centre, for switching between S12 & S21 measurements rapidly, a function that would not be possible with a mechanical relay. All the internal connections are made with semi-rigid coaxial cable, fitted with SMA connectors.

Rear Panel Connections

Rear Panel Connections

The back of the case just has the 25-way D connector for control, and a pair of BNC connections & 500mA fuses for DC biasing the output ports where required by the EUT.

Power Splitter & Directional Couplers

Power Splitter & Directional Couplers

Underneath the centre panel is where most of the RF magic happens. These two blocks, which are integrated with the test ports contain bias tees for each port, a power splitter for the RF reference back to the analyser & directional couplers for reading back the forward & reverse RF power from each test port.

Step Attenuator

Step Attenuator

The final component in here is a 70dB step attenuator, adjustable in 10dB steps.

Covid-19 Lockdown Projects – Extracting Diesel Heater Exhaust Waste Heat

Since I fitted the new “8kW” diesel heater to the camping power trolley, it has occurred to me that there is a lot of energy in the exhaust gas stream that ordinarily would be wasted into the atmosphere. Since we’re all still on lockdown here in the UK, I figured it would be good to run an experiment to see if it was worth recovering this energy – in the form of heating water.

Heat Exchanger

Heat Exchanger

Some time ago, I stripped an old gas combi boiler, and recovered some parts – most important here the HDW plate heat exchanger. This large chunk of stainless steel is a stack of formed plates, brazed together, that usually would heat Domestic Hot Water. In this instance it’s being repurposed to transfer heat from exhaust gas to water.

Brazed Connections

Brazed Connections

These heat exchangers are mounted in the boiler via a plate with O-Ring seals on, so they don’t really have fittings – just holes in the end plate. Solving this problem was simple – braze on some copper fittings with 55% silver brazing rod. The 22mm is the exhaust side, while the 15mm is the water side.

First Test

First Test

Cobbling together some random hose & fittings, along with a small water pump allowed me to run a first test. At this point there is no lagging at all on the exhaust system from the heater, so it’s going to shed a lot of exhaust heat into the air before it even gets to the heat exchanger. However I was able to get around 600W of heat into 15L of water, heating it up nicely. The heat exchanger is plumbed contra-flow here – exhaust comes in via the stainless tube on the bottom right, and water comes in through the speedfit elbow on the top left.

Lagged Heat Exchanger

Lagged Heat Exchanger

After the temperature of the water tank hit a plateau at around 45°C, I decided to insulate everything the best I could with what I currently have. I’ve wrapped the heat exchanger with some recycled PET insulation here, just to hold the heat inside. I’m not concerned about the exhaust outlet being in contact with the fluff – this system is so effective at pulling the heat out of the exhaust that the gas exiting the far end is totally cold!

Unlagged Exhaust

Unlagged Exhaust

Now it was time to get the exhaust system under the trolley insulated. This is the system removed from the unit entirely. This is constructed from copper pipe, brazed onto standard silencers. Deadening the sound from the unit is important, as this gets used on campsites!

Fibreglass Tape Insulation

Fibreglass Tape Insulation

An hour & some itchiness later, the exhaust is completely covered in fibreglass insulation, secured in place with stainless steel ties.

Exhaust Hanger

Exhaust Hanger

The exhaust originally passed through a close-fitting hole in the frame rail which would obviously not work now due to the thickness of the insulation layer, so this was modified with a grinder. Since there was now no support for this end of the exhaust, a pair of drilled holes & some stainless steel wire form a nice hanger!

With all this insulation in place (including around the tank & pump), the rig is now able to easily hit 65°C within a short time, so there has definitely been an improvement. At this point, it’s clear that waste heat recovery is worthwhile, so I’ll be building a proper rig to capture this energy for reuse!

Narrowboating Quickies – Webasto Heater Exhaust Rehash

Since rebuilding the burner for the Webasto water heater on board nb Tanya Louise, I figured it was about time I sorted the exhaust out as well. The standard Eberspacher / Webasto type exhaust system components are shit. Nothing is properly gas tight, no matter how you build the system, due to how the pipe is constructed – it’s spiral ribbed stainless flexi tube, and even proper clamps don’t exert enough force to create a gas seal on the fittings, leaving gaps in the spiral for exhaust to leak out. Unfortunately I don’t have a photo of the old exhaust setup – it was however awful.

So to fix the problem of the messy setup, and to fix the issue of leaking exhaust gases, I got to work creating a custom system from 22mm copper pipe, brazing all the joints together.

Completed Exhaust

Completed Exhaust

Here’s the completed system, matched to the location of the heater unit in the engine bay, and the exhaust skin fitting. The ends of the pipe are expanded with a hydraulic tool to allow them to fit onto the heater & skin fitting, these being too large for 22mm pipe normally.

Brazed Muffler

Brazed Muffler

The muffler is also fully brazed to keep exhaust gases inside the exhaust. These are supplied just crimped together as they’re intended for use under vehicles. A sealed marine grade exhaust silencer is available, but very expensive. Again the copper pipe ends are expanded with the hydraulic tool to allow them to fit into place on the stainless tails. Brazing was done with 55% silver brazing rod.

Fibreglass Wrap

Fibreglass Wrap

To keep the heat away from sensitive parts in the engine bay, the entire assembly has been wrapped in fibreglass insulation tape, and secured with stainless steel ties. It’s important to use only stainless in these applications – the fibreglass wrap will hold any moisture in contact with all the parts, and mild steel will rapidly convert back into Iron Oxide 😉

Heater End

Heater End

The heater itself is on the other side of the plywood board in the photo, the cooling water pipework can be seen on the lower left, along with the diesel dosing pump. The main fuel tank is just visible in the bottom right corner.

Skin Fitting Connection

Skin Fitting Connection

The other end is sized for a snug fit onto the exhaust skin fitting, just astern of the old oil cooler. This is set to be removed at some stage, and be replaced with an engine bay blower for ventilation.

Silencer

Silencer

In the corner, next to the bulkhead sits the silencer.

In all, this setup also made the heater quieter, probably due to the longer length of exhaust pipework, which is now about 1.5 metres from the heater outlet to the skin fitting. This is a bonus – the exhaust of these heaters without any silencing sounds like a jet engine!