Hi, gang,


Days are short, the nights are cold, and I’m tinkering with a? new project—a 50Ω dummy load. Yeah, I know… been there and got the Tee shirt.

This project is based on someone else’s design I found on the ‘net. My goal is to package it using pcbs to make assembly easier, quicker, and cheaper.

The design used 94 (yup, you read that right!) 94— 4.7k 3W resistors in parallel. That combination results in a 282W 50Ω load. Fan forced cooling might push power to 500W for under 30 seconds.? Now before anyone gets their fingers a typing away, I know, I know. There? are 50Ω microwave termination resistors that can be found on ebay for under $20. You need a heatsink for these, something I don’t want to mess with.

So far, I’ve laid out? a pcb that will hold 30 — 3W resistors. Then three boards stacked on top of each other with a final pcb holding the last four resistors and a simple monitor circuit to drive a meter. The pcb, as of right now, would be about 4” by 3”

Now I’m not a RF engineer. I have the traces at .3” and I wonder if that’s wrong. Should I make the traces thinner so the added inductance (caused by the pcb traces) won’t mess with the 50Ω or leave them wide to handle the RF current? Would double sided pcbs put the kabage to the circuit as I would have a really nice capacitor built into each pcb.

Finding three watt carbon resistors maybe a challenge to find. Would demons raise from the basement if metal film resistors were to be used?

Anyway, I’m up for suggestions. Days won’t start getting longer until after the 21 of the month.

Mike, WB8VGE
The Heathkit shop

Mike,

Does the design info you have include any impedance vs freq plots? I have tested several different designs over the years and found that some can have very disappointing?performance, especially if you are hoping to use the device above 10M. Designs that use large parallel plates to mount the resistors can exhibit a surprisingly?large amount of capacitance. Here is an example of one on eBay??that looks like it is usable up to 2M, but it only?uses 20 resistors. I would be reluctant to pursue a design that uses 94 resistors without seeing some specs or measured performance data.

Also, for designs?that use a large number of resistors it can be hard to get the heat out of the resistors that are bunched near the center. This can result in an SWR that varies with time while power is being applied, and in some designs, overheating of some of the resistors.

Bill N0CU

On Thu, Dec 7, 2023 at 10:43?AM mike bryce <prosolar@...> wrote:

Hi, gang,


Days are short, the nights are cold, and I’m tinkering with a? new project—a 50Ω dummy load. Yeah, I know… been there and got the Tee shirt.

This project is based on someone else’s design I found on the ‘net. My goal is to package it using pcbs to make assembly easier, quicker, and cheaper.

The design used 94 (yup, you read that right!) 94— 4.7k 3W resistors in parallel. That combination results in a 282W 50Ω load. Fan forced cooling might push power to 500W for under 30 seconds.? Now before anyone gets their fingers a typing away, I know, I know. There? are 50Ω microwave termination resistors that can be found on ebay for under $20. You need a heatsink for these, something I don’t want to mess with.

So far, I’ve laid out? a pcb that will hold 30 — 3W resistors. Then three boards stacked on top of each other with a final pcb holding the last four resistors and a simple monitor circuit to drive a meter. The pcb, as of right now, would be about 4” by 3”

Now I’m not a RF engineer. I have the traces at .3” and I wonder if that’s wrong. Should I make the traces thinner so the added inductance (caused by the pcb traces) won’t mess with the 50Ω or leave them wide to handle the RF current? Would double sided pcbs put the kabage to the circuit as I would have a really nice capacitor built into each pcb.

Finding three watt carbon resistors maybe a challenge to find. Would demons raise from the basement if metal film resistors were to be used?

Anyway, I’m up for suggestions. Days won’t start getting longer until after the 21 of the month.

Mike, WB8VGE
The Heathkit shop