Introduction

This is/was an attempt at converting a relatively low end SWR bridge that was meant for the CB radio crowd and is not very sensitive for the lower frequency bands. For this project I am specifically using a "Workman HP202S". This is a Cross Needle SWR/Power Meter (50 Ohm, 26-30MHz). I picked one up at a ham fest for $10 and liked the look of the meter.

For sensing forward and reverse currents, the HP202S uses a Monomatch style sensor bridge. These are good sensors, but they are frequency sensitive. They work well at high frequencies, but their sensitivity falls off for low frequencies, which is where most hams want to operate. I built one in the early 80's but I only operated on the 15 Meter band, so it worked fine. But, I thought that a better sensor would help this meter be more useful.

The sensor that looked the most promising is known as a "Stockton Type". This sensor has been in publications since the early 90's. I don't particularly understand how it works, but that shouldn't matter as long as it just works. If you really want to know, do a Google search. But be ready for some pretty deep theory and equations.

The transformer secondaries should have an impedance that is 10 times the impedance, at the lowest frequency of use.

Powdered Iron and Ferrite Toroidal cores are both usable. Ferrite requires fewer turns for the same inductance.

Powdered Iron doesn't saturate as easily as Ferrite.

Using the coax shield as a Faraday Shield to minimize capacitive coupling between the primary and secondary.

The transformers are dotted to indicate the winding direction. Primary and secondaries should go through the core starting at the same point. After that, winding direction, clock wise or counter clock wise, should make no difference.