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Dual Wideband Loop Controller

dual wideband loop controller

I am almost ready to experiment with diversity reception. My latest project is a dual wideband loop controller which works well.

My objective is to feed two identical receiving loops into my two channel Afedri SDR radio. Almost there!

You can see my completed dual wideband loop controller circuit in the picture above. Shielded CAT7 Ethernet cable uses two pairs for signals and one pair for power. Outside, I have a splitter which combines cables from each of two loops into a single run to the shack.

Each antenna then feeds through a switchable high pass filter, which attenuates signals below 1.7 MHz by 35 dB. I can use this feature to reduce possible strong signal interference from local AM broadcast stations.

Finally, each of my antenna channels is connected to the receiver through a broadband balun. The transmission lines and filters are designed for 100Ω balanced performance, which I convert to 75Ω unbalanced to feed the receiver.

This time around, I selected through-hole rather than surface mount construction on a double sided PCB. Terminal blocks are used to connect power and relay switching, and the whole design fits into a 3D printed control box.

Dual Wideband Loop Controller Parts Selection

Filtering is switched using HK19F 12 volt relays, which cost $1 each on e-bay. I selected these DPDT relays on the basis of cost and ease of mounting in a modified 16 pin DIP socket. My filters are made with miniature THT capacitors and inductors using an Elliptic filter design.

The shielded RJ45 jack is another cheap wonder from e-bay. My circuit board is designed to attach the CAT7 shield to the BNC output shield.

Finally, after some experimentation, I settled on a UTP to BNC Video Balun for impedance matching the antennas to the receiver. These cost $2.50 each and provide 100Ω to 75Ω matching, balanced to unbalanced. While this balun is only specified to 6 MHz, I found them to work well through all of HF.

I designed the circuit board in KiCAD, sized 90 by 80 mm. It cost $5 plus shipping from PCBGoGo.

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