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Wideband Loop Transmission Line Choices

loop transmission line

With a little ingenuity, you can select either coaxial or network cable for your wideband loop transmission line. Either will work, but I am going with twisted pair. 

You have a number of choices for loop transmission line. This is the cable that connects you antenna to the radio some distance away. Historically, most people have used coaxial cable. In my case, I am going to use a balanced line, specifically CAT7 network cable for my loop transmission line.

Network cable contains 8 wires, or four twisted pairs of 22-26 gauge wire. The twisting helps reduce interference and sets line impedance. If you check out the various CAT cable types, you will find earlier CAT 3 cable was not shielded and had very restricted bandwidth, i.e. 16 MHz. The newer cables have much higher bandwidth (up into the UHF range) and various type of shielding. Recently, I picked up some CAT 7 S/FTP in 30 meter lengths for around $20 each.

The S/FTP designation means that each pair of wires is shielded in a foil wrap, while all four pairs are inside a metal sheath, as shown above. I will use two pairs for signals, and one pair to power the amplifier.

To check transmission line performance in LTSpice, you need to add a model for an equivalent circuit. The model for lossy transmission line requires four parameters: length, and resistance, capacitance and inductance per unit length. In my case, the CAT7_30m model I used was LTRA(len=30 R=0.1 L=430n C=43p). Immediately, LTSpice will show the effect of standing waves. You mitigate this by adding some resistive termination to the cabling, in this case a 50Ω resistor to source each line in the pair.

The other challenge is how to connect your loop amplifier to the cable. Since my amplifier is single ended, I need a current balun to transform the unbalanced amplifier output to balanced line. I will start with 10 turns of bifilar wound magnet wire on an FT-37-43 ferrite core. This 1:1 current balun should force equal currents to flow on each wire. Everything checks out in LTSpice so I am hopeful the real thing will work well.

Loop Transmission Line – Coaxial Cable Choice

With some slight modifications, my system should also work with 50Ω coaxial cable. Check this out for yourself in LTSpice using a model for 30 meters of RG58. Losses for the two cables are similar. However, I found much more standing wave ripples with coax, and reduced these with 50Ω source and termination resistance. Unfortunately, I found that this approach adds an extra 6 dB attenuation, requiring a bit more gain in the base amplifier.

You also need to add a Bias-T network at each end of the coax to send power for your amplifier. Not hard, but more work.

I am hopeful that my balanced line choice will provide better rejection of common mode signals close to the ground, and therefore less interference leaking in to my system.

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