I built my diversity antennas vision around two wideband loop antennas, at least 100 feet apart, all connected with shielded CAT 7 cable for signals and power.
So, I have a dual channel coherent receiver, perfect for diversity reception and spatial filtering. Now I need some antennas optimized for this purpose, and they have to fit on a small city lot. What to do?
Like they say, there is more than one way to skin a cat. (Although I admit I don’t really understand what that means!) So, it helps to come up with my own diversity antennas vision, something to steer my thinking forward. Mine is comprised of two magnetic loop antennas, spaced at least 100 feet apart so there is a phase difference between their received signals.
With careful design, magnetic loop antennas can operate in a wideband mode enabling coverage of 500 kHz through 30 MHz. The downside is very low signal levels which require amplification. You need to translate magnetically induced currents, in the pico-amp range, up to micro-volt level signals usable by a sensitive receiver. The upside is a very small footprint antenna that works well at near ground level and is less sensitive to nearby man-made interference. More on these topics later.
A typical wideband loop is about 1 meter in diameter and made from aluminum tube. You insert current-to-voltage amplifier into the loop and then route the received signals to your receiver. If you build two identical loops and use equal lengths of transmission line for each, diversity reception should be quite straightforward.
Diversity Antennas Vision – Twisted Pair Replaces Coax
Amplified loop antennas require power, which is traditionally piggy-backed on a coaxial cable using a Bias-T diplexer at each end. With this approach, DC power and radio signals live together on one cable. Another approach is to use separate wires for the power.
As I built my diversity antennas vision, I read that many experimenters have switched from typical RG8 or RG6 coaxial cable to using Ethernet CAT cables for receiving antennas. Recent improvements in these cables include improved shielding, high speeds and lower prices. Each cable contains four (twisted) pairs of wires. This means I can use CAT for multiple signals as well as power. Each pair has a nominal impedance of 100Ω .
So my design will involve using CAT 7 cable with separate pairs for the amplified signals from each antenna, as well as a third pair for 12 VDC.