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Initial Wideband Loop Testing

initial wideband loop testing

My initial wideband loop testing shows reasonable MW and Lower HF performance, but I need to address shielding and impedance matching to prevent the CAT7 from acting as an antenna. 

So far, I have built two identical copies of my mostly plastic wideband receiving loop. My plan is to use these for diversity reception. Each is a one meter diameter loop of aluminum core PEX connected to an LZ1AQ amplifier. I am doing initial wideband loop testing with a 100′ length of CAT7 cable into the shack, with the signal pair just connected to a short length of RG6 feeding into a software defined receiver, either my Perseus or Flex 6300.

At present, this test system does not contain any impedance matching or a balun. And, while my CAT7 cable is shielded (S/FTP), shielding is not connected to anything.  Finally, the loop is mounted near a metal fence which might effect pattern. I need to address these shortcomings when I build a proper system later. In the meantime, what are the results?

As expected, the loop provides tons of signal on Medium Wave and reasonable signals across Lower HF, up to around 12 MHz. Beyond the 20 meter ham band, very little. In my initial wideband loop testing, I will focus on lower HF.

My testing used four scenarios: Power On, Power Off, CAT 7 disconnected at the far end (loop disconnect) and CAT 7 disconnected at the near end. Power Off and Far End Disconnect scenarios were pretty much identical – weak signals and some noise. Near End Disconnect between CAT7 and RG6 was effectively “no antenna” and the SDR just showed its basic noise floor of around -127 dBm.

I also compared the loop to my nearest full size ham radio antenna, either my beam or 40 meter dipole on a 50 foot tower.

Initial Wideband Loop Testing – Strengths and Patterns

My findings? First, across 3-12 MHz, the active loop provides signal strengths and SNR within 6 dB of the full size ham antennas about half of the time. For the balance, it is much weaker. Performance compared to the ham antennas is best in early morning to Asia and evening to Europe. I am wondering if the inconsistent results relate to angle of signal arrival, or variations in the magnetic component of the radio wave.

Second, as presently configured, my Ethernet cable is acting as an antenna rather than a pure transmission line. The 100′ cable adds 10-15 dB to the receiver noise floor with the loop either powered on or off. The LZ1AQ amplifier does not add to the noise floor across lower HF. Also, with loop power off, signals are received at the radio at about 25 dB below when the amplifier is on. This is due to signal ingress on the transmission line.

So, good for the amplifier, and bad for the transmission line. I will experiment with different approaches to grounding the shield and matching impedance.

Third, I can get decent nulls on local RFI sources. Magnetic loop patterns are basically isotropic with pinched-in nulls off the plane of the loop. Also, the nulls and pattern vary considerably with angle of signal arrival. At lower angles, the nulls are great. For example, my neighbor’s halogen light RFI on 11 MHz disappears with the loop in use and the null properly oriented.

So, I think I’m off to a good start. But I must do more work on the transmission line.

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