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Author Topic: Low noise antennas  (Read 289 times)

Online Ray Lalleu

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Low noise antennas
« on: January 05, 2021, 1801 UTC »
More of my thoughts about receiving lines and antennas,
but that should be another thread.

Remember that T2FDs and small loops are low noise antennas by themselves, even when not away from the noisy shack.

What is in common between the T2FD and the small loop ?
Both have the high electrical fields kept low or in the smallest space as possible. The high electrical fields are near the ends of the wire, where there is a node of current. In the T2FD, the 'ends' are folded one near the other and connected by a resistor. In the small loop, the ends are also one near the other and connected by the tuning capacitor.

The electrical wires in a house are more or less like a tree, with many 'ends', where the RFI HF currents are at high impedance too (count the lines with 2 wires acting as mono wires). This sounds to me as poised to coupling with the antenna ends where the impedance is also high.

I know, modern houses have also RFI magnetic loops, with phone lines and TV cables, and that with the worst offenders (computers, TV, set top boxes) connected on two or more  kinds of cables/wires.

Nevertheless, I'm thinking in that way for practical low noise HF antennas, and have designs more or less prepared for tests.
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Offline ChrisSmolinski

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Re: Low noise antennas
« Reply #1 on: January 05, 2021, 1853 UTC »
There's two views of thought about T2FD and other loop antennas (Skywave antennas fall into this category): One that loop antennas are in inherently lower noise than antennas with high impedance ends (dipoles for example), and second that such views are nonsense and cannot be supported by theory.  My own experience is that while I cannot prove the former is true, in general I have had extremely good results with this category of antennas, and they're a major part of my DXing arsenal.

My northeast Beverage is also an excellent performer. Of course it is large (500 ft). It is also terminated with a resistor at the far end... hmmm...  ;D

Regarding T2FDs... The termination resistor in the T2FD helps to keep the impedance excursions relatively sane over a wide frequency range. It also makes the T2FD a very inefficient antenna for transmitting due to losses in the resistor, but for receiving this is not an issue.
Chris Smolinski
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eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 300 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree