All,
AirSpy Youloop with 2 turns and radius about 1/2 meter constructed of two RG402 cable limbs with a copper? shield in diameter 3 mm has following parameters:
total calculated inductance that includes both loop inductance in free space and inductance of 2 turns of innner cable conductor = 8.65 uH and cable self capacitance about 190 pf ( that uses solid PTFE dielectric e=2.02 ).
The calculated self resonance frequency of untuned Airspy's shielded loop will be at 3.93 MHz. Steve Ratzlaff's posted impedance plot shows the same value of the self resonance frequency.
Also, the calculated radiation resistance? at 1 MHz is significantly less than 1 ohm ! That is typical case for small loop antennae.
?
So, AirSpy loop antenna cannot be matched to RF amplifier with input impedance of 50 ohms, it is not intended even for HI-Z amplifier.
So great mismatch creates? antenna gain is equal to the impedance mismatch factor and the gain in the practical sense is measured around -47 dB.?
The antennas inductive reactance is larger than the radiation resistance.?
I revied Owen Duffy's plot where he presented radiation resistance? and inductance reactance variations in frequency range from 0 to 10 MHz. Based on his plot the radiation resistance is in a range of 12 to 14 ohms ?!
How is that achieved ?
Also, the inductance? is significant lower about 80 nH derived from the presented plot by Duffy.
For Airspy loop antenna it is impossible to have radiation resistance values shown on that plot. Please see work of other author Ofer Aluf where he provides formula for radiation resitsance estimate, which I have used to calculated this value.
Regarding Martin's remarks that "Moebius loop is an unnecessary complication".
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?I read other user's comments where he states about the differences in performance between Moebius vs, Conventional Split Shielded Loop:
- Moebius is better below 1.1 MHz by 6 dB
- similar performance in range 1.1 - 1.3 MHz
- conventional split is better above 1.3 MHz
I could not find any performance preferences in SW bands using Airspy Youloop. I guess, since the self resonance frequency is around 3.93 MHz the antenna should perform as a short untuned whip antenna having low radiation resistance but with capacitance reactance as well.
Regards,
Raphael
