multi-section magnetic loop antennas for low loss and hand-effect - Page 1
Forum section: electronics / rf-microwave

author: OwO
author_rank: Super Contributor
timestamp: October 31, 2019, 03:21:01 pm
content: Background: most hams probably know about the magnetic loop antenna, which is an electrically small antenna that keeps most of its near-field energy in a magnetic field. Having low E fields near the antenna is desirable because it reduces the hand effect.All resonating antennas must move all its stored energy back and forth between electric and magnetic fields within each cycle, and in a magnetic loop a capacitor stores the E field energy. The loop and the capacitor form a resonant tank similar to a lumped LC tank, and it is the magnetic field created by the loop that radiates. For a small loop it's possible to keep most E field energy confined within the capacitor and away from environmental effects.For a given magnetic field strength the radiation loss increases as the loop area is increased. Therefore it is desirable to maximize loop area. However ordinary magnetic loop antennas have a size limitation set by the circumference of the loop. When the circumference approaches 1/2 wavelength, the loop resonates itself without a tuning capacitor, and all energy that used to be stored in this capacitor are now in the E fields surrounding the antenna. This effect is significant way before 1/2 wavelength, with large magnetic loops reaching resonance with small tuning capacitors, spilling E fields into the environment nearby. In practice larger magnetic loops have significant hand effect. Moreover this transmission line effect also causes uneven current density along the loop conductor (the current is lower near the capacitor end of the loop), which leads to uneven magnetic field in the loop, limiting the effective radiating area.So far I haven't seen magnetic loop antennas applied to VHF and above, mostly because of the low bandwidth caused by the size limit (bandwidth improves with larger loop area), plus the hand effect advantage is eliminated at larger electrical sizes (it approaches a folded dipole antenna which has field patterns similar to a dipole).I have been experimenting with splitting the resonating tank into many sections like pictured:(20x20cm, 110MHz)Each "dot" along the loop is a 30pF capacitor. Each section of wire between two capacitors is a resonator but with magnetic field sharing with nearby sections. Since each individual resonator is small, larger caps are used which leads to lower hand effect. (Hand nearby will add up to ~0.2pF of stray capacitance.) In simulations I have verified that the structure resonates with nearly uniform current density along the loop, with all currents in phase, even with total loop lengths in excess of one wavelength. Since each transmission line section is short, the self-resonant frequency can be pushed far above the operating frequency. The bandwidth, which is 0.5%, is the same as an ordinary magnetic loop of the same area.Near field (magnetic):Here is a more extreme example (electrically large), currents and magnetic field still uniform, with 5dBi of gain and 3% bandwidth:(140cm x 10cm, 179MHz)In real life I have tested a small 900MHz magnetic loop with 3 sections and verified the much smaller hand effect compared to inverted F antennas. I will be testing the larger antenna designs soon. I also did some quick calculations of capacitor loss based on the simulated currents and Q factors of commonly available 1206 caps, and so far it is small compared to conductor losses.

author: OwO
author_rank: Super Contributor
timestamp: October 31, 2019, 04:58:36 pm
content: After a little bit of matching the 140cm long 180MHz antenna reaches 19% 3dB fractional bandwidth, which is comparable to electric dipoles but with the advantage of low hand effect. This can make a good indoors/portable antenna with gain. In simulations it's possible to get up to 8dBi of gain by adding more sections until the current starts to become non uniform.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2019, 08:35:23 pm
content: I think this counts as reducing the local speed of light with metamaterials. Consider the resonant frequency of a given segment -- supposing you could get the inductance of just a segment without also forming a loop, of course -- each segment has the same resonant frequency but a low impedance, and the total series connection of all is the nominal (design) impedance. It's narrow band (metamaterials are usually (always?) resonant), but it can behave like a normal (long) resonant antenna in certain respects.The cancellation of local E fields due to the complex local field (2n-pole, i.e., one cap is a dipole, two caps opposed is a quadrupole, etc.) is a nice feature, yes.Tim

author: iMo
author_rank: Super Contributor
timestamp: October 31, 2019, 09:07:09 pm
content: Those frequencies do not match ham radio bands, afaik. What is the purpose then?What are the capacitor's like, for what voltage?I made 1500miles long QSO 59 SSB in 6m band, 100W into 35cm diameter mag loop off my window. I like mag antennasThe voltage at the capacitor was many thousands volt, however..

author: m3vuv
author_rank: Super Contributor
timestamp: October 31, 2019, 10:23:50 pm
content: had a 9ftx 15mm loop in my loft,had a 500pf wide spaced cap driven by a stepper motor with a 10:1 homemade reduction gear after a while i noticed the current draw from my ts-850 was only about 2 amps on tx,id blowd up the pa drivers,after doing some tests with a friends ant analyser the bandwith was only about 2kcs on 3.775,the bandwith of the ssb signal was to wide for the loop,thought of building another but cobbling a multi band verticle instead,i live in a flat with no garden,antennas are a big issue here,at the moment im shunt feeding my tv antenna pole via a 9:1 unun,its 30ft approx and seems to match on 5 bands with the rigs atu,the counterpoise/ground is all my water pipework,not ideal but works! 73 m3vuv

author: OwO
author_rank: Super Contributor
timestamp: November 01, 2019, 04:27:06 am
content: Yes, one way to think of it is a transmission line structure with series capacitors cancelling out the series inductance, giving highphasevelocity. There is a complement which is a transmission line loaded with shunt inductors (for example imagine a balanced feedline but with shunt inductors once in a while). Unfortunately high Q inductors are much harder to come by than high Q capacitors, so one is more practical than the other. IIRC waveguides also show a similar phenomenon where the phase velocity blows up near the boundary frequencies of a mode.Quote from: imo on October 31, 2019, 09:07:09 pmI used random "convenient" dimensions for simulation; I will be scaling the antennas before doing real life measurements.The capacitor voltage rating scales inversely with loop size; larger loop area => lower currents for a given radiating power => lower voltage across capacitors. For the 20x20cm 110MHz antenna each 30pf cap sees about 700V at 100W, 400V at 30W, and 220V at 10W (peak voltages). I would rate this antenna at 30W max because there is also a 14% conductor loss (based on skin effect figures; actual loss may be higher because of proximity effect). You can reduce the voltage requirements (and loss too) by adding more sections/increasing loop area. The 140cm long antenna, for example, has only about 170V peak and 1.2A rms at 100W. I will most likely be using 1206 1KV capacitors to handle most of the current (cheap ones have a Q between 400 and 1000), plus a surface mount variable cap in parallel for tuning.

author: radiolistener
author_rank: Super Contributor
timestamp: November 01, 2019, 05:06:18 am
content: the main cons of magnetic loop antenna is extreme high field strength in the near-field zone of antenna. This is because it needs to have very high Q in order to work the same as a full size antenna. And field strength is multiplied with antenna Q.So, if you're using 100 W transmitter, it's not safe for health to be near such antenna at TX time, keep it away as far as possible

author: iMo
author_rank: Super Contributor
timestamp: November 01, 2019, 09:44:08 am
content: In my setup above, 1.5m off the tuning capacitor hanging fluorescent bulb (no wires) lit while I keyed CW.I sat 2.5-3m off the antenna during that QSOI had to use a gimmick capacitor wired in parallel to increase the capacitance of the variable capacitor, so I did with a piece of RG58, it burned in flames immediately after I keyed..PS: I am still alive as I've been wearing the Pukao Hat during my experiments