magnetic loop bandwidth

[p.larner]

good evening,is there any method of increasing the bandwith of a mag loop antenna,ie thicker diameter loop material etc?,cheers.

[TimFox]

Actually, using fatter wire reduces the series resistance of the loop, increasing the Q, which decreases the bandwidth.
When I needed to increase the bandwidth of a resonant circuit (ca. 10 MHz), I used feedback on the next amplifier to reduce its input resistance (in parallel with the resonant circuit), which reduced the loaded Q and increased the signal bandwidth. while reducing the signal output at the resonant frequency.
However, that does not improve the SNR at frequencies outside the unloaded bandwidth of the circuit.

[p.larner]

thanks for the reply,its a shame it doesent answer what i asked tho!.

[TimFox]

Please read what I actually wrote, including punctuation and spelling.
The feedback-reduced-input-impedance amplifier increased the bandwidth of the resonant circuit formed by the loop and its tuning capacitance.
I patented it for a specific application.
Perhaps you didn't really mean "bandwidth", as in wide frequency response?

[p.larner]

Hi,i meant how to make the 2-1 bandwith point wider on a loop,hope that clears things up?.

[szoftveres]

thanks for the reply,its a shame it doesent answer what i asked tho!.

Yes, that response did answer your question - everything was in there, reduced Q, wider bandwidth, even an example of loading by reducing the input impedance of the amplifier with feedback.
Either you don't understand the answer, or didn't ask what you really meant and didn't explain what kind of information you were looking for.

[EggertEnjoyer123]

Maybe write what you are trying to do with the loop antenna, what it looks like, etc?

[paul@yahrprobert.com]

The magnetic loop antenna is the answer to the question: "how do I make an antenna much smaller than a wavelength but which will actually radiate some power and not dissipate it all in its own losses?"
The way it does this is by being as high a Q circuit as possible, so that with a small amount of power I can make huge circulating currents and enormous electric fields.
  So it seems that any effort to reduce the Q, with losses not provided by radiation resistance, will throw away some of your power.  So go ahead, put some extra dissipation in the system (series resistance, capacitor losses, whatever) and you'll increase the bandwidth, but you'll also reduce your radiated power.
   One possible avenue would be to make an array of loops each tuned to a slightly different frequency, and fed with some suitable network.  Maybe you could get more bandwidth that way.  Hmm, you could even arrange them like a log periodic array and get some gain!

[TimFox]

Hi,i meant how to make the 2-1 bandwith point wider on a loop,hope that clears things up?.

That's what I did with my feedback circuit:  you don't have to follow my advice, but don't say I didn't answer your question.
I assume you meant making the frequency span wider for half-power response (- 3 dB).

[Wallace Gasiewicz]

In my minimal experience building and using a mag loop, it is not practical to increase the bandwidth for transmission.I am just an amateur, not an engineer. 
The efficiency of the loop ultimately is the surface area of the loop and very low resistance.
The bigger the diameter of the radiator, wire or more likely pipe, the better.The thing is just a big LC Circuit. Any resistance lowers the Q and also eats up power.
Any resistance that increases your bandwidth will decrease the radiation efficiency and eat up your hard won watts.Much of the amateur development of homebrew loops is to Decrease the resistance in all connections and in the radiator.Typically no soldering is used, rather welding or silver soldering. The goal is the highest Q possible and as a result the lowest bandwidth.
I used a ten foot piece of 3 inch diameter copper pipe and a vacuum variable cap.As far as I know there are no physical alterations, like the shape,  to the loop that will effect a higher bandwidth while maintaining efficiency.I do not know of any way to increase bandwidth while maintaining efficiency,Any resistance will increase bandwidth and decrease efficiency.

In a receive only loop sometimes people put a variable resistor in the circuit to widen the bandwidth. This does not help sensitivity.
There are at least two groups on Groups.io that discuss these loop antennas.

There are lots of calculators on the web that let you put in different values and see the results.

[TimFox]

Since the OP did not specify, I assumed he was using the loop for receive.
My specific feedback circuit works for receive only.
For both purposes, decreasing the Q of the resonant circuit (loop plus tuning) itself reduces the efficiency of transmit and the sensitivity of receive.
The feedback-damped receive circuit has good sensitivity at the free-resonance frequency, and a wider signal bandwidth, but no improvement in SNR away from the center frequency (compared with no feedback).

[Wallace Gasiewicz]

Yes Tim, I suppose you are correct to assume that   
but it would be helpful if the OP would tell us what he is using the loop for.

[p.larner]

its for transmitting,its bandwidth was at most 1.5kc,it caused issues with the rig swr power foldback/protection on ssb,made it unusable!.

[EggertEnjoyer123]

What frequency?
What dimensions are your loop antenna?
Is your loop antenna close to a quarter wavelength in size?

[geggi1]

A larger diameter radiator will to some extent give you a larger bandwith.
Download the spreadsheet in the link and do some tries with diffrent sizes of raqdiator.
You can even use the goal seak functrion in excel and have the spreadsheet finding the size if you have a desired BW.
https://www.aa5tb.com/loop.html#cal

[Andy Chee]

Have you had a look at VK5SFA Steve Adler's 160m transmitting loop?

https://web.archive.org/web/20201218104218/http://members.iinet.net.au/~sadler@netspace.net.au/tmla.html

His antenna has a 4kHz bandwidth @ 2:1 VSWR.

[Geoff-AU]

its for transmitting,its bandwidth was at most 1.5kc,it caused issues with the rig swr power foldback/protection on ssb,made it unusable!.

Make the loop bigger or try a multi-turn loop.  Your loop is too small and the efficiency will be completely miserable.

[Wallace Gasiewicz]

VSWR Problems with magnetic loop antennas are usually caused by the feed system. Usually a small loop is used to feed the loop. If this is not adjusted correctly, there will be mismatch. There are various ways to do this. Sometimes people have even put in a motor to change the position of the feed loop to tune it remotely. 
This group has lots of suggestions:   

https://groups.io/g/MagLoop/messages

[radiolistener]

just add some small resistor in series with magnetic loop. It will reduce Q-factor and as result bandwidth will be increased.

[Helix70]

1.5Khz? On what band? There are many styles of magnetic loop, can you share more information about it, maybe a few photos?

[p.larner]

i dont have it any more but was 3m dia of 15mm copper in an octagon,the cap was a 500pf tx type and i was trying to use it on top part of 80m.

[Wallace Gasiewicz]

Unfortunately that is a less than ideal setup for 80 meters.The efficiency of the radiator is maybe 1%, Not good at all.If the octagon was made with soldered joints the resistance would be much too high, making the antenna even worse.The capacitance does not match the calculations either.You had a pretty crappy antenna for the 80 Meter Band.
Here is a link to the QST Antenna Design Contest winner 2018, a dual magnetic loop for 40 and 80 M   

https://www.arrl.org/news/2018-qst-antenna-design-competition-winners-announced
You need to be a member to get articles however.

[Andy Chee]

i dont have it any more but was 3m dia of 15mm copper in an octagon,the cap was a 500pf tx type and i was trying to use it on top part of 80m.

What is the diameter of the coupling loop?  If you read VK5SFA's 160m magnetic loop webpage I linked above, he discovered that increasing the coupling loop diameter from 600mm to 900mm, would lower the SWR.

[p.larner]

i was not using a normal coupling loop,tried a gamma match first but found a 3 inch powdered iron torroid with some windings on the primary with the loop passing thru its center as the secondary,gave up on it in the end,i think loops for 80m that are a reasonable size and can handle the bw of an ssb signal dont exist,40m and up maybe but 80m no.

[Andy Chee]

i was not using a normal coupling loop,tried a gamma match first but found a 3 inch powdered iron torroid with some windings on the primary with the loop passing thru its center as the secondary,gave up on it in the end,i think loops for 80m that are a reasonable size and can handle the bw of an ssb signal dont exist,40m and up maybe but 80m no.

You might have to define "reasonable size".

If you look at the VK5SFA design I posted up thread, he uses two turn loop for 160m.  The same principle of using two turns, can also be used with an 80m loop design.

Incidentally, how hot did your 3 inch toroid get during your key down tests?  If I had to guess, it was this toroid that is causing your narrow SWR problems.

[p.larner]

well i have ordered 10m of 22mm dia copper tube,would that be ok in an octogon shape for an 80m magloop for ssb?.

[Wallace Gasiewicz]

Don't make an octagon, you will have silver solder or weld. Just bend it into a circle. fill it with sand and bend so you do not get any kinks.With the right capacitor you should get 80 M.

[radiolistener]

If you're planning to use it for transmission, you're needs to know that this is not safe for your health to be near magnetic loop antenna during transmission, because it has extreme high fields strength which exceeds safety limits even if you push just 10 Watt, for 100 Watt it will even worse. This is because such kind of antennas are too small relative to it's working wavelength and needs to accumulate a lot of RF energy in their near field region in order to compensate it's low radiation efficiency due to small size.

And this is why it has very narrow bandwidth, because it spending many cycles to accumulate energy, so it works like bandpass filter.

If you want TX antenna there is serious reason to use large size antenna to avoid damage to your family health.

[iMo]

My first 6m band antenna was a magloop, aprox 35cm ring in diameter made of an 8mm copper tube
The mag loop is a difficult antenna, I would not recommend to mess with it unless you can tune it with a special High Voltage capacitor coupled with a fine remotely operated mechanical stepper.
The bandwidth is quite narrow and you have to tune it always precisely.
Otherwise it works, but its effectiveness is rather low (depends on its diameter vs the wavelength ratio).
Also mind the voltages and currents around the tuning capacitor are Huge (it is a high Quality LC circuit with a LOT of kiloVolts RF around the tuning capacitor even with 100W into it).

[radiolistener]

yes, when you push 100 Watt into magnetic loop antenna, there is high voltage up to 200000 Volt and high magnetic field strength more than 70 A/m near antenna.

[Andy Chee]

For reference, here's a simulation of E and H fields for a transmitting loop operating 1W @ 7MHz

https://owenduffy.net/blog/?p=1842

And dipole for comparison

https://owenduffy.net/blog/?p=2064

The simulated 1W loop above required 1.45 meteres safe operating distance (according to Australian radiation standards), operating at 100W would require even more distance.

In summary, definitely keep your distance from transmitting loops!