Mag Loop Antenna Aperture - Page 1
Forum section: electronics / rf-microwave

author: ronsch
author_rank: Newbie
timestamp: January 16, 2016, 04:14:45 pm
content: I have a Pixel receive-only amplified mag loop antenna and it is working great. However, I am always hoping for improvements. The antenna is about a meter in diameter and made of aluminum pipe about 1/2 inch in diameter.Does anyone know whether the aperture of the antenna appears like a solid disk, or would increasing the diameter of the tube make a significant increase in antenna gain by increasing the surface area?I am obviously not an RF engineer, so I am interested in opinions.Thanks, RonK2RAS

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 16, 2016, 04:34:20 pm
content: For what frequency range? Is it tuned, resonant or "small loop"?There is a theorem, relating the ratio between equivalent aperture and physical size, to the percent bandwidth, of any antenna. (Size being determined by the minimum bounding sphere for balanced types, or hemisphere (the flat side being common with the ground plane) for unbalanced types.)In general, the aperture depends on overall size. Bandwidth can always be narrower (and/or efficiency lower!), and too-small conductor diameter is a factor in this. An electrically small loop, tuned (with loading capacitance) or not, will have low bandwidth or efficiency regardless, and conductor size will only affect I^2*R loss.So if it's a naturally resonant (~1 lambda) loop, bandwidth will more or less follow conductor size, much as it does for the dipole. If it's a small (< 1/8 lambda) tuned loop, then conductor size will play a bigger role in efficiency than bandwidth. If it's small and untuned, then efficiency will be crap regardless and conductor size will have little effect.If it's for, say, a few MHz or below, it's certainly not a naturally resonant loop, but as long as the amplifier is moderately low noise, it should still be limited by atmospheric noise (not amplifier or antenna noise), which isn't going to be improved upon by any (relatively low directivity) antenna design. (High directivity antennas will improve SNR to a given station quite a bit, but we're talking fractional miles of arrays to work the low MHz, so screw that.)Tim

author: ronsch
author_rank: Newbie
timestamp: January 16, 2016, 04:48:29 pm
content: The antenna is definitely non-resonant and is receive-only, therefore there is no coupling loop. I use general ham frequencies 2Mhz to 28Mhz. Interestingly, the antenna works great down to shortwave, but it is increasingly deaf above around 20Mhz. I would have thought that the antenna would work better at the higher frequencies, since it is at least a little closer to the wavelength. There is a 30 db LNA at the bottom of the antenna.With this information do you think the antenna could be improved by increasing the size of the antenna tube?Thanks, RonK2RAS

author: cdev
author_rank: Super Contributor
timestamp: January 16, 2016, 04:50:46 pm
content: Ronsch- (for above 20 MHz) try a much smaller diameter resonant loop! But - with a resonant design, if you plan on transmitting, use a good quality capacitor. or it might fail, even at low power.Non resonant loops are very different than resonant loops. I have only done a bit with them. Would like to do more. I have some FETs on order which I am going to use in an active non-resonant receiving loop. Ive had good luck using MMICs but would like to try the designs I see online which typically use FETs.I really like resonant loops for receiving because they work really well with SDRs, eliminating some of the SDR's problems very handily. (by means of preselection and noise resistance)Increasing both the diameter of the conductor and the area enclosed both offer a lot of advantages and don't reqire heavy pipe to acheive, especially in a recive only scenario (but shouldt for transmitting either!)**because of the skin effect** it can be accomplished easily with light conductors which are applied over a light frame such as a foam pool float. All that is necessary is keeping it aligned so you could easily use foam core overlaid with copper or aluminum high temperature "duct" tape. Some kinds of aluminized Mylar tape even might work (and be super strong)I made a really good receive only mag loop out of aluminum tape, using the unrolled up ends as the capacitor. There is no reason that couldn't be done for transmitting too, but you should take care as to the kinds of plastic or foam you use. Ideally, use clear plastic (like clear tubing sold for lab use) or pool floats. Wrap the tape in a loose spiral. As far as the ends, bear in mind (if you use aluminum) that it may be difficult to get an electrical connection as good as you would like so there (at the ends) I would mix aluminum (cheaper) with copper and take care to make the area where they cover each other big enough and flat enough so that you're sure that even if the conductive adhesive fails to do its job, capacitor effect will.Then there is no reason you cannot them solder to the copper portion. Or if you use the tape as your capacitor you wont even need to do that.Try it. Its really really easy! You don't have to take the paper backing off the tape for one made with a simple strip of aluminum tape. So you are not even "using" it up. You can make a simple compression type capacitor for the ends. (or use a trimmer or varicap of your choosing, ideally a design where there is no reliance on a bushing to carry current. If you have a lot of noise, using a multi ganged capacitor where the rotating part is grounded (so the loop is balanced) is best. In that scenario you may want to construct a shielded pickup loop out of coax.The simplest way is the best. Visualize a book which gets squeezed to increase the capacitance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 16, 2016, 04:58:51 pm
content: Ironically, it might be improved by making it smaller, and tossing more gain in front (again, assuming the SNR works out). I think the lower atmospheric noise by 20MHz and up, might make that a tough goal, though. (That is, with less background noise, the antenna + amp SNR is much more critical.)I don't actually know offhand what a loop antenna does around fractional wavelengths, whether it has strong cutoffs or what. I would think it's null at 1/2 lambda. For sure, a meter diameter is a bit over 3m circumference, so would be ideally suited (without LNA, and for bidirectional service) for the 3m band (FM radio!). 30MHz (1/3 lambda) is too close to be in "small loop" territory. 20MHz being < 1/4 lambda seems to imply it's going into "small loop" territory by then.The LNA might also simply not be made for the required bandwidth, I don't know.How about doing diversity, of some sort or another, with a nice little (naturally resonant) whip, dipole or loop?Tim

author: cdev
author_rank: Super Contributor
timestamp: January 16, 2016, 05:09:47 pm
content: T3sl4co1l - yes, exactly, the small amplified loop has both better coverage at the top and is also more broadband, but needs more gain. That much I do know. Also, its got excellent noise immunity, compared to a wire antenna. Its got the potential to act like a differential scope probe, which is how it should be set up. I think.Did you know that some SDR software can use multiple receivers as you describe?That works really well, I think.If you made two (or even three) loops that had the same center but which were offset 90 degrees from one another, and used equal lengths of coax to feed three receiver front ends which were all fed by the same clocks, to maintain phase and timing relationships between the signals, you likely could use that information with software like Linrad (there must be others too, I am pretty sure Winrad also has that) for things like noise cancellation really effectively.Okay, for the following, keep in mind that I am talking about a resonant magnetic loop situation, which is different than what the OP describes. because its what I know and also because its what I find works best for receiving HF. YMMV.A meter diameter resonant loop in my experience, is optimal in the core daytime HF spectrum, from around 6 or 7 MHz up to around 15 Mhz. You can extend the bottom part of the range lower by using more capacitance. The Q of a well made loop is often really high and that improves its performance in my opinion.A non-resonant, ampified loop is a completely different animal, it maintains the noise resistance of the resonant loop but becomes broadband and is often able to receive all of HF well, but it needs a good low noise amplifier. Many MMICs made for microwave use work well at HF too. Google for some designs.Another option which emphatically does not need any amplification, is to make a varactor tuned balanced loop, that will give you literally volts of output at your desired frequency - which makes the right kind of design critical with varactors- (you may prefer a stepper motor design with conventional capacitors, even for receiving?) as well as preselection and ability to null out noise.Google "Chris Trask" active loop antenna, (but in my experience you really dont need the amplifier part. You may get a benefit from it though, if you live somewhere really quiet, which i don't)

author: AF6LJ
author_rank: Unknown
timestamp: January 16, 2016, 05:24:35 pm
content: The loop is a mobius loop (see below). The loop is a balanced device and in the base of the loop there is a balun of some kind to match to the preamplifier.It is my opinion you won't gain anything by increasing or decreasing the size of the loop

author: cdev
author_rank: Super Contributor
timestamp: January 16, 2016, 05:34:07 pm
content: Wow, what an interesting looking design. (the pixel loop) please forgive my previous postings as they clearly don't apply. That kind of balanced differential approach looks very useful. Going to try making that right now.

author: AF6LJ
author_rank: Unknown
timestamp: January 16, 2016, 06:20:29 pm
content: Quote from: cdev on January 16, 2016, 05:34:07 pmI have a Pixel loop they work very well, all the way down to 50KHZ.in the actual Pixel loop there is a broadband transformer or balun of some kind.I cannot access it since it is potted in silicone in the bottom section of the loop above the F connector.One thing they do in order to avoid ground loops; the loop and the amplifier are isolated from the mast by means of nylon washers.

author: cdev
author_rank: Super Contributor
timestamp: January 16, 2016, 07:59:08 pm
content: Ronsch,Perhaps if its non-resonant, the usual reasons for increasing the diameter (reducing resistance, improving efficiency, making it more broadband, etc) probably don't apply as much if at all.Look at that Chris Trask active pickup loop for its two step balun design, that is likely to be more appropriate, maybe.

author: modelman
author_rank: Newbie
timestamp: January 20, 2016, 01:35:19 am
content: One advantage of the "magnetic" receiving loop is its deep null perpendicular to the plane of the loop. The null is very useful for rejecting local sources of noise (e.g. plasma TV QRM, etc). In order to preserve the broadside null, the circumference of the loop must be less than a quarter wavelength at the highest frequency to be received. By rejecting the local noise source, the S/N ratio is improved. Increasing the gain may not necessarily improve the S/N.The concept of antenna aperture is a measure of its gain, and not necessarily its physical cross-sectional area. The gain of the mag receiving loop is very low and requires a preamp at the feedpoint of the loop.ModelmanQuote from: ronsch on January 16, 2016, 04:14:45 pm

author: cdev
author_rank: Super Contributor
timestamp: January 20, 2016, 02:52:08 am
content: The nonresonant loop and resonant loop couldn't be more different when it comes to output level.A high-Q resonant loop receiving a signal on its tuned frequency is often putting out over a volt or two in signal. So much its scary.But that window of bandwidth is very small and whenever you change frequency you need to change the tuning. As we speak I'm fooling around with a varactor tuned loop circuit which I have a ten turn pot attached to for tuning. Something like that is necessary or you would miss the peak. It runs off a 9 volt battery which should last years because its used to back bias the varactors which requires practically no current.I am a little concerned about nonlinearity but in the last one of these that I built, it didn't seem to be a problem. Definitely does not need a preamp if its resonant, although one might help for really weak DX in a very very quiet location.

author: AF6LJ
author_rank: Unknown
timestamp: January 20, 2016, 04:34:26 pm
content: Quote from: cdev on January 20, 2016, 02:52:08 amThat is purely dependent on the input signal level a resonant loop where all the stray resistance has been eliminated will only work slightly less than a dipole in terms of gain, high Q or not.QuoteThere are a number of issues with using a varactor;First; they are relatively low Q variable caps.Second; the DC voltage differential across the diode must exceed the RF voltage across the varactor by enough to exceed the peak to peak RF voltage, otherwise the diode will conduct and introduce noise into the received signal.I've been researching tuned loops; mainly small transmitting loops. The principals are still the same the only difference is the currents and voltages one must deal with.In my case my next loop project will be an experimental 14MHZ STL capable of handing the legal limit.Loop antennas are great for situations where space is an issue or when one has interference that is from a point source that needs to be mulled out.Passive loops are no substitute for a full size resonant antenna.

author: cdev
author_rank: Super Contributor
timestamp: January 20, 2016, 06:29:05 pm
content: Sue, what kind of capacitor will you be using? I am guessing you want a fixed frequency loop to start?The systems with large vacuum variable often use a stepper motor for tuning. (very expensive considering)As far as gain, here is an interesting question. What would be the effect of side by side loops, with one loop in the center being the "driven loop" and the loops to either side being tuned slightly above and slightly below the frequency of interest?My question being - basically, might it be possible to create something akin to a beam with loops? (That kind of is like what happens with a conical log spiral, which electrically, resembles a log periodic dipole array.)Here is an easy way to make a loop that can be fixed to a single frequency to try it out that likely would handle significant amounts of power. Use a single piece of sheet aluminum like the kinds they use for aluminum siding and fold it back on itself putting a piece of glass to weigh down its base and then have the glass also be the dielectric. To adjust the frequency move it back and forth until you find the peak. (so more or less aluminum is over the glass and in close proximity to the other side.) Then its all one piece of aluminum so very good conductivity. if you are not transmitting, use a sheet of paper. You can also use aluminum foil but then you must figure out a different way to support it. One way is wrapping it around something like tubing. Make sure not to break the continuity-i.e. use a single piece of aluminum foil.The loop described by AF6LJ looks like it could be optimal for a pickup loop on a tuned loop system like that and if its made with heavy coax it likely could handle a KW.Using aluminum tape with backing is extremely easy and if the tape is stiff enough to hold a loop shape makes a ten second loo, basically.Aluminum tape makes a great receiving loop with its own built in capacitor, itself. I would not rely on it to handle any real power. it would be interesting to see how it would fail, it might just - poof, vanish with a flash of light!Plusses of tuned loops:Sizerotatabilitylow cost to build (comparable to untuned loop)Can be used for transmitting with bigger capacitorsRelatively low angle of radiation- considering their height.Noise rejection as good or better than untuned loopNarrow bandwidth pre-selectionDoes not have to be heavy, necessarily, if designed with the skin effect in mind. (could use something really light covered with copper foil)Negatives: narrow bandwidth (but that can also be a plus as its also a band pass filter for receive (and also for transmit but I dont think its acceptable to depend on that property to make a bad signal compliant- thats an interesting question)Untuned loop:Requires preamponly for receiveGood noise rejectioncan be rotated for best receptionDont know about angle of radiation.

author: AF6LJ
author_rank: Unknown
timestamp: January 20, 2016, 08:46:30 pm
content: Quote from: cdev on January 20, 2016, 06:29:05 pmI'll be using a vacuum variable with a gear reduction drive w/ a DC reversible motor,To start with.As far as gain, here is an interesting question. What would be the effect of side by side loops, with one loop in the center being the "driven loop" and the loops to either side being tuned slightly above and slightly below the frequency of interest?That would be a similar to a loop yagi or a quad, assuming you are using the loops like are parasitic elements with a driven element.Quotethis has been done...QuoteI'll bet this has been tried; but I would point out Q is Everything when it comes to these types of antennas. These types of loop antennas have high currents in the loop element itself and high voltages at the tuning capacitor.A loop that would handle the "legal limit" in terms of power in the US can have in excess of a hundred amps of circulating current and voltages approaching 25KV at the tuning capacitor.I have seen a number of these loops that were made up of sections of copper pipe and joints soldered together, this is not optimal in terms of keeping the resistance of the loop low. A small transmitting loop that is capable of handling the legal limit should be made from one continuous piece of copper pipe with welded flanges on the ends to mate up with the vacuum variable. There needs to be appropriate support to take the load off the tuning cap. Not to mention a reasonable means to couple energy to and from the loop such as a small coupling loop, or a gamma match. (the loop is preferred)QuotePeople have made copper tape loops, and they work reasonably well at low power levels. They do have relatively high DC resistance that limits their efficiency.It should be pointed out;A small transmitting loop is nothing to trifle with; even at a hundred watts the RF fields are quite strong and people can be hurt badly if they come contact with the loop during transmission sections. One that handles 1500W can be very dangerous if precautions are not obeyed.

author: cdev
author_rank: Super Contributor
timestamp: January 21, 2016, 06:37:24 am
content: How far should people be kept from a 1500 watt transmitting loop on HF? I think around one loop diameter should be the minimum. Otherwise the loop starts getting detuned, and that must mean heat is perhaps going somewhere.

author: AF6LJ
author_rank: Unknown
timestamp: January 21, 2016, 01:14:42 pm
content: Quote from: cdev on January 21, 2016, 06:37:24 amI don't know for sure. The good news is; RF at HF frequencies doesn't carry the same potential for damaging health effects as VHF / UHF radio frequencies do. With that said; exercise common sense. I wouldn't run a small transmitting loop even at a hundred watts where someone could come in contact with it.