| Electronics > RF, Microwave, Ham Radio |
| Mag Loop Antenna Aperture |
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| ronsch:
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, Ron K2RAS |
| T3sl4co1l:
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 |
| ronsch:
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, Ron K2RAS |
| cdev:
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. |
| T3sl4co1l:
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 |
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