If you you are looking for a portable self-supporting antenna that is multi band and functions well, independent of terrain, it is really hard to beat a portable magnetic loop like the Alex Loop.
I have used Magnetic Loops for at least the past 10 years (I have an MFJ and an Alex Loop and I have built a few homebrew ones for portable use) and I can say that the only real compromise when using a well built 1 meter diameter loop on 20m through 10m is bandwidth. Efficiency on the higher bands is good to excellent (usually ranging from about 30% up to about 90% on 10m). The efficiency does suffer when pushing a loop this small to cover 30m and 40m but it isn’t so bad as to make it impossible to make QSOs at the 5w to 10w level, even on SSB. The problem with 1/4 wave verticals (including loaded ones) is that they rely on radials / earth to complete the other half of the antenna and unless you want to spend a lot of time deploying radials (which I think defeats the whole purpose of this thread) then results are going to vary a lot depending on the soil conductivity. I think that most Hams would be quite surprised at how inefficient a short vertical with few radials really is, especially over rocky terrain. Using them near salt water is whole different story. Horizontal antennas require supports, so they are by definition not self-supporting. On most of the HF bands reasonable height is required to achieve any kind of half decent take-off angles for DX. Over poor soil you don’t have the efficiency issues you do with verticals. Moxon in his excellent book "HF Antennas for all Locations”, suggests that if you are on a mountain/hill you can use the terrain to your advantage by mounting a low inverted-vee doublet, partway down the hill in the direction you want to work. This will significantly lower the take-off angle and can be quite effective for working DX in this sort of environment … but it still requires a support and this only works on a hill. So what does a well constructed 1 meter diameter Magnetic Loop give you : PROs - continuous coverage from 40m through 10m with a very close match to 50 ohms (SWR under 1.5:1 usually at resonance) on all bands - good efficiency from 20m through 10m and reduced, but usable, efficiency on 30m and 40m (assuming proper design and construction of the loop) - both high angle (useful for close-in NVIS contacts on 40m) and low angle radiation for DX on higher bands (note that the radiation pattern is a donut standing on its end) - no need for an antenna tuner (in fact it is not recommended; all tuning should be done by adjusting the capacitor on the loop) - at heights in excess of 1 radius (i.e. 1.5 feet from the bottom of the loop) very good ground independence, which makes tuning predictable regardless of terrain. - self contained, self supporting (with small tripod), compact and lightweight making it quite portable - directionality. A magnetic loop has bidirectional radiation pattern with lobes in the plane of the loop and a fairly deep null broadside - very quiet antenna on receive and from my experiences even a slight rotation of the loop can drastically reduce nearby electrical interference - 5 minute (or less) setup and takedown time - fairly low wind resistance (you will realize the importance of this if you have ever operated on the top of a mountain ! ) CONs - mediocre performance on 40m / 30m (for a 1 meter diameter loop) - narrow bandwidth (typically around 10 Khz 2:1 SWR bandwidth when tuned to resonance on a given band) This varies from band to band but you can expect a wider bandwidth on the higher bands and narrower bandwidth on the lower bands. This means that sitting on a frequency and calling CQ works great, while S&P is a little more work as it requires frequent tweaking of the loop capacitor to resonate the antenna as there frequency changes - requires a means of either measuring SWR or Field Strength to adjust the tuning capacitor to resonate the loop for lowest SWR. - need to consider RF exposure and limit power to 5W to 10W for a manually tuned loop. Also the operator should not be closer than about 1 meter from the loop during TX at these power levels (I personally try to always sit broadside to the loop, no closer than arms length) - possible RF burn hazard. This is especially true for a home-brew loop with a bare radiator (i.e. copper or aluminum) and not as much of an issue for an antenna like the Alex Loop as the coax jacket provides protection. Observations - manual tuning works well with a little practice and often it is possible to achieve a 1:1 SWR just tuning by ear and peaking band noise. - for S&P, moving off frequency a few KHz from the desired station and quick re-tweaking is pretty quick, but doesn’t work with a heavily occupied band (think Field Day on 20m) unless you want to be rude and TX over someone else (not recommended). - I can’t stress the importance of proper design and construction of a loop if you are home-brewing. Everything should be soldered/braised/welded and you must use a split-stator or butterfly capacitor otherwise efficiency will suffer greatly. - wide 2:1 SWR bandwidth on a home-brew loop is an indication of very poor efficiency. The trade-off with a Magnetic loop is between efficiency and bandwidth … you can’t have both at the same time. One area for improvement, in my opinion, would be to apply some smarts to (semi)-automatic tuning of the loop. With the technology available to us today it should be possible to (semi)-automatically tune a magnetic loop to resonance as the user changes bands and frequencies. I could foresee something like a mini VNA housed inside the same box as the capacitor, being fed band/frequency data from the rig via Bluetooth (hey, I might as well think big ! ). Loop tuning should be possible without transmitting a signal from the rig itself. Also because of the previously mentioned ground independence, it should be possible to go through a calibration process to allow the loop tuning circuitry to figure out what capacitor position corresponds to a given band and save that information as a preset for future use. This would make band to band QSYs happen a lot more quickly as it would have a good guess at a starting point for tuning each band. I think that smart tuning technology would go a long way towards addressing what I see as the one major issue with a portable Magnetic loop which is the narrow bandwidth and the need for frequent retuning. Michael VE3WMB / VA2NB P.S. The standard disclaimer, I have no personal or financial interest in Alex Loop. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
