Magloop woes
I've finished my magloop, and have a few questions.
I made the antenna according to Reg's magloop program. Octagon loop, 7.3 meters diameter 19.05 conductor diameter 100 watts output 3 meters above ground (though not for testing) average soil The diameter of the coupling loop is .420 meters in diameter. Since I saw this param changing all over the place as I changed frequencies in the program, I just chose a diameter from somewhere in the 40 meter band. The loop is done up as a Faraday loop from coax, circular, and placed in the traditional position. Continuity has been measured. All seems to be well, but when attaching the antenna analyzer to the thing, I get infinitely high SWR in most places (understandable) with a foray into around 100-300 ohms on what I think may be the tuned frequency. Anyone know why my measurements could be so far off? At first I thought the loop could be cut wrong, but just using the program shows a wide variation in loop sizes, so I doubt that is correct. TIA - Mike KB3EIA - |
Mike,
I don't see any mention of a fixed or variable capacitor. I take it that there is one fitted! Just an observation, Roger, G7JAQ, |
"Roger Adam" wrote -
I don't see any mention of a fixed or variable capacitor. I take it that there is one fitted! ================================ Neither is there any mention of frequency. Where is the antenna analyser connected? What parameters are being measured. How long is the feedline, etc. ? Does the antenna work when connected to a transmitter or receiver? If it doesn't what are the symptoms? A magloop has an extremely high Q and narrow bandwidth. Behaviour approaches that of a quartz crytal. If you don't get the expected measurement results then the most likely reason is the limited capabilty of the analyser. A precision, scientific-grade instrument is needed. Reg. |
On Sat, 08 Nov 2003 16:06:36 GMT, Mike Coslo
wrote: Octagon loop, 7.3 meters diameter .... ...somewhere in the 40 meter band. On Sat, 8 Nov 2003 18:16:19 +0000 (UTC), "Reg Edwards" wrote: Neither is there any mention of frequency. .... A magloop has an extremely high Q and narrow bandwidth. Behaviour Really? You two are talking past each other. 73's Richard Clark, KB7QHC |
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Roger Adam wrote:
Mike, I don't see any mention of a fixed or variable capacitor. I take it that there is one fitted! Of course! I have a 4 element trombone cap, powered by an electric screwdriver. - Mike KB3EIA - |
Reg Edwards wrote:
"Roger Adam" wrote - I don't see any mention of a fixed or variable capacitor. I take it that there is one fitted! ================================ Neither is there any mention of frequency. Where is the antenna analyser connected? What parameters are being measured. How long is the feedline, etc. ? The frequency I chose for the *coupling* loop size was around 7 mhz in the 40 meter band. I chose this because of the wide variation in loop sizes called for by the program. the antenna overall is designed to hit 75 meters, and at the other end, 17 meters. But 40 meters is the mainstay of this antenna. The analyzer is connected to a piece of coax that is connected to the coupling loop, which is a Faraday loop. The feedline for the testing is around 5 feet of 50 ohm cable. I don't doubt that my proximity to the antenna might have some effect, but I don't expect it to be that extreme. Does the antenna work when connected to a transmitter or receiver? If it doesn't what are the symptoms? Haven't connected it yet. I wanted to see if I was in the ballpark. Initial tests suggested I probably aren't. A magloop has an extremely high Q and narrow bandwidth. Behaviour approaches that of a quartz crytal. If you don't get the expected measurement results then the most likely reason is the limited capabilty of the analyser. A precision, scientific-grade instrument is needed. Now I am really perplexed! If I need an instrument such as you describe, how will I ever get this thing to tune with my lowly IC-745? It certainly isn't a scientific instrument. What I used to test the loop was a MFJ-259 analyzer. Not "lab grade", for sure, but not at the bottom of the barrel either. It has worked competently for everything else I've used it for. - Mike KB3EIA - |
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Mike,
Ignore the old wives. The extra resistance due to soldered joints around a copper octogon is unmeasurable. Wipe off any visible surplus solder while hot or use fine glass-paper. I finish off with metal polish and a coating of clear yatch laquer. A beautiful, long lasting appearance . The usual conductor material in commercial magloops is an aluminium alloy. Its lighter, cheaper, weather resistant and you can't tell the difference in radio performance. By far the best way of improving magloop performance is to increase the circumference of the main loop. Double the circumference, half the capacitance and the difference will astound you. However there's nothing more to be gained after the circumference has increased above about 3/8ths wavelength at the frequency of interest. Just make the main loop diameter, capacitor value, conductor diameter and coupling loop diameter according to program Magloop4. Ideally, there should be no direct connection between coupling loop and main loop. Otherwise RF currents will flow on the coax feedline and can make a mess of the null in radiation pattern. A few ferrite sleeves over the antenna end of the coax will help. The coax used in the coupling loop can be junked. The Faraday thing is a waste of time and material. Just use a length of wire stiff enough for it to be self-supporting. It can then be easily adjusted for diameter. Alternatively, if made a little oversize, it can easily be rotated a little relative to the main loop to vary the coupling coefficient. If the loop diameter is adjusted for the 80m band it will be near enough for the 160m and 40m bands too. If you do nothing with the program except change frequency you will see the diameter of the coupling loop changes very little from one band to another. But if you have some wierd, bulky, home-brew contraption for the tuning capacitor then in practice things may behave less conveniently. You won't need an antenna analyser. Just connect a transceiver with its ordinary SWR meter to the antenna and it should work right away. Don't take undue notice of the SWR meter till exceeds 1.5 : 1. It is the transceiver with its own SWR meter which has to be kept happy - NOT the analyser. If you've never used a magloop before then it may take a little time to get used to the VERY narrow bandwidth especially on 160m and 80m. Just go ahead, finish construction and stop worrying. ---- Reg, G4FGQ |
On Sun, 09 Nov 2003 14:56:14 GMT, Mike Coslo
wrote: A precision, scientific-grade instrument is needed. Now I am really perplexed! If I need an instrument such as you describe, how will I ever get this thing to tune with my lowly IC-745? Hi Mike, Don't get hung up in the Cassandra cries of impossible. Your receiver will tell you quite well where the loop is resonant. You will get much more background noise as you tune the band across that frequency; and if you cannot tune your receiver with such restraint to observe this, prospects for transmission tuning are far bleaker. Shelling out bucks for "scientific-grade" instrumentation does nothing to improve the situation when your receiver has more than enough "precision." There are zero-power tuners on the market that have adjunct white noise generators to allow you to tune an antenna system using the receiver employing this same method. But as that is for antenna systems of broader Q's, their the additional noise is useful for emphasizing the resonant peak. Your Loop comes with an advantage (more Q) in that regard. 73's Richard Clark, KB7QHC |
Mike,
to celebrate the change in my license conditions which allowed me to access HF freqs this year (no code experience), I decided to put up a long wire transmitting antenna (30metres long). This was connected via an Icom ATU connected to the Icom IC 725 rig. I also decided to have a go at building a Magloop as an alternative, to experience the trial and tribulations of getting it to work at minimum cost. I could also gain experience as to the ability for the loop to "null" out interference. Having no experience of operating on the HF bands, I was not sure what range of frequencies this antenna should be designed for. Initially, using freeware software, I could see the efficiency rapidly changing for the worse, as the operating frequency went lower, for a given size of loop. Having decided to choose a square loop of 1.33mtrs sides, using 22mm copper pipe for practical reasons, I set about making a piston capacitor. This consisted of approx 2x300mm of insulated 15mm pipe, fitted within 22mm outer tubes, being part of the main loop at the top. I had hoped to cover the 20metre to 40 metre bands. It actually did this, but the piston caps were either fully in or fully out and tuning at these extremes was very, very touchy. I discarded the piston idea and bought a single vacuum variable cap (5-100pf), a 12v geared motor, insulated coupling spindle and built a pulse width modulator kit from Vellerman to drive the motor. It was at this time that I decided that the way forward for me was to make this a monoband antenna (20M) with a reasonably high efficiency (The 40metre band seemed so busy at the time). I used some external 3 core copper cable (pyro) for the gamma match and using a MFJ 259 analyzer via 3mtrs of feed coax set about setting it up. A very obvious VSWR dip was found at an indicated resistance of 50 ohm, around 14 MHz. The analyzer could track the changing frequency when volts were applied to the motor. The antenna was tested in open space, but at ground level + 1 metre. This system is now installed at the top of the garden and via its connecting cables can be tuned from the shack. In operation, I usually find a reasonable station around 14.250 on the long wire, connect the MFJ and set it to the chosen freq, connect the magloop feed to the analyzer and adjust the PWM up and down to find best VSWR. Then switch the magloop to the rig and wait for the opportunity for a "break". It's fair to say that the received signal appears a few "s points" down compared to the long wire. The predicted spec of the antenna is as follows:- Circumference=5.33mtrs Conductor dia=22mm Band =20m (around 14 MHz) Bandwidth=44.1kHz Cap value 15.4pF Cap voltage=3.6kV Efficiency=87.3% Inductance=-4.325uH Inductive reactance=390ohms Loop area=5.8mtrs Loop dia=1.3mtrs Loop Q value=325.2Qres Radiation resistance=0.524 ohms Resistance loss=0.076 ohms So from my experience, you should see a tuning dip when using the MFJ Analyzer. I'm getting around 1.2:1 at around 50 ohms at the chosen freq. But I will say that when coming away from that freq, the VSWR rockets skywards very suddenly. As REG points out, the Q is extremely high and the bandwidth very narrow In the early days, if I attempted to transmit without getting the magloops' vswr down to a reasonable level, the Icom IC725 would go into protection mode and then it was impossible to determine in which direction the variable cap should be adjusted. I have never tried using the receiver to tune the loop. Well that was a long "over" from me, maybe some of it may help. The icing on the cake would be if we establish contact via the loops on 20 metres!! regards, Roger G7JAQ |
Roger Adam wrote:
a bunch of good info snipped Thanks for all that Roger. The icing on the cake would be if we establish contact via the loops on 20 metres!! That would be great, Roger! I'm in the middle of Pennsylvania, and I take it you're in Great Britain. The path should be good in my afternoon/your evening. I'm taking Reg's suggestion and skipping the Farady loop, and going to plain wire. My coax just keeps getting shorter... and shorter, so I'll switch before I have to buy new! - Mike KB3EIA - |
Mike,
I sure you're right in thinking that the coupling system is the cause of your problems. As far as the ability to "null" out any unwanted sigs, I have fitted a Yaesu rotator (which when bought was fully rusted up inside!) to the vertical drainpipe support of the loop, but haven't yet tried it out. Thanks to TAM WB2TT for help on the Yaesu. Friends have said why don't I position the loop horizontal and do away with the rotator. I may post a request asking for REG's advice on that! I was working on the principal that with the long wire being horizontal, then the loop should be vertical to give me a different polorisation. However, after looking through the books and 'net, It appears that the initial polorisation becomes changed within a short distance of being launched and that there is little point in spending a lot of time on this aspect. As far as a possible future QSO is concerned, I'll be monitoring around 14.325MHz from 20.00hrs UTC most evenings. That will most probably put you under pressure to get the loop up and running! By the way, my QTH is Surrey, 10 miles south of London. Good luck, Roger G7JAQ |
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