Anyone modified an MFJ 1786 Tuneable Loop to work on 40 meters?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4SC, Dec 8, 2015.

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  1. AF7ON

    AF7ON Ham Member QRZ Page

    I'm not disagreeing - the low efficiency of small magnetic loops is widely agreed upon - I'm merely trying to reconcile the differences in approach (theoretical vs measurement) and trying to understand where the energy goes. I am sure you'll agree that electrical energy that is not transmitted as electromagnetic waves must be dissipated as heat. If the tests referenced to in Radcom are correct, the heat does not appear in the antenna, coupling or tuning elements. So where does it go? The conventional wisdom is that it is in fact dissipated as heat in the antenna - the measurements I refer to say not. Who is right? I'd be interested in finding some other workers who have done actual measurements to see where the energy really goes! Surely the military, who use these antennas widely, must have some data?

  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    Well, the 2nd link that I posted for you a while back had every mathematical equation that you need to figure it from the Navy. Read all 3 likks that I posted & then see if you have any questions. I have studied magnetic loops for a couple months, since N7HRO decided to do some pipe bending. I have about 30 links on them & 11 links about different manual & automatic control systems.

    If you measure efficiency in thermal loss, how can you explain the difference between an incandecant light bulb & an LED light bulb?

  3. AF7ON

    AF7ON Ham Member QRZ Page

    Ed: I'm not sure your simile of the incandescent light bulb vs LED is a good one. An incandescent light bulb is about as efficient as a magnetic loop antenna (!) - you get a few watts of light and many watts of heat. LEDs are intrinsically more efficient and put our the same light as the incandescent for a much lower input power. They still get a bit warm, but dissipate a lot less.

    My basic question is, if a loop antenna dissipates 95% of the input energy as heat, what get's warm? It's the physicist in me wanting to know where the energy goes!

  4. WA4SIX

    WA4SIX Ham Member QRZ Page

    Again, please read the links that I put up. A magnetic loop can easily be 90+% efficient. It all depends on construction methods & materials. The comparison of incandecant & LED is very similar. You are using watts per watts, when you should be using watts per either lumens or candlepower. Same as you should be using heat dissipated & RF energy dissipated.
    You are basing all of this on 1 flawed article. Read the dozens of articles that seem to agree on the published data.

  5. AF7ON

    AF7ON Ham Member QRZ Page

    All the standard articles (and there are very many, as you correctly say) describe the losses as being due to the relatively high resistance (ohmic and skin) in the antenna compared to the very low radiation resistance. They all are based on the the theoretical formulas of radiation resistance of small antennas (which I am sure are correct). This approach leads to the losses being dissipated as heat in the antenna, which does sound eminently reasonable.

    My original question was based on the thought that tens or hundreds of watts of heat dissipation should result in quite a significant temperature rise in the antenna or tuning components, but this does not seem to be reported in the literature that I have read. The low efficiencies quoted appear to be theoretical and not measured. The one article I quote did some measurements, the results of which are very different from the accumulated wisdom. What did they do wrong?

    Perhaps the actual owners of antennas could report if they got hot when transmitting?

    This is such an interesting topic, perhaps it deserves a separate thread?

    I assure you that I am not trolling, just trying to get my head around it!

  6. AK5B

    AK5B Ham Member QRZ Page

    I would go out and feel my loop element after xmitting full bore at 600 watts or so sometime EXCEPT for the fact that mine is not so inefficient as to warrant such a test.

    If it was only 5% or 10% efficient on the bands it is designed for maybe I would feel a slight increase in temperature (but I still suspect not---that's a lot of surface area that those happy little electrons go skiing over).
  7. KK4NSF

    KK4NSF Ham Member QRZ Page

    I've got a home-made magnetic loop, 42"diameter, built much like the MFJ 1786, except that it's tuned for 40m. It works very well indeed. With 100 watts, it does not get hot..... it doesn't even get warm. I just used mine for a long rad-chew this morning, and it's still at room temp. I can't say for sure that all of the energy gets radiated into usable signal, but it didn't get converted into heat.

    actually.... that is no longer the case. Enough experimenters have tested the Magnetic Loop, and found that the Chu-Wheeler assumptions are incorrect, so there is no longer the same consensus of opinion as there used to be. Some published papers put small loops as high as 90%, IF the loop is constructed within certain limits on it's size . Even the calculator shows how it is quite easy to hit close to 90% on 20meters with a 36" loop. On 40 meters the loop needs to be a bit larger.... closer to 60" diameter or so.... to reach that level of efficiency. I'm guessing mine is closer to 30% to 40% or so, based on ARRL published formulas.
    Last edited: Dec 17, 2015
  8. KK4NSF

    KK4NSF Ham Member QRZ Page

    oh... I forgot the original topic: yes, increasing the capacitance will lower the operating frequency. I don't konw that the orignal capacitance on the MFJ is.... but with a 30"loop you'll need a total of ~285pF.... and you'll wind up with an efficiency of between 9 and 14%. IF you could increase the diameter to 36", the cap requirments goes down to 205pF and the efficiency goes up to 14-19 percent..... and so forth.

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