[Elecraft] Mag Loop

[Fred Jensen]

Well Morgan ...

Your fluorescent bulb is responding to the voltage/current nodes around 
your octagon.  Your loop is coupling to free space using the H-field, to 
which the bulb is pretty much insensitive.  Because an electrically 
small loop couples via the H-field, it tends to be fairly insensitive to 
the ground and objects around it, so long as the permeability of the 
objects is roughly 1.0 [trees, rocks, you, anything non-ferromagnetic]. 
The ultimate radiation pattern resembles the winding on a toroid, 
expanded, with the null through the center.

I worked a couple of DL's using my K2 @ 5W and AlexLoop from a summit in 
SE New Mexico on 15.  Kind of sobering to realize that all the lines of 
force going everywhere several thousand miles away come together through 
the center of the little loop directly over my head, and a tinfoil hat 
won't help.

If you're parked on the tracks in a railroad yard, things will be 
somewhat different. :-)

Be careful with your loop, even very moderate power levels will involve 
very high voltages and circulating currents.  There's a neat little 
calculator at
http://www.66pacific.com/calculators/small_tx_loop_calc.aspx

73,

Fred K6DGW
- Sparks NV DM09dn

- Northern California Contest Club
- CU in the Cal QSO Party 7-8 Oct 2017
- www.cqp.org

On 10/20/2016 12:37 PM, Morgan Bailey wrote:
> Having played with a Loop, I made one octagonal about 64 inches across, and
> mounted it vertical or perpendicular to the ground, I observed sharp nulls
> through the center of the loop. Additionally, I took a 4 or 7 watt
> flurescent bulb and while transmitting (low Power) ran the bulb from top to
> bottom and found that about ( reference feed point at the bottom, 6 o'clock
> position, im calling it zero degrees and tuning cap at the top, im calling
> 180 degrees or 12 o'clock position) 60 degrees about at 4pm point the bulb
> started to glow and was brightest at about 100 degrees 2:30 position and
> dropped back to dim to extinguished at 180 degrees. The loop was made for
> 20 and 40 meter operation. It is physically too large for 15 meter
> operation.  When I made a tuning cap to resonate it on 80 meters knowing
> the efficiency would be maybe 3 percent, the only place the bulb lit was
> near the top with no glow from zero degrees 6 o'clock to 150 degrees or
> about 1-130 o'clock positions. All NVIS on 80 meters with a small loop.
> This was reinforced by its behavior on 20 and 40 meters. 40 had about a 5
> or 10 degree higher angle of radiation over the 20 which was more evenly
> current distribution on the loop, with the main (brightest) lobe lower in
> the loop.
>
> So when we look at loops and directionality, the frequency of a multi band
> loop does change the radiation pattern. The bigger the loop for the band
> the more evenly distributed current was distributed along the loop. Up
> until the physical size of the loop does not follow the loop formula and
> gets too big to be considered a magnetic radiator. On 40 it radiated
> brightly at the 3 to 4 oclock positions but on 20meters, the current was
> more evenly spaced from about 430 to 1 oclock positions. On 20 it was
> brightest at about 245 positions.
>
> Maybe this is not scientific but my believe is that, this
> is demonstrating the angle of the lobes coming off the loop. It seems that
> it radiated well in the 20 to 30 degree take off angles on 20 and 40 meters
> with 40 meters being a bit higher more like 25 to 45 degree take off
> angles. Having this loop in my dining room on the floor of my house over a
> 3 foot crawlspace, I was able to work Europe repeatedly during the
> afternoon hours. I did this more than one time months apart. It was not a
> propagation fluke. Rig was 75 watts CW from a Yeasu FT 450d. I use it as my
> test rig.
>
> I built the loop to prove that they do not work....yet again I was proven
> wrong.
>
> My loop was made out of 1.5 inch by .25 inch thick Radiused edged aluminum
> bar. I got this from onlinemetals.com. I used radiused bar because RF does
> not flow around corners well. And, structurally, it was self supporting
> with a small bracket for a stand. And, it is just easier to work with flat
> stuff rather than round stock. By using the calculation on the web it
> performed identical to a 1.5inch copper tube. It was cheaper and much
> easier to work with. Additionally, I made it so it could be disassembled,
> yes I know the problem with resistance, don't go there, after all it is a
> compromise antenna and I was building it for portability and sturdiness.
> This loop really performed beyond my expectation.
>
> Was it better than a full sized vertical or a trap vertical with
> radials...Oh hell no, but I did make good contacts and even checked into
> many 40 meter and 20 meter nets from having it inside my house on the floor
> of my living room. I positioned the plane of the loop east west and was
> easily able to work both coasts and into Europe. Most of my contacts on it
> were on CW. See the attachment of it in my living room.
>
> When I rotated the loop 90 degrees on the axis the signals dropped from 2
> to 6 S units, mainly depending on the distance from my location. The closer
> in the less drop. I attributed this to the NVIS component of the radiation
> (the 1 o'clock radiating area).
>
> [ATTACHMENT TOO LARGE FOR FORUM STANDARDS, IF YOU WANT A PIC I WILL GLADLY
> FORWARD IT.]
>
> Well this is my experience with my loop and here is a picture of the loop
> in its final configuration (see attachment) and no I don't want to be
> flamed about the construction but it works and with some fine emery paper
> and no-al-ox on the joints. It has a very sharp resonance and narrow band
> width of maybe 5kc 2to1. After that, it ,the SWR climbs like a wall > 25 to
> 1  But even though the SWR may be high off the sharp resonance transmit
> frequency, it still was a good rx antenna broad banded 100khz with very low
> noise. I can not say it was immune to but, it did not pick up electrical
> noise like the full sized vertical and the noise floor was lower than the
> typical vertical antenna. I have a full sized 40 and 20 meter vertical with
> 60 radials to compare it to and it does not get the same sig reports as
> that vertical but I did not expect it to. On occasion, with a preamp for rx
> (dx engineering one) copy was better on some stations over my outside
> vertical---only because of the reduced noise floor. It was a fun project
> and I really enjoyed the construction.
>
> They are not a miracle antenna but any antenna is better than no antenna
> and for many this may be the only option to be able to operate. Hell when I
> get moved into a retirement home, I will use it for a clothes rack and
> transmit on it when no one is looking.  Since digital, PSK**, would be an
> ideal use for a loop as you don't need much power for the computer to do
> the heavy lifting. QRP signals are still good enough for computer ears, but
> not necessarily for ours.. LOL. In the end, it is another example of "QRP
> Works."
>
> Good luck with your Loops
>
> Vy 73,
>
> Morgan Bailey NJ8M