I just dug up an old design of mine where I was planning to use 7 Amidon R33-050-750 0.5 inch by 7.5 inch rods in a bundle. i.e. one rod in the center and six wrapped around it yeilding a "bumpy" 1.5 inch diameter bundle.
Somehow in my madness, I had calculated that this design would increase the effective antenna cross sectional area and improve the S/N.
I had calculated that I needed about 1000 turns to get 70 mhy. I found another of my old calculations that predicted 883 turns, AWG 38 spaced one wire diameter apart at 126. turns per inch. I had planned to wrap two strands at once, then coat with Q dope. When it was dry enough, I would remove one of the winding, then wrap the layer and start the next one.
Any comments, even those questioning my sanity, will be greatly appreciated.
Now that I am semi-retired, I thought I'd give this another try, but I wanted to run it by the experts here.
why do you think a larger antenna will improve the S/N. At low frequencies the noise floor is typically set by atmospherics. with a bigger antenna you pick up more signal and more noise.
I think you're right, Bob. It's quite likely this will yield great results.
You might also consider a loop antenna like the Palomar commercial units. Those are sweet - and they have a simple tuner. At those frequencies a simple tuner can help a lot to reduce off-frequency interference.
I haven't done a finite element analysis on such things, but I think you can approximate the same effect as a fat bundle by adding a big ferrite bead on each end of one rod, (pick one that just slips over the end) including the same inductance per turn. Inserting the rod about a diameter into the bead should be enough.
This gives you a lower resistance and lower stray capacitance coil than wrapping around a big bundle. I would also use a very thin coil form, like one made of a few layers of epoxy and paper wrapped over a few layers of Saran Wrap over the rod. The PVC pipe adds turn length without doing anything useful for it. The ferrite rods are pretty good insulators, all by themselves. If you can measure resistance between two points on one with an ohm meter, it is not RF ferrite.
"Bob Agnew" wrote in news:LLg6f.90478$lq6.23128@fed1read01:
At Tektronix-Orlando FO,the WWVB antenna we had for our Spectracom 8161
60Khz WWV receiver/comparator was only a ~6" ferrite rod with a preamp on a flexible PCB,all slid into a 2" PVC pipe.
We got a fairly decent reception,with the antenna indoors,as our building owner would not allow us a rooftop mounting.The hard part was keeping the antenna away from other instruments that generated noise.
Guess there's only one way to find out. Looks like its back to winding. Now if can only find those 7 ferrite rods ;=}} I may have to reorder. I was using the Amidon R33-050-750 rods.
OK -- I dug up the Amidon application notes. The reasoning that I used to justify a large bundle is as follows:
Loop induced voltage = 2*Pi*N*A*Mu_sub_epsilon*F / Lambda
where :
Mu_sub_epsilon = effective permeability of the rod F = Field strength in microvolts per meter N = number of turns A = cross sectional area of loop in square meters Lambda = wavelength in meters
The idea is to maximize A. The usual assumption is that the dominant noise is the noise in the receiver bandwidth and not atmospheric noise. When atmospheric noise dominates, the strategy fails as someone has pointed out.
Loop piping and fiber junction were assembled and soldered. Then slit all the way around to facilitate winding. Slit was squeezed closed and then soldered. Crude but effective ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
The rod is intercepting the energy in a volume by steering the flux passing through some cross sectional area larger than the area of the ends of the rod, through the length of the rod, and thus, through the coil. I say an area larger than the area of the ends of the rod, because the high permeability of the rods gathers flux that would have gone past the rod if it has been air. Larger end areas allow flux that is further from the rod's center line to detour through the rod without having to crowd in near the more central flux as much. So larger ends gather more flux. A simple rod may be easy to manufacture, but it is not the optimum shape to gather flux from space and transfer energy into a coil.
My point is that the *effective* area of flux intercepted by the rod is related to the areas of its ends, not to the cross sectional area of its middle, as long as the permeability of the middle is high enough that the flux isn't held back by too small a center area.
This is from the "Rod Permeability vs. Rod Length divided by Rod Diameter" graph on:
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Note that for a permeability 10 rod, it poops out when the length reaches 10 times the diameter. At that length, flux near the rod has little incentive to dip towards the ends and crowd in along with other flux to go through the rod, and will just as well run along side the rod. I have added a factor of center area to end area to this graph which has assumed this factor is always 1.
So, for 33 material, with a permeability at least 800, as long as the overall length is much shorter than 800 times the end diameter times the middle area divided by the end area, most of the available nearby flux that would have passed through a constant diameter rod will take the short cut through the rod (and through your coil), even though it is necked down in the middle.
For example, if you used 1.5 inch beads with half inch holes on each end of a half inch rod, the combination would gather as much flux as a
1.5 inch diameter solid rod as long as the overall length was much less than:
800*1.5*(pi*0.25^2)/(pi*0.75^2)=133 inches, so your 7.5 inch rod extended by, perhaps a couple inches, by the beads, is well within "much shorter than". Gluing two rods end to end would more than double the output because the volume of space that flux is gathered from would more than double. This is because the effective area of flux gathering is bigger than the end areas, and how much bigger depends on the rod length, which is not included in the formula. The formula looks pretty suspect to me (like it is for an air core coil) because there is no reference to rod length. It gives the same result for a zero length rod, unless "effective permeability" does not mean rod permeability, but some combination of rod permeability and length.
Doing it my way should cut your winding resistance and capacitance down by a factor of about 4 (with the thin form) while keeping the intercepted energy a little higher (assuming the beads have an outside diameter 3 times the rod's diameter and extend its length to something like 9.5 inches). To me, that equates to higher Q and higher energy output.
Yes there can be an advantage. I don't remember all the details. Ir gets quite complicated. First of all,at 500 Khz., the principal mode of propagation is the ground wave. I don't remember what the mode is, TM0 etc. The second problem is the ionospheric problems with the D-Layer. The ground wave propagates differently that the sky wave.
Here in Arizona that's the only style I've found effective against all the man-made noise. Mine was 8" diameter of 1/2" copper pipe and used a "fiber-tee" junction to avoid the shorted turn.
See the S.E.D/Schematics page of my website for the circuit I used. Unfortunately I don't have a picture of the antenna (this dates to
1974).
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
The ferrite rod antenna can be very well shielded by a grounded layer of foil that wrapped around the coil, but is slit lengthwise. It also helps to wind a coil that is symmetrical with respect to its ends and connect it to a fully differential amplifier with matched input impedances, so any noise that is common to both ends is rejected. This makes shielding much less important.
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