Maybe try posting in rec.radio.amateur.antenna. It's been a few years since
I peeked in there, but there was a whole lot of knowledge and expertise
underlying the flammage.

"Bob Agnew" wrote in message
news:NTy6f.90657$lq6.40604@fed1read01...
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.



"John Popelish" wrote in message
...
Bob Agnew wrote:
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.

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.