On 10/29/2014 7:45 AM, Wimpie wrote:
El 28-10-14 21:33, rickman escribió:
I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked the idea of a large shielded loop made of coax tuned with a
capacitor. My goal is to get as large a signal as possible from the
antenna and matching circuit to allow the use of a receiver with very
low sensitivity... in fact an all digital receiver.

I spent some time simulating antennas in spice and was able to get a
bit of a feel for the circuit, but I'm not convinced it would work the
way I want. Just before I set the project aside I was told I needed to
model the radiation resistance. That has the potential of wrecking the
Q of the circuit. I am counting on the high Q to boost the output
voltage. If the radiation resistance is at all appreciable I would
lose the high Q and need to start over.

Anyone have an idea of how to estimate the radiation resistance of a
tuned, shielded loop antenna?

The other factor I don't understand how to factor in is the
distributed capacitance of the coax. Is that a significant influence
on an antenna or is it in the noise compared to the tuning capacitor.
The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of
this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center
conductor, or would the resistance be a round trip measurement of both
inner conductor and shield? I assume the shield has a much lower
resistance than the inner conductor but I don't know that for sure.


To get some idea of the output voltage of a loop you need to know:

The fieldstrength of the desired signal at your area. This is probably
given in V/m (dBuV/m, etc). As a first guess use E/H = 377 Ohms to
convert this to H-field [A/m].

EMF = n*A*u0*w*H gives you the EMF for a loop with area A and n number
of turns, w = radian frequency, u0 = magn. permeability for air.

This is new to me. I guess I have been mistakenly using the E field
formula. The field strength at optimum times is estimated at 100 uV/m
at my location which is at the weak end of the CONUS map. I will plug
the numbers into your H field version of the equation.


The EMF is boosted with the Q-factor of your tuned loop. Guessing the Q
is the difficult part. You can't just use resistive loss (even when
corrected for skin effect). As you have a multi-turn loop there is an
eddy current loss due to proximity of the turns (the so-called proximity
loss). At these frequencies loss due to radiation is negligible, unless
you make very large coils.

I have not seen the proximity effect taken into account in any
calculations for similar antenna, so I assumed it was also not
appreciable at this frequency. I'm not at all sure about the radiation
resistance. I will be plugging the numbers into the equation I have. I
assume this resistance would be in parallel with the inductor so a high
value is better. Or would it appear in series with the inductor and a
low value is better?


Practically spoken you can't model the proximity loss in spice. In my
opinion you should measure the Q of your loop, or do some search on
Q-factor of VLF/MF coils for your coil geometry. That result you can put
into spice together with the induced EMF.

I'm surprised you feel the Q can't be calculated. When originally
digging into this I found that the calculation of inductance is an
amazingly complex thing. There are lots of equations out there each of
which simplifies some aspect of the phenomenon and have different
applications. I would not expect the proximity effect to be any more
complex.


At these frequencies, external (induced) noise is the dominant factor,
think of man made noise. Only the resistive loss part of the capacitor
generates thermal noise. Using a coaxial cable as tuning capacitance
will not give the highest Q as you have a long/thin conductor. A
parallel plate capacitor has less resistive loss.

Q is important, but not the only factor. The coax was chosen to be
inexpensive and easy to work with. RG-6 with an 18 ga solid center
conductor is just slightly bigger than the skin effect and so is about
as usefully large a conductor without it being hollow. So I'm not sure
what might be better. I suppose Litz wire could improve the Q, but I'm
already looking at a Q of ball park 100 or more. Once you get a very
high Q it become hard to use the device without ruining the Q.


Are you able to use good quality RG58? As far as I know RG6 for consumer
CATV has low copper content and may have a CCS center conductor.

I picked an RG-6 with a solid center conductor. The specified
resistance is 6.5 mohm per foot. Funny, I'm sure most RG-6 is used for
cable TV where the center conductor is steel for strength with copper
plating for conductivity at high frequencies. One vendor argued with me
that solid copper cores were not available in RG-6. lol

BTW, I measured the resistance of my 50 foot of cable and it is in the
right ball park for 6.5 mohm/foot. The shield measured in the same
range as well. I thought the shield might have had a lower resistance
because it would amount to a larger cross section, but I guess not. I
don't think the shield resistance factors into the Q, but I'm not
certain of that.

--

Rick