Phil, I though I explained the shortcomings of my "experiment" and am not claiming it was scientific or good engineering practice. Nonetheless, the observed signal strength was greatly reduced as described in my prior post.
The loop I used is not resonant so detuning would not be a factor. It is possible that the capacitance shunted the signal.
If someone would perform a proper experiment I would be interested in the results.
Neil
ETA, I found an article which supports my earlier statement about the shield cannot remove the E field without removing the H field.
Quoting: First, the shield doesn't shield or filter the time-varying electric field. It cannot do so without also removing the magnetic field. Since neither the electric or magnetic field passes through the many skin depths of shield wall thickness, any claim the shield passes one field and limits the other is wrong. The mechanism of field behavior is described in detail in Concentric and Coaxial Transmission lines and skin depth. pages.
The article also discusses other aspects of the shielded loop antenna which should be of interest. Here's the link:
http://www.w8ji.com/magnetic_receiving_loops.htm
Hi Neil,
I'm sorry if I came across as harsh. I was aware that you said your experiments were not scientific. I just didn't want the subject to end there.
Could you describe your loop a bit more. I just assumed you were talking about a loopstick antenna inside a radio. Loopstick antennas are tuned by one section of the tuning capacitor.
In my searches for "shielded loop antennas", I also found the site you mentioned: http://www.w8ji.com/magnetic_receiving_loops.htm. This site is unique in its analysis which is totally different from conventional analysis of shielded loop antennas. So, either he is right and everyone else is wrong, or vice versa. I am mainly going with my first hand experience with that RCA clock radio that had a Faraday shield around the loopstick.
Phil
No problem Phil. I just wanted to make sure that we all understood that my experiment was pretty crude and no definitive conclusions should be drawn.
The loop I used was not a ferrite antenna and I now understand your comments about retuning the receiver front end. The loop is from a Kenwood receiver and was snap mounted to the rear of the Kenwood and a zip cord connected it to the differential input of the receiver. It is plastic encased, rectangular and measures 6 by 3 by 1/2 inches. It does not appear to have any shielding or ferrite material but I can't see inside.
One thing that reassures me in the cited article is the author mentions that the gap is necessary for operation of the antenna because it allows the energy induced on the outside of the shield to couple into the inside of the shield where it induces current in the coil. This allows for the observed operation of these antennas by others while satisfying my concern that the shield blocks both E and H fields.
I am mainly going with my first hand experience with that RCA clock radio that had a Faraday shield around the loopstick.
Were the ends of the rod shielded or open? I have never seen one which was totally enclosed in a shield.
Things are not always as they appear are they?
Neil
I live in a house, but I am near sputtering
power lines and all sorts of other offending
RF noise sources. (Our lots are really close
together.) At one point the RF noise was
so bad I almost gave up listening to the
radio! And the intermittent noise drives you
crazy!
All of the above info you guys have posted
is really good!
For whatever it's worth, I have receiving
phasing units for the AM BCB and for the
SW range. They were expensive but worth
it for getting rid of noise. However, the
shortwave unit has failed and I haven't had
time to investigate the problem. I am on
80 and 40 meter QRP cw and have been
using a DSP speaker, which also has been
somewhat helpful.
But if I can build something like you guys
have described, that is really good too.
On a humorous note, I had sort of a reverse
situation. I was running about 10 watts on
80 meter cw. This caused a touch lamp in
our living room to turn on and off. Then one
day the lamp just stopped working.
Anyway, thank you for the info! Now I don't
feel so alone, because I really am in a bad
RF receiving area!
Best Wishes,
Bruce, MICRO1690/1700
Neil,
Quote: "Were the ends of the rod shielded or open? I have never seen one which was totally enclosed in a shield."
I wanted to reply before this thread goes into oblivion. I don't honestly remember if the shield over the loopstick included the ends of the loopstick. I do know there was a gap, maybe 1/32in visible along the entire length of the aluminum shield. That gap would prevent the loopstick from being completely encased in the shield whether the ends were enclosed or not.
I really wish I still had that radio. It did perform way better than any other loop- antenna radio I ever used in the basement in the presence of so much electrical noise and the weak signals.
I think a thorough evaluation of a SHIELDED loop antenna would have to be made very carefully. Firstly, the loop antenna you describe attached to your component receiver is already a very good, directional antenna. Loop antennas are, by their nature, excellent noise and interference rejectors. In fact, they are much more common than wire or whip antennas for AM reception. The trick is to evaluate the electrical noise rejection of a SHIELDED loop vs. a non-shielded loop. I haven't found anything on the web that provides that information.
So anyway, all I can say is that the weird radio I had a long time ago seemed magical in its noise rejection. It was just a tiny clock radio that is long gone. I wish I still had it! I have searched the web for info, but it doesn't show. It was a vintage 70s or 80s, seemingly run of the mill small LED clock radio. I can only assume that the shield over t he loopstick has long been deemed an unnecessary manufacturing expense.
Phil
Phil,
Your description of the shielded rod makes sense in that the gap would prevent the shield from forming a shorted turn which would effectively short the coil on the rod. Also, if the ends were open then the magnetic field passing in a direction perpendicular to a plane parallel to the rod would couple magnetically through the rod inducing a voltage in the coil.
As I mentioned, I have not seen one as you described. Commonly there is a foil bonded paper slipped between the rod and the radio circuit board or digital display. It appears this is to shield the rod from the radio itself rather than external interference.
I understand your regret parting with that radio. There are some thing I gave away or trashed which I wish I had back. On the other hand I am a bit of a pack rat so this regret is minimal.
Neil