Re: NEC-LIST: CFA in short

Chuck,

In a recent e-mail you wrote:

>Paulmoody_at_onaustralia.com.au wrote:
>><< I operate a 6 foot loop antenna on 3.5 MHz. This antenna is well
>> below the 'normal' size of antennas for this band. Is this loop a
>> dipole ?>>
>
>to which Chip <Fractenna_at_aol.com> replied:
>> Yes. It is a form of folded dipole, with length far less than a
>> wavelength. Not all 'dipoles' are 1/2 wave in length
>> electrically. I understand the confusion in the term and am happy to
>> be further enlightened by others here.
>
>A small loop may be regarded as a magnetic dipole but differs strongly
>from a so-called "folded dipole" in that current flows in the same
>direction (e.g., clockwise) all the way around a small loop, whereas
>in a folded dipole there are current nodes at both ends, and in
>between the current flow in the parallel conductors is in the parallel
>sense, e.g., left to right in both wires, meaning clockwise in one
>wire and counterclockwise in the other.
>
>-Chuck W1HIS

Be careful Chuck, you are commenting on a subject that needs careful
attention to what you are saying.

I am (still) involved in a debate, you might recall my earlier
correspondence to Jerry Burke on this subject (reference NEC-List memo
d. 18 June 98), prompted by an attention grabbing title to a paper by
Prof. Mike Underhill, G3LHZ and M.J. Blewett, G4VRN, entitled:

"Magnetic Loop or Small Folded Dipole?" [reference IEE Seventh
International Conference on HF Radio Systems and Techniques,
Conference Publication No. 441, pp. 216-225, 7-10 July, 1997].

This controversy is on-going around-and-around, in the open
literature, in Pat Hawker, G3VA's column Technical Topics, Radio
Communications. I think a small loop is a loop, G3LHZ seems to think
that even for small loops the dipole mode dominates. Neither of us
seem likely to change our minds.

In fact an awful lot of hot air has been expended on the subject.
Engineering sense would indicate that if the loop is small enough it
will behave like a loop, but if its circumference approaches a
wavelength it will behave like a folded dipole (but depending on how
you feed it). At intermediate frequencies the current will be a
combination of both modes. Alan Boswell, G3NOQ has used NEC to
examine this issue.

I put brackets around "depending on how you feed it". Large (full
wavelength loops) are usually quad or delta shaped, and it makes a big
difference for a vertical loop if you feed it at one corner, lower or
top corner, or the centre of one side --- and again which side (a
vertical or horizontal side).

For small loops (the topic of current discussion) I could say also it
depends on "how you tune it". Electrically samll loops are tuned by a
capacitor, but what is not generally known, is that the current
distribution, and so the pattern for vertical loops close to the
ground depends on whether this tuning capacitor is located at the feed
point, or on the side remote from the feed point. The patterns
(according to NEC-4D) can be quite different, even for rather small
loops.

Jerry Burke does not trust results for small loops near the ground.
He states that as the loop becomes electrically small the NEC
impedance matrix becomes increasingly ill-defined. This (Jerry
states) can be a problem without a ground, but with the limited
accuracy of the table-lookup used for the field due to ground, the
error can easily overwhelm the solution.

But I think Jerry is pessimistic. I have measured the field strength
for a loop whose perimeter is 0.07-wavelength, which agrees with that
calculated to within a few percent (easily accounted for by coupling
loss --- the loop has to be matched).

And, noting the difference between the unloaded Q-factor of small
loops (the Q-factor NEC predicts) and the observed loaded Q-factor
(the high Q-loop is driven and loaded by the source impedance of the
transmitter), we find this difference in accord with my experience
with electrically small (VLF) monopole antennas --- at least for loops
as small as 0.035-wavelengths --- and perhaps smaller. The smallest
loop we have used has a diameter of 80 cm, for a frequency having a
wavelength of 80 metres (perimeter 0.03-wavelength).

I could go on, since loops are a very interesting antenna to model.
But this is a subject for another day. Today it is CFAs.

73, Jack, VE2CV

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Tue Mar 02 1999 - 18:51:32 EST