[Fwd: LF: Loop antennas]
Wed, 17 Jun 1998 15:03:06 -0400
Prof R. Jennison wrote:
> I am a little cocerned that many practical amateurs may be
> discouraged from experimenting as a result of all the theoretical bla-bla
> that has been popping up of late. We really need more experiments. I
> quoted the parameters of my present hook-up in an earlier message just to
> encourage them that loops can be neither difficult nor costly. One is not
> necessarilly trying to achieve perfection and it is fun to experiment. For
> example, I am fully aware that one of my conductors is too thin - but it
> works. One day I shall replace it with a hundred metres of 10mm earhing
> cable which I have just obtained from an electrical wholesalers. It cost
> £32, sadly far more expensive than boot sales but very much cheaper than
> Sainsbury's Home Care where an identical reel is £59.99 !
> I have great respect for Andy, G4JNT, and there had to be a reason for
> his choice of a 2 turn loop for his first transmitting aerial on 73. I had
> not encountered a multiturn loop used in the transmission mode before and
> his seemed to work wonders, so I was intrigued.
> I looked at it theoretically and it appeared that it ought to work,
> but theory is one thing and practice is another. I therefore experimented
> on 73 kHz with a very crude and somewhat lossy loop constructed from 2
> adjacent loops of ordinary house wiring cable (2 single strand conductors
> and an earth wire in each. With this I was able to try loops with one, two
> and four turns. The system was obviously lossy, especially in the 4 turn
> configuration, but it worked. There was not a lot of difference in the
> performance in each case, measured over a standard range of 18 km, but
> there seemed to be slightly better results in the 2 turn system despite the
> resistive loss. All these tests were at constant power input with the
> input tapping point on the antenna or on its chain of capacitors chosen to
> give the best match directly to the anode of the output valve of my
> Many months later, I repeated the one and two turn tests on a
> slightly better loop at 137kHz. I repeatedly requested comparative reports
> from my fellow hams, whose numbers had considerably increased and who were
> at much greater distances than in the 73kHz test.
> The reports were in favour of the two turn loop.
> Recently the LF net has been inundated with all sorts of theories, some
> quite good but some highly questionable. Most have concentrated on
> radiation resistance and skin depth but no one has yet suggested using
> superconducting loops, no doubt that will come! It is ammusing to apply it
> to the case of slicing the loop into lots of thinner loops.
> All this must be getting very confusing to the less mathematical hams.
> For their sake, may I just mention that radiation resistance is an
> engineering systems parameter which depends upon the geometry of the aerial
> and is not fundamental to the region of space occupied by an aerial, this
> is well demonstrated by a folded dipole which has 4 times the radiation
> resistance of a simple resonant dipole. Ultimately the criterion is to
> match the system into the characteristic impedance of free space which is
> an absolute physical parameter. It may be argued that, by tapping along the
> capacitor which tunes the loop, and thereby forms an integral part of the
> aerial, one may have any radiation resistance that one likes, insofar as
> that is the impedance that is presented at the output terminals of the
> aerial in that particular configuration. However one will, of course,
> transform the other resistive losses at the same time.
> The high powered theoretical discussions on the LF net do not seem
> to fit the observed facts, and some, I dare to whisper, are not quite
> right. In all of the discussions, one parameter that has been completely
> ignored is the loss of energy in the local induction field. On 80 metres
> or top band this is not a problem but at 73kHz the induction field covers
> about 50 square kilometers and if one operates in a built up area there are
> countless steel framed buildings, underground sevices and the like which
> can gobble up some of the power and put a sizeable unknown into the
> equations - and I am refering to the H field, not the cursed E field. One
> may recall that a family near the Hilversum transmitter ran all the
> household electric appliances from a loop in the attic for many years
> before they were ultimately nobbled!
> I'll stick to my guns. The proof of the proverbial pudding is in the
> eating and on my lousy rig two turns are better than one, or perhaps I
> should say that one good turn deserves another!
> Roger, G2AJV.