Dear Mike,
having experimented with LF aerials since summer 1997, when the first two
special LF licences have been issued for Germany, I would sum up my findings as
follows:
The diameter of the wire I do not regard important so far. It should be chosen
to cope with the mechanical drag. I am using normal litz wire for mains wiring
so far, wire diameter about 1 millimeter, total with insulation 2,5 mm (there
are even knotted and soldered connections in my present aerial).
On my estate I have space to erect a T aerial with a total top length of 25
meters, and a downlead of 10 meters. This total 35 meters of wire gave a
capacitance of about 210 pF against ground. Therefore I always count with 6 pF
per meter wire.
To measure the capacitance I have used a tuned resonant circuit, the variable
capacitance of which has been measured with and without the aerial connected. A
simple RF ground or some wires on the ground must be provided to make these
measurements reliable.
To get higher capacity I have built a double T aerial with the same dimensions,
using three bamboo spacers of 1 meter length. The centres of both Ts were
connected by a wire 1 meter long, and the ends of the top were also connected by
1 meter of wire. Each T centre had its own downlead, thus forming a V-shaped
downlead. With a total of 73 meters of wire, capacity was 340 pF or 4,66 pF/m.
This showed that even with 1 meter spacing of the tops the capacity ratio
reduced considerably.
The traction on the two ropes which carried this aerial was considerable,
including the weight of three bamboo spacers. Therefore I tried to simplify the
construction. As seen from above the aerial now resembles the letter H, with the
ends on one side being spaced more than in the centre, therefore resembling
something between letter H and X (or the form of the "Orion" star
configuration). The ends of the tops were not connected any more (also for
mechanical reasons) and the spacing wire between both tops (forming the centre
of the H) had been increased to 1,5 meters. On one end the spacing of the top
ends is also 1,5 meters, on the other end the top ends are spaced perhaps two
meters. The V-shaped downlead has been retained. With a total of 70 meters of
wire the capacity now is 370 pF, giving a ratio of 5,2pF/meter. This is the
optimum which I can realize at present in the given environment.
These experiments show that wide spacing of the top wires is desirable,
especially at the ends, to retain the ratio of 6 pF/meter of the single wire T
aerial as far as possible. The feasibility to do so depends of the realization
of suitable rope holders, of course. On my estate on one end I have several high
trees, the other two ropes are hold by an unused television Band III long yagi
forming the highest point on the roof. Springs have been inserted into the ropes
carried by the trees to decouple the aerial from excessive traction in heavy
winds.
Concerning the connection of the ends of the top wires I would still employ such
a connection if it were mechanically feasible. But this is not the case under my
present conditions.
One detail which I still will change is the V-shaped downlead. Near the ground
the capacity of the aerial to ground should be held at a minimum. Even the EZNEC
programme which DL9KCE tried to simulate my aerial has run wild as long as these
conditions have not been expressed clearly to the programme. Therefore I have
the idea to change the V-shape into a Y-shape soon. This may slightly reduce
total capacity but it should be possible to tune out the difference by the
aerial variometer.
I have no experience with inverted L aerials so far. Sloping down the far end of
an inverted L too much may convert the aerial into a lossy loop, including the
ground. In order to know more about the efficiency of my aerial I have also
measured the resistive impedance under tuning conditions using a home-brew
resistive bridge and a 3 watts PA. At present my ground loss is about 80 ohms,
and I do hope that summer will be sufficiently mild to allow more experiments on
the ground system.
I hope these explanations will give You some ideas to try out on your
own aerial.
73 Ha-Jo, DJ1ZB
QTH JN68GN, 120 km ENE of Munich.
Mike Dennison schrieb:
I have done very many antenna experiments over the past 2-3
years to get a better signal at 73 and 136kHz from a small garden.
I have experimented with inverted-L Marconis. These have mainly
sloped down from the top of the vertical section and I have on two
occasions increased the height of the far end with improved results.
It seems that at LF you cannot get away with drooping the far end
of an inverted-L like you can at HF. This is presumably because
the current distribution is approximately linear for a short Marconi
and sinusoidal for something a quarter-wave or more.
One a similar topic, what influences the capacitance of a multi-wire
top section? Is it simply wire diameter so a thick wire has more
capacitance than a thin wire? What about multiple parallel wires -
these are often suggested to be a metre or so apart - why? If the
distance makes a difference, do I assume that the further apart
they are the better the effect, and if so why is this?
I used to run three top wires about 300mm apart but replaced these
with a single wire of the same diameter of each of the three (for
other reasons) and it seemed to make little difference. Was I doing
something wrong?
Also, if I run three top wires in parallel, should I join them at the far
end? Or perhaps I could join them so they make a single zig-zag
wire down the garden, back again and down again.
The antenna is getting better all the time, but I am sure there's
more progress to be made. Does anyone have some practical
experience to pass on?
Mike Dennison, G3XDV
Publications Manager
* RadCom * Ham Radio Today * RSGB Books *
* GB2RS News * RSGB On-Line Web Site *
Radio Society of Great Britain
Lambda House, Cranborne Road
Potters Bar, Herts UK, EN6 3JE
E-mail: [email protected]
Tel: +44 (0) 1707 659015; Fax: +44 (0) 1707 645105
RSGB - UK AMATEUR RADIO
|
