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To All from PA0SE
This is an addition to my report of Sunday
April 16, 22.13, here repeated.
The shape of a vertical aerial has a lot of influence on
the reactive part of the impedance at the feedpoint but the real ("Ohmic") part
only depends on the height of the aerial and hardly on its shape. This makes it
possible to find the radiation resistance of the ex-Decca mast at
Puckeridge by computer modelling, only using its height of 325
feet.
We find Rrad = 0.641 Ohms on 137 kHz and Rrad = 0.175
Ohms on 72 kHz.
The aerial current was given as 9A.
This results in a radiated power of:
Prad = 9 * 9 * 0.641 = 52W on 137
kHz
Prad = 9 * 9 * 0.175 = 14.2W on 72
kHz
This is within a few dB of the values found from
field strength measurement by Geri, DK8KW and myself, assuming a radio path over
perfect ground.
In the modelling no top loading was assumed.
>From the description of the mast it seems some form of top loading is actually
present: wires extending from the top of the mast down to the surface. When
these are connected to the top of the mast that seems to be quite wrong as
the current flowing via the wires is in opposite direction to the current in the
mast, counteracting the field radiated by the mast.
But the boys at RACAL-DECCA certainly must have
known what they were doing. It would be interesting if we could obtain some
more info on this matter.
The discrepancy between radiated power found by
calculation and from field strength measurement is much smaller than in the
case of amateur installations, as reported by SM6PXJ, DK8KW,
M0BMU, PA0SE ans possibly others.
The same is the case for the measurements by Gamal
and Geri of the field of DCF139.
This confirms the earlier expressed opinion that
in amateur stations quite a bit of power is lost by radiation into the
ground and absorption in surrounding objects.
In commercial installations the situation - extensive
earth mat, unobstructed view - is much more like the ideal
one used for the CCIR curves: power radiated from a short
vertical aerial over perfect ground.
73, Dick, PA0SE
----- Oorspronkelijk bericht -----
Verzonden: zondag 16 april 2000 22:13
Onderwerp: Operation from ex-Decca station at Puckeridge
To All from PA0SE
I only could monitor transmissions from the
ex-Decca station today, Sunday April 16.
The result is:
Time (UTC) Call Freq ERP
Iant S-report Fieldstrength
1000
G3WSC/P 136kHz 50W 9A S9 +
20dB 171 µV/m
1446 G3GRO/P 73kHz
? 9A S9 +
7dB 70 µV/m
The distance between Puckeridge and PA0SE is 317km, for
the larger part over sea.
The CCIR curves show that for this distance at 136 kHz
attenuation over sea water and "land" hardly differs from a path over perfect
ground. Assuming perfect ground we can apply the simple rule that 1kW
transmitted from a small aerial produces a field strength of 300 mV/m at 1 km
distance and this decreases in a linear way with distance. Working
backwards from the field strength measured we find at Puckeridge 45W was
fed to the aerial on 136kHz and 7.5W on 73kHz.
This is EIRP and not ERP of course.
In order to enable the station receiver to be
used as FS meter I measured the FS of DCF139 (138.83kHz) and DCF77
(77.5kHz) at two locations in open field, using a home made one turn
screened loop of 1 m^2 area and a Wandel & Goltermann SPM-12 selective level
meter, fed from the car battery.
I could have saved myself the trouble because the values
measured at the two locations and the ones in my shack were exactly the
same:
DCF139: E = 1.93 mV/m
DCF77: E = 2.85 mV/m
It shows that the magnetic component of the field is not
noticeable influenced by wiring, central heating system etc. in the
house.
When using my home made portable FS meter, described on
G3YXM's website, I had noticed that moving around outside over a small
distance sometimes made considerable difference in FS measured. I attribute this
to underground cables, pipelines etc. These extend over a great distance, pick
up the signal as a long aerial and re-radiate it, interfering with the primary
field. (On LF the field penetrates over tens of metres into the ground). The
dimensions of objects inside the house apparantly are too small to show
this effect to a measurable extent. (Inside an appartment block with concrete
walls it may be different.)
When I measured the field of DCF139 and DCF77 I selected
locations where no such visible and invisible field-disturbing objects
were present.
I sincerely thank all those who took part in organising
and performing this interesting experiment and in particular Walter
Blanchard who was instrumental in making it possible.
73, Dick, PA0SE
JO22GD
D.W. Rollema
V.d. Marckstraat 5
2352 RA Leiderdorp
The Netherlands
Tel +31 71 589 27 34
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