Hello Tony,
measuring voltages on untuned antennas is "tricky", in particular with small electrical antennas (compared to the wavelength) as these tend to have large reactive components.
On 60kHz the L-antenna you described has a reactive component of about 10000 Ohm while the resistive part will be some 10's to some 100's Ohm (mostly loss resistance). So properly tuning the antenna will increase the RX voltage
by several S-points.
Ground loop antennas on the other hand seem more broadband.
Maybe that explains why they perform better at lower frequencies ( compared to the untuned L-antenna).
Anyway, your L-antenna should perform well on 500kHz.
73, Rik ON7YD
Hi Roger.
The soil here is well drained peaty topsoil about 400mm - 500mm deep on a mixture of slate and shale and although I am 500m from the sea, I am 75m above the water.
I have no idea what the electrical conductivity is but I imagine it's probably lower in the winter when my windows get a covering of salt during storms. It would be interesting to pick on one reliable ground wave transmission and monitor it through various
weather conditions. It would also be interesting to see how it works lower in frequency (sub 50 KHz), which is something that I will definitely look into.
73,
Tony, EI8JK
On 11/08/2010 11:32, Roger Lapthorn wrote:
Thanks for this Tony.
Do you know what sort of soil/rock you have beneath you there? Here I am on relatively low conductivity chalk/clunch with clay a few miles to the north west under fenland peat.
If the earth electrode antenna is behaving as a loop (a debated theory) then it is most effective is the "loop in the ground" is as large as possible, which would be the case with low conductivity soil/rocks underneath: the return path between electrodes would
be forced to take a longer route deeper into the ground. If the soil between the electrodes has good conductivity then the return current would flow directly making the effective loop size small.
In the last few days we've had a lot of rain here and the results on 500kHz last night with the earth electrode antenna suggest the rain made little difference to performance with reception several times by PA0A. This is counter-intuitive to me, as I would
have expected levels to be weaker if the soil was wet (loop formed being smaller etc.). Of course it could have been that the contact resistance of the earth probes was lower and overall the two effects cancelled?
Whatever the theory says, the earth electrode "antenna" has some mileage especially when, like me, there is little space for large "in the air" antennas. Sure, a big vertical or large loop in the air would be better (I think), but this is about experimenting
and discovering the limits of possibilities.
Good luck and keep everyone posted if you do further tests.
73s
Roger G3XBM
On 11 August 2010 10:34, Tony
<[email protected]> wrote:
I have finally found the time to get some (radio) work done here and got my 2nd tower finished and I erected an inverted L, 10m vertical and 30m top rising to 15m at the far end. I still have the "earth antenna" which is just a length of wire laying on the
ground 80m long and terminated directly to an earth stake and laying roughly in the same direction as the top wire of the L .
Comparing the two gave some very interesting results.
10 MHz CW L = S7 earth = S1
7 MHz CW L = S9 earth = S3
R. Bristol 1566 KHz L = 0 earth = S2
Donebach 153 KHz L = S6 earth = S8
DCF77 77.5 KHz L = S3 earth = S5
MSF 60 KHz L = S4 earth = S8
All very non-technical I know, neither antenna was matched or tuned in any way and was all done about 13:00z.
There was no noticeable difference in the noise level but when I tried it before the earth antenna was very much quieter after dark. I will try and repeat this tonight and see what the difference is then.
Tony, EI8JK.
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