Dear LF Group,
I have now added a further 130 or so points to my spreadsheet of
field strength measurements. The original measurements were for
an 8m high inverted L on 136kHz; for the new results I repeated the
136kHz experiments with the antenna in 17.5m high inverted V
configuration, and also on 71.62kHz using both configurations of
antenna. The top wire length was 40m in both cases.
As before, plotting ERP vs. distance shows that at distances
greater than 1km, and up to the maximum distance measured
(about 6km) the ERP calculated from the field strength is fairly
constant for each antenna and frequency. At distances below 1km,
there is a rapidly rising trend in ERP as the distance decreases,
indicating that near field effects are dominating the field strength.
Again as before, there were no obvious directional effects when
ERP was plotted against bearing from the transmitter.
The same antenna current of 2A was used in all experiments; the
average ERPs obtained are given below. I also used EZNEC to
calculate the radiation resistance, and so, using the measured
antenna current, calculate the expected ERP. I have also included
rough measurements of the loss resistance for each antenna
(excluding the loading coil):
8m inv L, 136kHz
Rrad = 0.0198ohm
ERP (calculated) = -8.5dBW
ERP(from FS measurements) = -11.2dBW
difference (measured - calculated) = -2.7dB
17.5m inv V, 136kHz
Rrad = 0.0464ohm
ERP (calculated) = --4.8dBW
ERP(from FS measurements) = -5.8dBW
difference (measured - calculated) = -1.0dB
Rloss = 27ohms
8m inv L, 71.62kHz
Rrad = 0.0058ohm
ERP (calculated) = -13.8dBW
ERP(from FS measurements) = -17.8dBW
difference (measured - calculated) = -4.0dB
17.5m inv V, 71.62kHz
Rrad = 0.0129ohm
ERP (calculated) = -10.3dBW
ERP(from FS measurements) = -12.5dBW
difference (measured - calculated) = -2.2dB
Rloss = 47ohms
Some conclusions can be drawn about the "missing decibels" (ie.
the difference between measured and calculated ERPs). The fact
that the measured ERP is constant at distances greater than 1km
indicates that the decibels must already be missing by this
distance, ie. the losses occur within the near field of the antenna.
There is less difference between calculated and measured ERP for
the higher inverted V antenna than for the inverted L. This is
consistent with the intuitive idea that the missing power is
absorbed by objects obstructing the antenna - the higher antenna
benefits because there are less obstructions for the signal as it
propagates into the surrounding space.
An interesting thing is that, with a vertical transmitting antenna
tuned against ground (ie. an "electric" antenna) and a ferrite rod
receiving antenna (ie, H field sensing), the apparent field strength
rises sharply at short distances. The usual statement seems to be
that in the near field of the electric antenna, the H field is
proportionally smaller and the E field larger than in the far field - ie
E/H is greater than 120pi ohms.
It would now be interesting to try a similar series of measurements
with a transmitter in an urban QTH, since the "missing decibels"
losses seem to be much larger in these conditions.
If anyone would like to look at the data I have collected so far, I
have put it all together on on one spreadsheet, which I can E-mail
in Microsoft Excel, plain text, and various other formats.
Cheers, Jim Moritz
73 de M0BMU