Topband: INVL FSM 160m

[cris blak]

Hello all,

Yesterday I made a FSM (field strength measurement) on my INVL antenna with different configuration.
The setup were TS450 as a transmitter with 10W (efective output power on CW VSWR = 1.2:1 and 20m RG213) into antenna (INVL for 160m). Another TS450 with calibrated S-meter using an A/D interface to the PC and a loop antenna with 0.7m its side made by 0.5mm copper wire at 3m over the ground connected with 3m of RG58 CoAx as a calibrated mobile receiver.
I chose several point 1, 2 and 3.5Km away from the antenna on a flat terrain having line of site with the INVL antenna.

The INVL antenna have 15m vertical part and about 26m horizontally toward West and were feed by a 1:2 toroidal transformer.

First I connect only 2 radials about 15m long each, pointing one to the west and the other to the east. VSWR were about 1.3:1
The received signal at 2Km west = -75dBm and 2Km east = -68dBm;

With 6 radials on the ground (ranging 15 to 20m each) and 18 radials about 3m above the ground, the received signal on the loop were almost the same (+/-1dB)! This is strange to me but is a fact.
As a fact, the 18 suspended radials were facing west , 3 ground radians also facing west, and the other 3 ground radials is pointing to east, north and south.

The last test were with the above 24 radials in place and another 6 by 15m long each spread equally between east and south direction.

Here is its what I achieve with this last configuration:
2Km west = -77dBm
2Km east = -77dBm

As a conclusion, there were no significant difference on signal strength between 2 and 24 radials but there is 7dB between west and east direction.
Adding another 6 radials on the ground as described above, the west east difference disappeared but the signal strength drop by 2 to 5dB as before.

What I don't understand is why is the 2-5dB drop in the signal strength adding the last 6 radials instead of increasing it? Is there some relation with the radiation angle?
Forgot to mention that the test were made at noon local time to minimize the sky wave interference.

I will appreciate any comments from the theory or your experience.

Thanks.

73 de YO3FFF
Cristi