Saturday, 31 January 2015

G7FEK antenna SWR measurements & LDG RT-100

I noticed that the LDG RT-100 is struggling to tune some bands on the G7FEK.
The antenna is a nested Marconi with two wires tuned for 80 & 40m, with two elevated counterpoises.
G7FEK aerial


80,40,17,15m are ok and the RT-100 re-tunes from memory in fractions of a second.
30,20,12,10m the RT-100 has to perform a full tune to find a match (5-6s, sometimes longer), even when it found a tune previously. I guess this is because the loads are a the limit of what it is capable of. The wind moving the vertical radiators probably does not help either.

I decided to do some measurements today with the VK5JST aerial analyser at the base of the antenna.
At 80m, where the antenna, the measured resistive impedance is 70 Ohm, which I think indicates a poor ground. Radiation resistance should be close 35 Ohm and therefore Rg (ground resistance) is estimated 35 Ohm, giving an approx. efficiency of 50% .

On the higher bands, I don't fully understand why the R is so low. More antenna theory to read & experiments to perform... elevated counterpoises?




F (MHz) R(Ohm) X(Ohm) Impedance(Ohm) SWR
3.600 70 0 70 1.40
7.100 100 55 114 2.74
10.120 22 132 134 18.50
14.050 23 108 110 12.70
18.080 24 55 60 4.88
21.060 29 54 61 4.07
24.890 13 54 56 8.47
28.040 16 38 41 5.05

No comments:

Post a Comment