C Andersson wrote:
Hello LF group,
I just want to share my last findings in the chase of "the missing dB's".
Yesterday I changed the antenna ground plane and added 10x6 m chicken wire
around the antenna base plus 12 radials 20-25 m each. The chicken wire was
solderable with a 300W soldering iron.
The feed impedance went down from 25 to 22 ohm. Not very much difference.
If you (re)match for the same applied power, the gain by having lower
loop resistance is 20log(R1/R2). Your data indicates a gain of +0.7 dB
from resistance ratio.
I have not used radials before, just a 8 m steel pipe + a various other ground
connections in parallel.
Today's measurements however show a noticeable improvement of 1,6-2 dB (with
the same antenna current).
So the actual improvement is more than resistance ratio. For the same
applied power, you should also get more current still by rematching to
22 ohms. So there is an additional gain (the 0.7 dB from resistance
decrement ratio?)
Now, after all, the estimated ERP seems to correspond with measured values.
Interesting that the ground plane in some way improves the far field signal.
Perhaps an explanation is in relation to CURRENT DENSITY near the base
of the antenna. Lowering the loss to a surface sheet of current around
the base of the vertical is where "copper plating" can be a rewarding
improvement?
73, Bob ZL2CA
| 