Hi Joe,
Am 09.01.2014 17:07, schrieb Joe:
Strong wind from the west or southwest
squeeze the 3 wires of the Top-C and let them jump or dance :o) wich
causes a change in the antenna impedance. Z is jumping around 20 – 80
Ohms.
But it is the magnitude of Z, actually the imaginary part, or the
reactance, or the C :-) You could build an automatic variometer which
compensates this changing C.
My antenna is also changing its reactive component but it is no
problem.
Ever thought about building an automatic variometer?
Another point ist he earth conductivity which
change extremely when it geting wet.
I tuned my antenna last night before starting my
transmission. I allways do this with an miniVNA, which works perfect.
It was tuned to 137.600kHz, this morning after
the rain, it was tuned to 144,900kHz! Due to the small BW it makes no
sense to transmit under such conditions.
Yes, but if the resonance changes, it does not mean that the earth
conductivity changed extremely.
Let's assume your earth loss is 60 Ohm, then it may change to 50 or 70
Ohm but not to 10 or 200 Ohm...
You can see the changing earth losses by this test:
Dry: P=100W, coil losses = 20 Ohm, antena resonated to 137 kHz. I= 1A
=> Earth losses ~ 80 Ohm.
Wet: (retune the antenna to 137 kHz by reducing the inductivity) P=90W,
coil losses still 20 Ohm, now 0.9A => Earth losses ~ 90 Ohm.
Oh, this leads me to the question how you're matching your antenna to
the transmitter. Is it by a variometer and a ferrite transformer in
series to the cold end of the coil, like i do, or is it maybe with a LC
network or a tapped coil or so?
I assume that your overall resistance (the R of that Z=R+jX) does vary
by less than 20% which would be OK for the PA, as long as the X
component is permanently compensated.
73, Stefan/DK7FC
|
|
