Corn, you should add that a tuner is needed
even if the antenna is resonant. The antenna
resonant impedance may not be 50 Ohms.
My 362 feet of wire is resonant every 0.633 MHz.
Every other resonance is useless because it is
too high impedance to match. I can only use the
low impedance resonances. So the useful resonances
are about every 1.27 MHz. The low impedance resonances
on my antenna are about 200 Ohms. I added a 4:1
transformer to lower the impedance to near 50 Ohms.
This makes it easier to use 50 Ohm coax. If I had
an open wire line the transformer would not be
necessary. Part of the tuner is the transformer.
Transformers are much better ways to change
impedance than reactive networks. The Matchbox
is a transformer and that is the reason it is
lower loss than any reactive network can be
if the impedance you are matching is 100 ohms
and larger. A dipole is going to have twice
the impedance of my wire. The first resonance
of a dipole is near 72 Ohms in free space.
The first resonance of my antenna is 0.68 MHz.
This is a problem since it is in the AM broadcast
band. I need a high pass filter to keep AM stations
from overloading my transceiver. The third resonance
is actually 1.9MHz and at a impedance of 200 Ohms.
That is good for 160M. The fifth resonance is 3.17 MHz.
I added a 270 pF doorknob capititor in series and the
resonance is 3.5 MHz and 50 OHms. The 11th resonance
is 7 MHz and 800 Ohms. The 16th resonance is 10.1 MHz.
As the frequency gets higher the impedance is lower on
the even resonances. The vswr is about 2.8:1 on the high
side. The 22nd is 13.9 at 200 Ohms. The 29th is 18.4
at 200 Ohms. The 33th is 20.9 at 200 Ohms. 39th is 24.7
at a impedance lower than 200 Ohms so the VSWR is about
2:1. The 45th is 28.5 and the VSWR is 2.8 at 28.
Regards, Jim FitzSimons W7ANF
Sherrill WATKINS wrote:
>
> Denny: Your comment is incorrect.
When the tuner is used, the system is resonant.
It is an insignificant fact that the antenna or
transmission lines by themselves are not resonant
because the system is resonant. - 73's- Corn -k4own.
--
FAQ on WWW: http://www.contesting.com/FAQ/tentec
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
|