Jim, to further put the numbers in perspective, using the 10 to 1 example,
the impedance could be between 5 ohms and 500 ohms (50/10 - 50 X 10). The
nominal 30 dB notch with .05 ohm short would have attenuation from 20 dB to
One would be smart to make sure the notch is placed at one of the 500 ohm
points along the cable.
Wonder how you go about determining where that is? Could you assume the
plate load capacitor is a point of minimum impedance and go from there?
If so, the high impedance points would occur every odd quarter wavelength
(at the suppression frequency) along the cable, right?
In a message dated 3/30/2010 5:43:29 P.M. Central Daylight Time,
On Tue, 30 Mar 2010 18:10:41 EDT, TexasRF@aol.com wrote:
>This is a very simplistic analysis as no adjustments were made for the
>mismatch losses incurred in the example.
Your analysis and description is excellent as far as it goes. But let
me take it a bit further (and closer to reality).
The other part of the problem is, "what is the impedance on the
transmission line at the point where the stub is connected, AT THE
FREQUENCY THE STUB IS SUPPRESSING?" That is NOT an easy question to
answer, but it has a LOT to do with how effective that short circuit
Let's take the example of a line that has a mismatch at the antenna of
perhaps 10:1 at the harmonic, and a mismatch at the transmitter of
perhaps 20:1. These are VERY realistic assumptions -- both the
transmitter and the antenna provide a fairly low SWR at the operating
frequency, but almost never do so at the harmonic (unless the antenna
happens to be something like a log periodic).
At various points along that line, the impedance varies from a few ohms
to several hundred ohms, and at all points other than the peak and
null, has both resistive and reactive components. Without knowing the
impedance at both ends and the electrical length of the line, there's
no way to know where the peaks and dips in the impedance fall. If you
happen to pick a point on the line for your stub where the impedance is
low, it does almost nothing. If you happen to pick a point on the line
where the impedance is high, it does a LOT. And anywhere in between,
the results vary from not very much to a lot. :)
Jim Brown K9YC
TowerTalk mailing list
TowerTalk mailing list