On Mon, 4 Aug 2003 15:52:47 -0000 "Paul Playford"
<w8aef@worldnet.att.net> writes:
> Gee, I'm not alone in receiving less-than-stellar performance from
> stubs. I
> have made several sets of stubs using various methods of tuning them
> and
> they just don't work at my QTH. But I took one of my stubs to a
> friends QTH
> and he was able to show me that my stub did indeed give 20 dB of
> suppression
> on the second harmonic - just like it is supposed to. And he loaned
> me one
> that he had made and used successfully at his location and it
> doesn't work
> for me either. I run monobanders and 4 square phased arrays and he
> runs a
> TH-11 and verticals.
Let's think about this for a minute.
Let's assume you are transmitting on 20 meters into a 20 meter
monobander.
A small amount of harmonic RF is also being transmitted on 28 Mhz
into the 20 meter monobander.
What is the swr of a 20 meter monobander on 10 meters? Answer: sky high.
Since the swr is sky high, there are 28 mhz voltage and current standing
waves on the 20 meter coax. You want to place your 10 meter stub at
one of the voltage maxima to achieve maximum effect. At the voltage
maximum, the impedance is highest, and your low impedance stub will
have the most effect.
There is a good chance that you happened to place your stub near
a voltage minimum, since you chose the stub location at random.
I would suggest moving the stub 1/4 wavelength (10 meter wavelength
in coax, considering velocity factor) one way or the other. This is
most easily done by adding a short coax jumper, or moving the stub to
some other point in the chain of indoor accessories.
Now, your neighbor, who uses tribanders, doesn't have this problem.
Since his 20 meter antenna also has a flat swr on 10 meters, it
doesn't really matter where he puts the stub, since the 10M voltage,
current, and impedance are constant along the line.
Move your stub about 1/4 wavelength and let us know if it works
better.
Dave Hachadorian, K6LL
Big Bear Lake, CA
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