Without going into the math, however correct or incorrect it may be,
the problem with using the math at all is that the actual velocity
factor and characteristic impedance of a given piece of coax vary from
"nominal" and can easily put your desired notch or peak well off your
intended frequency.
The last bit of coax I set up had a characteristic impedance of 54
ohms and a velocity factor of .693 (instead of 50 ohms and .67).
That's enough to put a math derived dip at 14.225 actually out at
14.713, and likely useless at 20m cw frequencies, if even effective at
20m phone.
The impedance thing is why a grid-dipper (however currently out of
favor and manufacturing) is really the best method for nailing a
section right to the quarter/half wave, etc. You can dip a section to
within a few kHz, which is a practical requirement to get one to work
for both the phone and cw segments on a band. Any real distance from
dead on middle band, and phone or cw end starts losing effectiveness
quickly.
If one wants to get a good dipper, the old Millen jobs, a still
functional workhorse, are available (for less than the analyzers) on
EBay. Or pass one around the local radio club.
73
----- Original Message -----
From: "Dino Darling" <k6rix@arrl.net>
To: "TowerTalk" <towertalk@contesting.com>
Sent: Friday, September 13, 2002 1:17 PM
Subject: Re: [Towertalk] Stub tuning
If you were trying to make a half wave stub, couldn't you just do
this...
11802.8/ (frequency in Megahertz) * (velocity factor of coax) /2.
This would give you an electrical half wave length of coax in inches.
Example: 11802.8 / 14.250 = 828.27 * .85 = 704.03"
^ ^
20 Meters LMR-400
(85%)
704.03 / 2 = 352.02 (Electrical half wave in inches).
Or am I totally off here?
Dino...K6RIX
k6rix@arrl.net
----- Original Message -----
From: "Steve Katz" <stevek@jmr.com>
To: <towertalk@contesting.com>
Sent: Friday, September 13, 2002 9:05 AM
Subject: RE: [Towertalk] Stub tuning
> It will, if you are trying to make a 1/2-wave, open-ended stub.
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