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[Towertalk] Stub tuning

To: <>
Subject: [Towertalk] Stub tuning
From: (Guy Olinger, K2AV)
Date: Fri, 13 Sep 2002 13:43:28 -0400
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

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.


----- Original Message -----
From: "Dino Darling" <>
To: "TowerTalk" <>
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

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
704.03 / 2 = 352.02 (Electrical half wave in inches).

Or am I totally off here?


----- Original Message -----
From: "Steve Katz" <>
To: <>
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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