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In "The DBJ-1: A VHF-UHF Dual-Band J-Pole" (Feb 2003, pp 38-40), the length of the RG-174 matching stub should be shortened a bit to get the antenna closer to a 1:1 SWR. Using the formula for line length versus frequency and wavelength,
L = (VF×984×N) / f
where
L = length (in feet),
VF = velocity factor,
N = number of wavelengths and
f = frequency in MHz
The velocity factor of RG-174 (Belden 8216) is 0.66 (Belden 7805 cable is 0.74) and N = 0.25 for a 1/4 wavelength stub. Using the above equation, the matching section becomes 4.33 inches, rather than the 5.25 inches shown. Thanks to Bruce, W3NJ, for pointing it out. He's using a stub length of 4.5 inches and getting 1:1 with that length. As was pointed out in the article, however, you should use an SWR analyzer or bridge for final tuning.
"The Dangers of Cathode Keying" (Nov 2003, pp 28-30) states that the cathode of the keyed stage floats up to plate potential. Actually, the open-circuit cathode potential will be limited by the grid-cutoff voltage, which in most 1950s-era tube transmitters is about 100 V dc. Thanks to P. Bertini, K1ZJH, for pointing it out. This will, of course, be the open-circuit key voltage. It doesn't change the argument for safety, however. That key-line is a good current source, as the full final plate current passes through the key (and more, if the oscillator is also keyed). It's possible, therefore, to draw 50-100 mA by touching the key contacts with moist skin. The bottom line? Use a keying relay if you can and be careful around the key when you're using a vintage cathode-keyed tube rig.
The Feedback for "The Doctor is IN" (Oct 2003, p 106) should read "...the formula for the LC constant should be used with L in µH and C in pF (not H and F) in addition to f in MHz." That derivation is straightforward if one accounts for the 10--6 term for µH, the 10--12 term for pF and the 106 term for MHz.--tnx Fred Grant, AA4NG
