Fw: [Antennas] Tuner Inductor Q

Sat, 4 Jan 2003 20:17:17 -0600

You do need to short the unused end of the inductor to the tap
for exactly the reason you indicated. I don't know what the 
effect is on Q for your particular roller inductor, but rather 
than guessing here are some quick results using a 40m ARC/5 
coil wound on a ceramic coil form. Q of tuner air variables is 
generally pretty high so that's not a consideration. 

Whole coil 14.7uH .....partial coil, tap at approx 5.7uH
Q of whole coil at 7Mhz is "480" (good as it gets)
Q of partial coil at 9Mhz, shorted to end "350"
Q of partial coil at 9Mhz, not shorted to end "370"
    ....however, because of the floating end, it becomes
    extremely sensitive to any capacitance (and the point 
    you made about high voltage). In other words, I'd always
    short the unused portion for a HF tuner.

The main thing on roller inductors is to make sure the high
band end has more turn spacing or the Q on 10/15m will drop
like a rock. That's why using an ARC/5 inductor without an
external coil for 10/15m is not a great idea.

Q of a coil on a good clean ceramic form is very close to
air wound (hard to tell the difference, even at 30Mhz). By
the way, CPVC and PVC is pretty good also. Anything 
that can absorb moisture is terrible. The biggest factor I've
found which affects Q is turn to turn capacitance (spacing
between wires), especially at the higher frequencies where
skin effect also comes into play). 

The other point that I would make is that the tuner will be 
much more efficient at the higher Q but the tuning will be
SHARP ...maybe you don't want it to be too sharp.  

73  Kees K5BCQ

--------- Forwarded message ----------
From: Cletus W Whitaker <[email protected]>
To: [email protected]
Date: Sat, 04 Jan 2003 19:56:58 -0500
Subject: [Antennas] Tuner Inductor Q
Message-ID: <[email protected]>

de WB2CPN South Central Pennsylvania   2003.01.04

On the subject of Roller Inductors vs Tapped Inductors, what are the
opinions on how to treat the unused part of a tapped inductor.  Let's
say half of the turns are used, the tap at the center is the "Ground"
terminal, should the unused turns be shorted to the center tap or
should they be left open?  If left open I would think that under some
conditions some hellashious voltages could be developed between the
unterminated end and ground.  But if we short the unused turns to the
tap, wouldn't the shorted turns drastically lower the Q?  

Related Thought:  Shouldn't a roller inductor on a ceramic form show
about the same Q as an air-wound coil of the same dimensions, except
for the losses attributed to the dielectric of the ceramic, and the
added capacity between turns.  Some roller inductors are close to
air-wound, most are not.       Tnx    73   Clete
- - - 

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