[NLRS] Calculating 1/4 Wavelength coaxial stub?

[John (JK) Kalenowsky, K9JK]

Scott (and inviting other comments, alternate answers),

I agree with John, K0PW, though I always *thought* (perhaps 
incorrectly?) that the "234" number (or "468" for half-wave) was 
taking the Velocity factor of  'free space' (which nets out to be on 
the order of 0.95) into account. "Radials", though, are not 
necessarily in 'free space' (thinking 'buried' or certainly near 
"earth") so it's very possible that 'end effects' is the 'cause' in 
that case. (Or possibly a 'win-win', 'end effects' is an explanation 
of why the velocity factor of 'free space' is NOT simply 
"1.000000000000000000000000000000000000000000000000"?).

Ignoring velocity factor for a second, the 246(feet)/f(MHz) 'formula' 
is 'closest' to an actual quarter wave, 492(feet)/f(MHz) for a half 
wave (I've also seen the 'half wave' formula as 5900(inches)/f(MHz), 
note that 5900 is not exactly 492 * 12, though). If you want to go 
directly to metric, 300(meters)/f(MHz) yields 'very close to' 0.26 
meters (26cm) for 1152 MHz, take a quarter of that, 6.5cm, and apply 
your velocity factor. The "inches" formula for a quarter wave, 
2950(inches)/f(MHz), yields 2.56 inches for 1152 MHz...'darn close' to 
6.5cm.

John's comment about variation in velocity factor of coax is also 
valid, manufacturer to manufacturer or even batch to batch from the 
same manufacturer. I even heard a one story about variation of 
velocity factor (and "significant" variation) within the same ROLL of 
coax from a manufacturer. The shorter lengths at shorter wavelengths 
compound those variations.

73, JK


----- Original Message ----- 
From: <hill195 at mchsi.com>
To: <nlrs at mailman.qth.net>
Sent: Tuesday, October 07, 2008 06:41
Subject: Re: [NLRS] Calculating 1/4 Wavelength coaxial stub?


>
>
> I have always used 246/f*Vf. I use 234/f for computing 1/4 wave 
> radials for a
> vertical antenna. No Vf applied. The smaller number is the result of 
> distributed
> capacitance applied to the radial conductor as a result of "End 
> Effect" making
> it slightly shorter. That, having been said, I never relied upon the 
> formula
> entirely though, due to nuances in Vf from one manufacturer to the 
> next or batch
> of coax to the next. I Always cut slightly longer, then trim and 
> sweep with an
> SWR analyzer or equivalent piece of test equipment and make my final 
> trims. This
> is especially important in coaxial power splitters on an array. 
> Those last few
> millimeters can have a profound effect on radiation pattern. I'll 
> craw back
> under my rock now :)
>
> 73, John, K0PW
>
>
> ----------------------  Original Message:  ---------------------
> From:    Scott <acepilot at bloomer.net>
> To:      NLRS List <nlrs at mailman.qth.net>
> Subject: [NLRS] Calculating 1/4 Wavelength coaxial stub?
> Date:    Tue, 7 Oct 2008 10:37:00 +0000
>
>>
>>
>> To make a shorted 1/4 wavelength stub, I would have thought that 
>> the
>> formula would be (234/f)*Vf where f is the frequency and Vf is the
>> velocity factor of the coax.  Searching google briefly, I saw a 
>> site
>> that had the formula as (246/f)*Vf.  Of course at 1152 MHz, that's
>> "only" a difference of about 2mm.  Which formula will get me 
>> closer?
>> Or...is there a different one alltogether?
>>
>> -- 
>> Scott N0EDV
>> http://corbenflyer.tripod.com/
>> Flying Corben Junior Ace - Building RV-4
>> Gotta Fly or Gonna Die
>>
>>
>> _______________________________________________
>> NLRS mailing list
>> NLRS at mailman.qth.net
>> http://mailman.qth.net/mailman/listinfo/nlrs
> _______________________________________________
> NLRS mailing list
> NLRS at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/nlrs
>