RE: [amsat-bb] RG's Velocity Factor

[<alan_bloom@xxxxxxxxxxx>]

I have always been uncomfortable with using a loop of wire to couple a grid-dip oscillator to coax, especially at VHF.  The loop adds extra inductance which causes the coax to appear "too long" (too low a resonant frequency).

My suggestion is to tune the loop to resonance with a capacitor.  Disconnect the loop from the coax and add enough shunt capacitance that the loop (now a tuned circuit) resonates at the expected resonant frequency of the coax.  (Use the GDO to measure it.)  Now when you connect the tuned loop to the coax no extra inductance is being applied.

An even better way to tune a stub is with a signal generator, a receiver, and a coaxial T connector.  Connect the signal generator, the receiver and the stub to the T connector.  With the other end of the stub open-circuited, minimum signal is received at resonance.  This method has the advantage that it includes the effect of the stub's connector.  If no signal generator is available, a strong off-the-air signal (e.g. a local repeater) can be used instead.

Al N1AL


> -----Original Message-----
> From: owner-AMSAT-BB@AMSAT.Org 
> [mailto:owner-AMSAT-BB@AMSAT.Org]On Behalf Of El Dep
> Sent: Tuesday, November 09, 2004 3:36 AM
> To: amsat-bb
> Subject: [amsat-bb] RG's Velocity Factor
> 
> 
> Hi to Group,
> 
>  looking for accurate "phasing stubs", just for experimentation
>  ie on circular polarization, I need your opinion.
> 
>  I cut a piece of coaxial cable RG58, following the usual formulas.
> Lamda/2 (half wavelength) for 145 MHZ is 1.034 m.
> So, the electrical length is 1.034m x VF (where VF the Velocity 
> factor of RG58=0.66).
> 
> Thus, 1.034 x 0.66 = 0.68 meters (or 0.34 for Lamda/4).
> 
>  I have a good GDM (Grid Deep Meter) and I tested if the piece of
> 0.68 meters is resonable at 145 MHZ as the theory said.
> 
>  On "Manual" of GDM I read that: 
>  the coupling with coaxial is possible by using a small loop (1 turn)
> between inner and outer contactor of coaxial,  close to GDM.
>  The opposite edge must be "shorted" for Lamda/2  and  "open" for 
> Lamda/4.
> 
>  Well, following these instructions I'm surprised because, the
> VF seems to be different from 0.66 !  
> 
>  The test repeated using another stub with RG213 and the result was
> similar ! Always the VF seems to be smaller, close to 0.50-0.55 and
> not 0.66 !  Always the pieces of 0.68m have resonance frequency
> around to 110 MHZ and not to 145 MHZ !
> 
>  The "Deep" on GDM is good and sharp only at this point, so 
> I'm sure 100% for the correct indication.
> 
>  That confirmed with both caoxial cables again and again, using
> L/2 and L/4 pieces of coaxial,  so there is not error with my 
> measurements.
> 
>  The GDM tested with a Digital Frequency meter, no problem - 
> the calibration is excellent on this between 100-200 MHZ.
> 
>  So, I wondering what is going wrong! 
> 
>  Any explanation? 
> 
> 73, John Smith 
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