[TowerTalk] RG-8X Crimp BNC Connectors

[w8ji.tom]

Hi Guys,

> >The impedance determining portions of the BNC connector are
> >exactly the same as those of the N type connector.
> 
> Agreed.  The proportions to the equation that determine characteristic

I grind the outer bayonet portion off male 50 ohm BNC connectors (they are
manufactured in TWO impedances) and use them as quick disconnects for
N-type 50 ohm females in testing. 

Also,  50 ohm N-male's will plug right into BNC females for quick
disconnects.   

BNC's all have the same "working end" center dimensions, but 75 ohm models
have air dielectric inside rather than solid dielectric. 

> >This is probably correct.  I also doubt that it is either a 50 or
> >a 75 ohm connector.  But the point is simply that it is NOT a
> >controlled impedance connector (for any particular impedance) and
> >as such is equally applicable to either impedance line.  Note
> >that they don't make different versions of this connector for
> >different impedances as they do for the controlled impedance
> >types.

If you are talking about BNC's, they DO make two impedances. Even if they
didn't, mixing could be a problem because the connector isn't centered at
the geometric mean of the two impedances, 75 and 50 ohms.

However, none of that matters below upper VHF unless the line has a
s---load of connectors, it is the distance of the bump in terms of
wavelength that is important.  

> Agreed again.  However, I am sure that the PL-259 can be calculated to
> *some* characteristic impedance using the formula to derive the
> characteristic impedance of coaxial cable.  Although, the PL-259 has been
> connotated as a "UHF" connector, sweeping it to the upper UHF frequencies
> reveals that (arguably) it should not be used in critical applications
above
> 100 MHz or so.

The PL-259 end is not the problem. That end is fine *FAR* up into the UHF
range. The problem is all in the SO-239. Typical SO-239's are about 35 ohms
or so. If you drill multiple holes in the dielectric, or remove dielectric
inside the threaded portion most SO-239's can be moved up to 50 ohms. They
also require use of a hood to minimize impedance bumps. They seep quite
well up to 500 MHz or more if the connector is modified.  

SO-239's (and the barrels, which also are almost always in the 30-something
ohm range) cause bigger problems in 75 ohm systems than 50 ohm systems,
because surge Zo is so low. None of that matters below upper VHF, unless
the application is reflection critical. The change in SWR and loss is
immeasurable at HF, even with an unmodified SO-239 and no hood.

73 Tom

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