> I've stayed out of the debate so far, but even Amphenol claims
> "nominal 50 ohm impedance" for their 50 ohm BNC connectors up to 4
> GHz. On their N-connector they drop the "nominal" but they don't
> elaborate.
BNC's are rated for a lower MUF because the mechanics of the bayonet lock
aren't ideal. The inner portion of the connector can pivot somewhat
depending on the angle the cable (or gravity) is pulling at. So, although
the dielectric and the mating surfaces of the connector would likely work
well above 4 GHz with creating a significant bump, the imperfections in
bayonet causes it to be rated lower.
In contrast, the TNC with its threaded attachment maintains the geometry
better, and is thus rated for a higher frequency.
The same issue exists between SMA and SMB connectors. Presumably SMA's are
rated for higher MUF because the threaded coupling maintains a more
consistent interface geometry as compared to SMB's which are "push on"
connectors which leave more "wiggle room" (pun intended).
N, BNC, TNC, LC, HN, SMA, et al were designed to maintain 50 ohms through
the interface, though none are truly precision connectors. Not even an SMA
is a precision connector. Precision connectors have virtually perfect
mating repeatability. The most popular precision connectors are APC/GPC
varients. They're more likely to be found on microwave test equipment.
UHF connectors weren't designed for constant impedence. They were designed
at a time when UHF was what we would now consider to be VHF-low, and the
impedence bump was less of an issue.
UHF connectors with Teflon dielectric have less of an impedence bump than
cheaper (phenolic, bakelite, plastic, etc.) dielectric types. Some really
cheezy UHF connectors have such high dielectric losses that they will get
warm and, under very high power at VHF/UHF, actually melt. So stick with
Teflon.
> I'll agree that maybe a BNC is not as good as an N-connector overall,
> but similar to your argument -- I'll bet no one here can measure the
> difference at VHF/UHF.
Bet I can on my network analyzer :-) Seriously, with a good VNA, you'll be
able to see changes in return loss at varying frequencies as you flex a BNC
connection. Likewise, using TDR (or a VNA with time-domain analysis), you
will be able to clearly see the bump in the line. Would you notice, or
could you measure, the slight variation in Z of a BNC connection when it's
flexed using a wattmeter or other gross measurement techniques at VHF or
UHF? Probably not. But it's there.
A UHF connector is tolerable on 70cm, providing it (and its mate) are Teflon
dielectric. I will still try to avoid them whenever possible and stick with
type N.
> Amp shows (much!) more RF leakage from a BNC than an N (which would be
> a detriment in some installations!)
Again, due to the bayonet.
One of the reasons you see UHF connectors on VHF and UHF mobile radios,
including commercial radios, is because it is a lot harder to screw a UHF
connector. You really have to work hard to bend a center pin on a PL-259,
whereas a type N is a bit more delicate in that regard. Some may argue that
a regular (solder-type) PL-259 is easier to install on a cable than a
regular (mil clamp type) Type N, so that's why they were used (personally, I
disagree, I always thought type N's were easier to put on). Nowadays, with
the proliferation of crimp-type connectors, there is no advantage to one
over the other.
--- Jeff
