[Elecraft] bnc connector

[Tom W8JI]

<<Most hams know that the standard BNC (not the mini 
version)
should handle 200-300 watts and not be pushed higher, altho 
likely will go
higher for some time.>>

The BNC, either 50 or 75 ohm,  is dimensionally almost 
identical to the 50-ohm style type N connector. As a matter 
of fact we use BNC males with the locking ring removed as 
push on connectors to mate with 50 ohm N females. All we 
have to do is very slightly flare the BNC mating ground 
shell to increase pressure a bit on the inside of the N.

The weak point is the cable with BNC's, not the connectors. 
The connectors, like a type N, will handle a few kilowatts 
into a matched load.

<<The So-239 is often rated at a Kilowatt of RF but clearly 
takes double
that in many many shacks.  However, at about 6 KW with a 
small
SWR, the SO-239 will begin to arc the pin to shield.>>

We high-pot SO-239/PL-259 combos at 5,000 volts peak. A good 
connector properly installed will easily make that number. 
5000*.707 is 3500 VRMS. That's 250kW into 50 ohms, so arcing 
is never the issue in a properly installed connector. They 
are current limited which is a heating problem, but will 
easily handle five times the current of a type N because the 
pin surface area is much larger.

Whenever we install connectors we high pot them to be sure 
we have no stray strands or other problems. We flatly reject 
anything below 3.5 kV, but prefer to have 5kV or more.

<The big one that looks something like a BNC, called an N,
has many advantages including taking higher RF, presenting
a lower SWR bump in the line, greater shielding of the 
shield line,>>

I disagree Charlie. There is a HN we use in plasma and other 
high power high SWR applications, but a type N like the BNC 
is really a weak connector. The N is a glorified weather 
tight BNC. I'd never consider a type N at more than 1500 
watts or especially into a high SWR. They are terrible for 
lightning and SWR related damage because they have very 
small center pins and very close internal spacing. The HN is 
much better if you want a reliable connector that is N 
style. That's all we use on high power plasma and medical.

The impedance bump in a UHF is limited to an area about 1/2 
inch long. The problem is all in the female, the male (like 
most males) is nearly perfect. You wind up with 1/2 inch of 
30-45 ohm line section for every standard properly installed 
UHF connector pair. The general electrical rule is a 
moderately sensitive system like a communications system can 
have about 1 degree of bump at that SWR, so the UHF 
connector should be good up to lower UHF with problems 
unless you get too many in the system. This is why Motorola 
never had a problem with UHF connectors on VHF gear.

As a matter of fact when I install hardline here I actually 
machine the N female ends and convert them to UHF females 
with Teflon insulation. We've had N's blow right apart in 
lighting hits, but the UHF connectors keep on ticking. For 
UHF weak signal I might consider an N, but they are 
unreliable at high power or with high SWR or lightning.

At low power where a cable needs to be quickly disconnected 
a BNC is fine, but I'd never use them here for anything 
outside of the test bench or a radio that needs a quick 
disconnect. Ham manufacturers did the right thing by using a 
UHF connector. It was generally a smart move.

73 Tom