[TowerTalk] Ameritron Remote Controlled Switches

w8ji.tom w8ji.tom@MCIONE.com
Tue, 10 Nov 1998 08:47:56 -0500


Hi Steve,

One 160 meter system of mine is fed via 500 feet of 7/8inch heliax and in a
rush I left an "N" connector on it. It feeds a pair of phased dipoles for
160, that's a fairly narrow SWR bandwidth antenna.

Operating up where the SWR is 3 or 4 to one, I blew the N connector out
with an AL-1500 amplifier.

I convert N's to SO-239's by machining the connector and adding the "guts"
from an SO-239. I dislike reliability of N's at high power with high VSWR
so much I actually machine perfectly good N connectors into SO-239's!

A phenolic 259-239 pair will handle more power than a Teflon N at HF!
(Radio Shack connectors are the exception, I have no idea what material
they use but it melts easily)

> Now, letís go to legal limit levels, and below.  This is where I reside.
> I've run 1 to 2 KW (intermittent SSB and data) into N's without incident
at
> HF freqs in the federal and industrial arenas for over 15 years.  So,
with
> my diligence in connector assembly and inspection, what are the chances
of
> running into problems if I continue to use N's at legal limit and below?

Don't take any of this to mean an "N" connector won't handle 1-2 kW at HF
into a reasonable SWR. The simple fact is those old SO-239's and PL-259's
are really pretty strong connectors. The only flaws are the SO-239
generally has an impedance of 30-40 ohms and the connectors aren't real
weather tight.

But I use 259/239's because:

1.) SWR of a line bump less than one electrical degree long is generally
not important (and the bump is only in the length of the SO-239 section,
the PL-259 is perfectly fine), so connector impedance isn't a concern below
UHF.
  
2.) I also put them in a "boot" that acts like a weather-hood open on the
bottom, so moisture is easily handled in outdoor environments. That way the
connector can breathe without getting soaked in rain.

3.) They are easy to assemble.

4.) They handle more power and higher SWR without failures from arcing and
heating.

> Have I been taking chances at HF?

Probably not, unless feedline SWR is really high. Remember this whole
thread started because I said "N connectors handle less power than
SO-239's", no one ever said N's are junk. I would like to re-enforce the
fact that "N's" are not much different than BNC's electrically. Have a look
at the
center sections of both.   

As a matter of fact, I grind the bayonet off BNC's and use them as quick
disconnect "N's"! A male BNC with no bayonet fits a female 50 ohm N very
well. Air gaps and conductor sizes are very close.

>  What are the safe power levels for these
> N's at VHF and UHF?  

In my experience, they are reliable at a kilowatt or so if SWR is low.
 
> Besides the UHF connector, is there another alternative to the N that is
> about the same size, but better at power handling, ease of installation,
> and ruggedness?  For example, use of the C or HN series?

Never used them. I mostly use UHF connectors, N's and  EIA flanges. 
 
> Here's perplexing question.  Why is the N rated for 1500V and the BNC
rated
> for 500V, with their dielectrics of air/TFE and TFE respectively
(excluding
> mating junctions)?  

No idea. Hi-pot tests here always prove otherwise.

>Could it be center pin size again?  Could the designers
> have limited the current capacity through the voltage rating, referenced
to
> 50 ohms with the BNC? 

I really have no idea why they did what they did with ratings, all I know
is how they work in practical systems.

I find nothing difficult about assembling a UHF connector, to me they are
much easier than an "N" connector.

73 Tom   

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