[HCARC] LMR-400 and PL-259 fittings

[Kerry Sandstrom]

Hi Gary,

Well this again is a long story.

Any kind of a discontinuity in a transmission line will cause part of the EM 
wave on the transmission line to be reflected.  Guess what?  That leads to a 
higher SWR.  The discontinuity can be anything.  Common ones are changes in 
the physical dimensions or dielectrics.  That is why splices in transmission 
lines are a bad idea.  An increase in SWR leads to a couple things: one is 
an increase in attenuation and the other is an increase in voltage at some 
places on the transmission line which can lead to breakdown.  That is the 
basics.

A common discontinuity on a transmission line is a connector.  At low 
frequencies like audio frequencies the discontinuity is very small in terms 
of wavelength and the losses are negligible.  At audio frequencies we can 
get away with RCA phono plugs, 1/4" phone plugs and very simple cables.  As 
we go higher in frequency, we can't get away with such simple connectors and 
cables.  Probably the first connector developed for coaxial cable was the 
UHF series of connectors.  Both coax and the matching connectors started 
becoming available during and immediatelky after WW II.  Before that, 
everything was parallel conductor transmission lines such as what we call 
open wire or ladder line now.  Radar was the big driver.  We think of radar 
as microwaves.  During WW II it was not.  Early radars were at low VHF 
primarily.  Our 2 m and 1 1/4 m started out as radar bands.  Incidently 
there are still a lot of radars operating around 150-250 MHz, just not in 
the US.  Because of the high powers and need for duplexers and sensitive 
receivers, reflections were a problem.

The characteristic impedance of a coaxial transmission line is determined by 
the ratio of the outer diameter of the inner conductor to the inner diameter 
of the outer conductor as well as the dielectric constant of the insulator 
between the two conductors.  What does that mean to us?  Well, to start 
with, if the characteristic impedance of the lines is the same, whenever we 
change dielectrics, the ratio of the diameters changes.  If we look at a 
coaxial cables with different dielectrics, even if the outer diameter of the 
cable is the same, the actual outer diameter of the center conductor will be 
different.  The dimensions of  solid dielectric cable is different than 
foam, dimensions of polyethylene dielectric cable is different than air or 
Teflon dielectric cable.  That is why sometimes you can't use a connector 
designed for one type of cable on a different type of cable.

With the exception of UHF series connectors, RF connectors are designed to 
have a certain characteristic impedance.  Typically they are either 50 Ohm 
or 70-75 Ohm connectors.  At HF it really doesn't make much difference in 
losses and SWR.  Where problems do arise is the dimiensions of the 
conductors at the connector interface are not the same for different 
impedances.  Unfortunately, the dimensions are close enough that you can 
force the connectors together without too much problem, but you will damage 
the connectors in the process.  The higher you go in frequency, the more 
these things matter.  VHF and UHF and higher, these things are important. 
Type N connectors are made in three basic lines.  Standard 50 Ohm which are 
rated to 11 GHZ, 75 Ohm and Precision 50 Ohm which are rated to 18 GHz (I 
think).  One company I have data sheets from manufacturer's 23 different 
Type N plugs for 10 different cable sizes in just 50 Ohm impedance solder 
type .  It has 17 more crimp style and 9 more for semi-rigid coax.

UHF connectors are not rated for frequency.  They shouldn't be used whenever 
SWR is important and they are definitely not waterproof.  There is a lot of 
variation in the insulators used for UHF connectors.  The old connectors 
used a tan colored insulator that I believe is phenolic or mica filled 
phenolic.  I've seen them charred black but never melted.  Many of the newer 
ones seem to have a white plastic with a very low melting point.  The latest 
ones are Teflon which has a high melting point.  I still primarily use the 
old style phenolic ones.  They are what I'm used to.

Incidently, the way you put a PL-259 on RG-8/U cable is you cut the various 
layers according to instructions, you tin the center conductor and the 
exposed copper braid and you screw the PL-259 onto the cable cutting threads 
into the plastic outer covering of the cable.  Then you solder the connector 
to the tinned braid and the center conductor.  I don't know how you would do 
that for LMR400.  it may require a crimp style connector.

Kerry