[Linux-ham] Is LMR-400 cable all the same?

David Black dave at jamsoft.com
Wed Mar 25 17:38:26 EDT 2009

----- "Tanner Lovelace" <kb4tye at arrl.net> wrote:

> At what point does it become a good idea to switch to something like
> an
> N connector?  At home, I have a dual band 2m/70cm radio hooked to a

I guess it's personal preference.  For me it can go either way around 70cm.  E.g. N connectors on a fixed repeater antenna fed by LMR-400, suggested by N connectors on the Vertex repeater to which it was attached.  The interconnects between the old duplexer cans were very short lengths of silver/teflon RG-58 with PL-259 connectors (better duplexers use N!).  My dual-band mobile rig has a PL-259 in back.  All these work fine.

> Ventenna antenna on the roof and to run to the radio I have to go
> from
> the attic to the crawlspace and then up to the radio.  I first tried a
> normal
> RG-8 cable and while that worked on 2m, on 70cm the length of the
> cable
> run made it so that no one could hear anything I transmitted.  I
> swapped
> that out for an LMR-type cable (don't remember the exact designation
> right now, but it was similar to LMR and on sale at RARSFest one year
> since I think it was basically what was left from a spool) and with
> the
> LMR-type cable doing the run 70cm works very well.  However, I am
> using PL-259 connectors on it, but the LMR cable does go directly
> to the radio (there's a short section of RG-8M in the attic to get to
> the LMR cable.

LMR-400 (~0.400 outer diameter) is very slightly larger than RG-8U.

Here's a nifty calculator:  http://vk1od.net/calc/tl/tllc.php

Just for fun, I ran two calculations - one RG-8U, the other LMR-400. 100 ft. of cable at 445 MHz, no mismatch (cable loss only):

RG-8U: -4.48 dB
LMR-400: -2.67 dB 

That's around 1.8 dB difference.  At 146 MHz it was only 0.7 dB difference.   If your RG-8U cable run was much longer you may have been looking at 3dB less signal getting out on 70cm, any connector or cable defects notwithstanding.

> So, basically, my question is would I have been better served by
> using an N connector? (My guess is the answer is no, but I'd
> love to hear the more detailed explanation of why that is.)

I don't have the numbers memorized, but two design benefits of N over PL-259/UHF connectors are:

. PL-259 doesn't try to maintain 50 ohms impedance through the connection: there's an impedance "bump" resulting in at least a little reflected power and resulting loss.  N does not have this problem and almost perfectly maintains 50 ohms.

. N connectors, if done right, are usually watertight.  PL-259 is not so you have to seal it.  People seal both anyway.

Dave  AD6Q

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