[Linux-ham] Is LMR-400 cable all the same?
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
> 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
> the attic to the crawlspace and then up to the radio. I first tried a
> RG-8 cable and while that worked on 2m, on 70cm the length of the
> run made it so that no one could hear anything I transmitted. I
> 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
> 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.
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