I see lots and lots of posts from people extolling the virtues of using LMR-400 coaxial cable in their ham radio setups. LMR (Land Mobile Radio) cable was developed by Times Microwave Systems as a high quality coaxial cable which can be used with excellent loss properties at frequencies greater than 1 GHz. If your ham radio interests lie in the 1.2 GHz band and above, I can see no reason why you shouldn’t use the LMR series of cables in your setups, since the more traditional RG series of cables often are out of specification above 1 GHz.
However, if you are running the usual 1.8 MHz to 54 MHz range of frequencies, I see no technical reason[*] why you should use the LMR series, especially LMR-400, based on the loss characteristic. Even at 144 MHz, there is not much difference between LMR-400 and good old RG-8 or RG-213. Sure, the double-shielded LMR series of cables has a better shielding characteristic, but you need to figure out whether this alone is worth paying the premium for.
Don’t get me wrong – I’m not saying that coaxial cable loss is trivial – the receive mode antenna system noise figure can be dominated by the feeder attenuation (loss), since for a passive device, the noise figure equals the insertion loss. However, I have noticed a lot of posts from people swearing by LMR-400 due to its low loss, even in the HF bands, and I somewhat disagree.
Here are some numbers via the Times Microwave Coaxial Cable Loss Calculator for 100 feet of cable:
To put this into perspective, if you’re running 100 feet of RG-8 or RG-213 and are thinking of upgrading to LMR-400 because of the loss you’re incurring, think again, especially if you mainly operate on the HF bands. Based on the above numbers, and a transmitter output power of 100W, the power arriving at the antenna using the same 100 foot length of the same types of cables would be:
So, if cable loss is your obsession, and that extra 4 watts is important to you on 40 meters, or if you think that extra 9 watts will make the difference to your QSO on 6m, then go ahead and use LMR-400. Remember, the numbers above are for 100 foot long lengths of cable, and it’s always good radio practice to reduce the length of your antenna feeder by as much as you can. When it comes to coaxial cable, less is more.
Also, remember that coaxial cables (even cable of the same type) are not constructed in the same way. Always examine a coaxial cable for shield coverage, and discard it if there is not complete coverage. Also, if you get the chance, heat up a piece of the shield braid and center conductor with a lighter flame and see how it is affected by the heat. Pure copper wire will glow while heated, but will hold its shape. Copper coated aluminium wire will glow, bend, and distort when heated.
[*] provided your antenna system is well matched to your transceiver. If you have a mismatched antenna and/or feed line, using higher quality (less lossy) coaxial cable can help mitigate the overall loss in the mismatched system by quite some margin.
Use of LMR-400 in Repeater installations
Use of LMR-400 in Repeater installations is not recommended. The braid over foil construction of the shield, while being a very robust shield, is made up from different metals. This dissimilar metal construction has been traced to sources of Passive Intermodulation when used in a repeater, or full-duplex operation when two (or more) signals of different frequency are present in the line, such as the received (inbound) repeater signal and the transmitted (outbound) repeater signal.