Lets logically end the LMR-400 debate.

[brandon7861]

The internet has no shortage of people arguing whether LMR400 is junk or not, and I am trying to get to the bottom of why because I know a few people that use it without issue (myself included).

I think the issue can be narrowed down to one of four things.
1) water ingress
2) damage from too much power
3) the braid over foil PIM issue.
4) counterfeit products

The first could likely be eliminated with a TDR trace of the coax to see the location and nature of the damage. Is it the last third that is wet, are the connectors good (you'll need a jumper to avoid the blind spot). I have two TDR tools, but my LMR400 is just fine, so I cannot test this. Is it a burnt spot (which won't appear on TDR at that low of power, might need a HV breakdown tester for that). The third would need a spectrum analyzer to identify. The fourth could be identified by close inspection/comparison to a known good cable.

A little more on the third possibility as it is usually encountered during duplex operation (TX and RX at the same time) when the TX forward biases any diode junction formed between the braid and foil, and the non-linearity causes mixing with the RX signal. This can happen on it's own or where something makes a dent in the cable and pushes the braid against the foil. I have long believed that this cannot be a problem on 11m as it is mostly simplex operation, but then I thought of all the splatterboxes out there transmitting on more than one frequency...

If you have bad LMR-400, can you either do some tests and report back here or mail your junk coax to someone who will? Lets solve this!

 

[L2]

No issues with my Times Microwave LMR400. I have one run to an old Imax 2000 and another to Diamond D-130 discone. The one on the Imax is about 15 years old. Still working great.

 

[Shadetree Mechanic]

I use lmr 400 from Jefa Tech and have never had a problem. The most I ever run into it is about 600w peak.

 

[FlatlandBusa]

I run 135' of Davis bury-flex 400, that is direct buried in my lawn. The ends are waterproofed per commercial standards and it has been in service for 2+ years now with zero issues. Currently running 1.2KW PEP through it, to an IMAX2000 that I use on 10, 12, 15, & 17 meters, on 17 meters the SWR is above 3:1 before the tuner.

 

[sp5it]

Currently running 1.2KW PEP through it, to an IMAX2000

Hard to believe, I fried Imax with 800W PEP

 

[unit_399]

When I bought my antenna system, part of the deal was 90 meters of Times Microwave LMR400. I typically run 500W PEP through it, and occasionally 900W PEP. SWR 1.4 or less. Never had any problems.

- 399

 

[Shadetree Mechanic]

@Unit 75 had a problem with the LMR a while ago.

Post in thread 'LMR-400 for Ham Radio/cb radio' https://www.worldwidedx.com/threads/lmr-400-for-ham-radio-cb-radio.259636/post-740084

 

[Blackcat630]

I have used Times, Jeffa and Shireen lmr 400 with no issues..

 

[Hawkeye351]

I run Digital Trends LMR400 on my base, original old antron (side mounted antron), no issues whatsoever.

A friend of mine runs Digital Trends LMR400 on his base maco v5000 running around 2kw at all times, no issues whatsoever. He achieved a 2db increase in receive and transmit after changing over from RG213 to tuned (nulled) lengths on his jumpers and coax of LMR400.

Our local amp tech switched over from RG213 to tuned (nulled) lengths of Times Microwave LMR400 on his maco v5000 running around 2kw at all times, no issues whatsoever.

Another local friend of mine switched over from RG8U to tuned (nulled) lengths of Times Microwave LMR400 on his solarcon and all his jumpers, running between 250w up to 2kw, never had an issue.

We all noticed big differences in our receive and transmit after switching over, and haven't looked back.

It's all in the VF (Velocity Factor), after applying the null calculation.

RG213 VF - .66, meaning only 66% of your power is actually making it to the antenna after cutting for that VF.

RG8U VF - .74, meaning only 74% of your power is actually making it to the antenna after cutting for the VF.

RG8X VF - .82............
LMR400 VF - .84........

You get the picture.

 

[Justme]

the velocity factor doesn't mean that amount of power gets to the antenna, that is the attenuation of the coax and length used.
You use the velocity factor in making coax baluns or calculate 1/4 or 1/2 wave lengths.

 

[brandon7861]

RG213 VF - .66, meaning only 66% of your power is actually making it to the antenna after cutting for that VF.

VF and loss are not related.

Thanks for all the positive reports, similar to what I hear. There must be a reason a few people end up having issues. I'd love to know what that issue is.

 

[Shadetree Mechanic]

The velocity factor is the speed of the signal in the coax compared to the speed of light in a vacuum, expressed as a ratio with 1.0 being the speed of light.

 

[Staybolt]

I use rg 213, there, i said it...

 

[Staybolt]

I find it funny how cbers get soo wrapped around the axle over coax and coax length, ect. I run what I run to my beam (125 foot) and I get out just fine...

 

[brandon7861]

I find it funny how cbers get soo wrapped around the axle over coax and coax length, ect. I run what I run to my beam (125 foot) and I get out just fine...

I can see the coax length issue for two reasons.. Into a shielded dummy load matching the coax impedance, length makes no difference.

Once you have mismatch, the coax will transform the impedance at the antenna to something else at the input (on the same SWR circle, discounting losses), but the complex impedance seen will change wildly. For people running a tuner, where that impedance ends up being could make the difference between matching it or not.

The other reason is the length of the common-mode path and it's resulting impedance looking down the braid from the antenna. If the antenna is not balanced (ex. any vertical GP antenna), there is the potential for common-mode currents. If the path down the outside of the coax has a low impedance, your coax becomes part of the antenna. This is why changing coax length (read as unintentional antenna length) sometimes changes SWR. Is the radio case grounded or is it floating and appearing as a capacitance hat at the end of an antenna? Take a half wavelength conductor, it can be a straight wire or the outer braid of your coax. If one end is shorted, the other end appears as a short. If one end is floating, the other end also has a high impedance. This is one (of two) reasons people prefer half-wave sections of coax to feed their antennas. Most radios (mobile-not grounded, rubber tires or base-usually two-prong outlet cord that people rarely ground, counting instead on the tower ground). So in most situations, the common-mode path will have the highest impedance when the coax is a half wavelength. If your radio is grounded, figure in that length and aim for an odd multiple of quarter wavelengths in total. This is, of course, in the free space theoretical sense. It gets complicated when other conductors are nearby, and there always are

The other reason people prefer half-wave cables is for ease of impedance measurement. As hinted to above, only a half-wave multiple of coax will not transform a mismatch at the antenna to something else at the input (well, it goes full circle, minus the loss). This allows one to measure the impedance at the feed point through the coax without calibrating out the transformative effects of that coax. There was a time when a rig expert could not be calibrated to the end of the coax, it only read at the input (reference plane at analyzer, not feed point).