I Bought KMR-400 Coax From Amazon | How Bad Can It Be?

Testing out the KMR-400 Coax Performance And Teardown.
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#hamradio #k8mrd #KMR-400

[Gruuvin]

A fair comparison would have been to run your same measurements against 75' of coiled LMR.

[DXCommanderHQ]

This sort of stuff really suits you Mike.. Although I have to say, doesn't matter what you do, it's always just great!

[notreallydaedalus]

Thank you for testing and providing objective results!

[macgyver]

There's a "law" if you will about using calculators that applies here:
"Garbage IN equals garbage out." Which I think you might have done right at the start by assuming the KMR has the same 84% VF as the LMR?!

Messing around building 9 element collinear antennas out of coax from scratch I learned you REALLY need an accurate velocity factor measurement to get the lengths right for those elements, performance and SWR can do wild things when you have 9 elements covering 4 bands and they all start interacting. Also learned an important fact about manufacturer's "VF numbers" vs post production reality... Not familiar how that Rig ex spurt stick works, but I unroll the coax and actually use a tape measure to get the precise length, a "known value". Then I hooked up my NanoVNA test leads and calibrate/zero those before testing the coax and doing some math on an ancient technology called 'paper'...😋

After testing a BUNCH of different batches of KMR-400 my samples all tested at about 4-6% faster VF than the 84% in print on the LMR data sheets. Over all I was pretty impressed with that coax and I'd say I got way more than I paid for.

[Starphot]

Retired RF bench technician here. I worked for a couple of fellows in the late 1970's in a mobile stereo/ CB shop when the CB craze was going. We had problems with some of the customer's vehicle installs with the intermittent SWR readings. The cable installed was the off-the-shelf Radio Shack Tandy Wire RG-58. We then replace those with an Amp brand cable with no further problems. We dissected the Tandy cable and found at even intervals along its length there was the center conductor just pinching on one side of the foam dielectric to the outer braid, causing intermittent shorts. Tandy did produce their own wire and cable, I believe in a factory in Indiana. Evidently they didn't have enough cool time out of the extruder before coiling the cable up. Their coax products did improve though. Tandy did make their own xtals for their CB radios and scanners as well. These shifted lower in frequency over time sometimes into the next channel.

[johnjones]

#1: Never test cable, especially coax with a foam die electric in a tight coil. Read specifications of LMR-400 & look at installation criteria. That coil of ‘KMR’ was coiled from China waaay too tightly & most likely distorted the positioning of the center conductor in relation to the sheath.
Bingo. There’s your excessive loss.

[tekdragon]

Once in awhile you search for info on something, and ACTUALLY get the exact answers you were looking for. THIS VIDEO is one of those times!! Excellent well-thought-out video with measurements, numbers, and an actual teardown. I couldn't swing the extra cash for the Times Microwave stuff this time around so took a chance on the KMR-400. At least I know now that I didn't get ripped off and it should do a decent job, even if it falls a little short of the Times Microwave version. Thank you!

[DonzLockz]

All things considered, id keep it as a backup feedline. Great test Mike!

[DrumCoversbyBill]

Thanks for sharing. I just received a 25 foot roll of this for my VHF/UHF base antenna (From the antenna to lightning arrestor at base of pole) Looking forward to see how it holds up.

[AlanElBee]

Great video, Mike - happy to see an honest review on something I use that might save me some $$. 73, K7KS

[gfodale]

Back in the 80's, I used RG 8 and RG 58 from Belden. Both in the Marine Corps, and personal use. Currently, for HF, I'm still happy with standard RG 8, RG 8X, and RG 58. For VHF and UHF however, I tend more to LMR 400, where the lower loss in those bands are substantial. I'm on a small fixed budget. I terminate my own cables and buy bulk cable as possible. Tailor the cost to the application is my thought. Nothing is perfect. We do what we can.

Also, as a side note, your coil looks like a loose choke from other videos. Others have commented, so I won't belabor the point. For those unknowing, please seek the other comments and search for common mode chokes. I've not done my own tests as yet, so I won't comment further. (tests are planned, just not today.... :) )

[daveNMXX]

Great video, thank you! "Better" is subjective, and this coax is likely "good enough" for most amateur radio use cases. It seems great for the price. But I'm using 75-ohm RG6 for my HF antennas, so there's also that.

[nicklikesradio]

Thats a nice induction coil you're testing.

[Silvertarian]

I was left scratching my head after watching this video.

I went ahead and ordered a 75' run of kmr400.

I tested it myself vs the old sma RG58 handheld extension cable I bought first 100' run(yes I'm dumb and learn the hard way)

Both cables were ordered from Amazon.

Here are the losses measured by my nano vna, and expected transmitted power from a 50w radio based on those measurements:

Cable Type: KMR400 (N)

146 MHz: -1.09 dB (42.25 watts)

450 MHz: -2.29 dB (34.8 watts)


Cable Type: RG58 (SMA)

146 MHz: -4.58 dB (27.8 watts)

447 MHz: -7.96 dB (14.84 watts)

[jackKFIT]

Been down the cheap coax route before and decided to do more research and testing and now I run Times Microwave LMR-400 on both my homebrew EFHW and DX Commander Classic. I haven't regretted that decision at all. I recently got on the M&P bandwagon and bought a 50ft run of Ultraflex 7 and an 18ft run of Ultraflex 5 for various portable setups. I haven't tested the M&P coax but I trust the YouTube guru's findings. Some cheap coax may be good but I will stick with brands I trust. Thanks for another great video.

[Aimsport-video]

Like you said… not bad for the price… great video. Thank you.

[SouthEasternCommunications]

Looks like a good budget cable for hf .. looks very much like a ll400 I sell here in Australia 🇦🇺
Good video

[aaronhope]

Due to the price difference, I thought it was worth the risk for what I'm doing. I'm glad it held up to actual scrutiny!

[WHJBill]

For the price, it looks good. A lot better than RG8. 😁

[thomasbjarnelof]

I bought somting like that that, and it was copper clade steal as inner conductor. A couple of years and the inner conductor corroded away from the outside connector. Indoor was not corroded.