Where should we use RG400 instead of RG58?

Discussion in 'Instruments / Avionics / Electrical System' started by wsimpso1, Nov 6, 2019.

Help Support HomeBuiltAirplanes Forum by donating:

Tags:
  1. Nov 6, 2019 #1

    wsimpso1

    wsimpso1

    wsimpso1

    Super Moderator Staff Member

    Joined:
    Oct 19, 2003
    Messages:
    5,893
    Likes Received:
    3,119
    Location:
    Saline Michigan
    OK, I have a roll of RG58, and I was intending to go from all of my VHF radios to their antennas with the stuff. VOR's, ILS, COM radios. Already have it in the wings for VOR/ILS reception, setting up in the fuselage for a couple VHF COM antenna, transponder, etc with it.

    But folks are hot on RG400 for some stuff, it is 50 Ohm, lower loss than RG58... and pricey too. So, is there any point in using RG400 over RG58? What advantages does it give? In what applications is it detectably better? Where is it just a waste of money over RG58? I am going digital and flat panel displays along with an IFR GPS/NAV/COM populated almost exclusively from Dynon. Is RG58 OK for VHF but RG400 is needed elsewhere, like on the Mode S Antenna?

    Back story is we went to install ADS-B in the wife's RANS S6 and they use RG174 for GPS antenna (two already in the bird, a third for WAAS is prewired with RG174 too. But our hangar mate and our A&P say RG400 is for the new transponder antenna. Please help me know which COAX I should use where?

    Billski
     
  2. Nov 6, 2019 #2

    Derswede

    Derswede

    Derswede

    Well-Known Member

    Joined:
    Jan 7, 2016
    Messages:
    839
    Likes Received:
    256
    Location:
    Central North Carolina
    For short runs, RG-58 works fine. I have pumped a half KW thru some without problems. The newer type of coax is lower loss, and most have a better shield that RG-58, and if you are having to fix it into a wing or similar, most of the newer coaxes have longer life sheaths. I use Mini-RG-8 for the majority of my antenna runs. That being said, I have several sections of plain old RG-8 and RG-8Mini that are over 20 years old that are in still excellent condition.

    The better shielding/low loss of LMR-400 or equivalent is great for "low signal" applications, but overkill in an airplane. Get a coax with decent shielding and a good sheath to keep the coax flexible and you should be good. One note on RG-58, I have a reel of it that was used for camera and data connections only. The braid shield is very light and will allow some RF to possibly make it to the center conductor. The mini-RG-8 I use for radio is Belden, not the cheapest, but it has good shielding qualities and is flexible and durable. Light as well. I have some that has been in the weather for 10+ years, still supple and no cracking or brittleness of the sheath. Most any ham radio retailer will have it at reasonable prices. Check for local "hamfests" as well, lots of antenna supplies cheap at those fleamarkets. Mounts, whips, etc are normally in abundance and an aircraft band antenna is only about 2 ft long for a quarter wave antenna (The typical aircraft band antenna).


    Derswede (N4ABA)
     
  3. Nov 7, 2019 #3

    Bill-Higdon

    Bill-Higdon

    Bill-Higdon

    Well-Known Member

    Joined:
    Feb 7, 2011
    Messages:
    444
    Likes Received:
    139
    Location:
    Salem, Oregon, USA
    If the Transponder run was like a lot of "big" and or military aircraft the yes us LMR-400 but for our birds a good quality RG-58 will work fine.
    PS I had to replace the Transponder cable on a UH-1M at Fort Bliss in the middle of the summer that was no fun.
     
  4. Nov 7, 2019 #4

    rdj

    rdj

    rdj

    Well-Known Member

    Joined:
    Sep 10, 2009
    Messages:
    293
    Likes Received:
    186
    Location:
    Northern California
    The Jim Weir column in the November 2019 issue of Kitplane discussed this. From the article: "Conclusion: For powers less than 50 watts, my cable of choice (if loss is desired or not a factor) is good old RG-174. But when it comes to powers greater than this, I’ll go with RG-58. RG-400 isn’t even in the picture."

    I don't always agree with Jim Weir, but have no quarrel with him here. In a GA plane, light weight and inexpensive is usually going to trump some nebulous few dBs of performance.

    What Jim also mentions in the article that I wasn't aware of before is this: "...some of the GPS/ADS-B machines require a loss of around 5 dB, which you can get with a 10-foot run of RG-174 that weighs about 4 ounces. Using the recommended RG-400, you will need roughly 30 feet of cable that weighs about 2 pounds. Not to mention finding space to coil up all that excess cable."
     
  5. Nov 7, 2019 #5

    Marc Zeitlin

    Marc Zeitlin

    Marc Zeitlin

    Well-Known Member

    Joined:
    Dec 11, 2015
    Messages:
    496
    Likes Received:
    681
    Location:
    Tehachapi, CA
    I only ever recommend RG-400. I don't give a crap about losses, or dB, or the RF issues (mostly because I'm an ME, not an EE). What I do care about, because I work on about 30 planes/year, many of which are 20 - 40 year old EZ types, is that the RG-58 that I see is almost always getting worn and stiff and the insulation on the outside is cracking and broken. I have never seen that on any RG-400 that I've installed or that is old. The dual grounding shields are also far more robust when installing end connectors. Maybe there's some very high quality RG-58 out there, but I've never seen it.

    So purely from a mechanical longevity point of view, I recommend the RG-400. With respect to cable length requirements, the only time I've seen length minima, it's been either 6 - 10 ft. and RG-400 is OK for those units. Always works, too.

    Don't skimp on the quality of any materials. You'll thank yourself 5, 10, 15, 20 years down the road, and so will the guy that owns your plane next (as well as the guy that works on it).
     
    wsimpso1 likes this.
  6. Nov 7, 2019 #6

    PW_Plack

    PW_Plack

    PW_Plack

    Well-Known Member

    Joined:
    May 25, 2015
    Messages:
    152
    Likes Received:
    71
    Location:
    West Valley City, UT
    More than power needs to be considered in choosing coax. The length, and the frequency of operation also affect loss. RG174 loses only about 1 dB in a 10-foot run at com frequencies, but loses 3 dB at transponder/ADS-B frequencies. A loss of 3 dB is losing 50% of the power transmitted through the cable.

    A reasonable upgrade to RG58 is RG-8X, aka RG-8 mini. It generally has 95% or better shielding, vs. as low as 50% for cheap CB-type cable, and lower loss, but isn't as expensive as RG-400. RG-8X is slightly larger in outside diameter than RG58, however, measuring .242 inches, same as typical TV coax (RG59), so connector shell sizes will have to be adjusted accordingly.
     
  7. Nov 7, 2019 #7

    Derswede

    Derswede

    Derswede

    Well-Known Member

    Joined:
    Jan 7, 2016
    Messages:
    839
    Likes Received:
    256
    Location:
    Central North Carolina
    Both Marc and Paul make excellent points. Life of the sheath is of major importance, as we have to add vibration to the package, and a poor sheath will age and fail much faster than quality cable. And do use a brand name. The cheap stuff that the CB shops sell tends to be Chinese and not only is shielding marginal, the life (and general quality) is normally poor. As Marc says, no one wants to have to dig out a cable which failed due to poor manufacturing. Most of my coax is Belden, most of the runs I am currently using are RG-8Mini and most are at least a decade old. My current mobile installation is 10 years old, the coax is still supple and no cracks or degradation of the shield. That is Belden cable. The center conductor is also important. you really do not want a solid wire center conductor anywhere that there may be some movement. Copper can work harden and break, even if the sheath and shield are still good. That can kill the transistors in the transmitter.

    Derswede
     
    Marc Zeitlin likes this.
  8. Nov 7, 2019 #8

    N804RV

    N804RV

    N804RV

    Well-Known Member HBA Supporter

    Joined:
    Jun 10, 2013
    Messages:
    162
    Likes Received:
    66
    Location:
    Mount Vernon, WA
    The Garmin install manual for the GNS 4XX recommends either RG142B or RG 400. For the newer transponders, you have to follow the manufacturer's requirements for minimum and maximum dB loss.

    RG-58 is fine for many applications. But, when doing avionics upgrades on our antique spam cans, I'd be hard pressed to leave old RG 58 in just to save a few pennies.
     
  9. Nov 8, 2019 #9

    wsimpso1

    wsimpso1

    wsimpso1

    Super Moderator Staff Member

    Joined:
    Oct 19, 2003
    Messages:
    5,893
    Likes Received:
    3,119
    Location:
    Saline Michigan
    Ok, I just bought a hundred feet of RG400, some ferrite rings and BNC fittings to replace the cable in place. Will commence rebuilding the wing wiring Monday.

    Thanks for the help with the thinking.

    Billski
     

Share This Page

arrow_white