Callsign
ad: dxeng
Page 1 of 2 1 2 LastLast
Results 1 to 10 of 17

Thread: What is the best antenna feedline length?

ad: L-HROutlet
ad: l-assoc
ad: l-Waters
ad: l-rl
ad: l-gcopper
ad: l-innov
ad: l-WarrenG
ad: l-tentec
  1. #1
    Join Date
    May 2009
    Location
    Toronto
    Posts
    184

    Default What is the best antenna feedline length?

    I need some expert help...

    I have an mfj-1775 dipole antenna for 40, 20, 15 meters and I am trying to get the most efficiency as I can so I did some research on coax feedline lengths and came up with 55.5 feet of RG8/U to be what I need, this is based on the formula: Length=(492*volFac of coax)/F(mhz). This length gives me some multiple of 1/2 wave length.

    I am confused as I play around with other lengths of the same coax I really don't notice much difference in signal strength or noise, example the 39.5 feet of RG8/U seem even better than the 55.5 feet of the same coax.

    So the question is this, what is the best length of coax that can be use to improve the performance of your antenna system, and I have I missed something here?

    Should I used the shortest 1/2 wave length that I can user?

    note: Though I can hear well, I am still have some troube being heard(at 100%)

    73,
    -Peter

  2. #2
    Join Date
    Jan 2000
    Location
    Kilowatt Alley
    Posts
    9,892

    Default

    As far as I am concerned, the best length of coaxial cable is the length that gets from the antenna to the rig.
    Used lots of antennas through the years and never gave it any thought beyond that.
    Never had any problems either.
    Chances are that you may always hear some DX stations that cannot hear you. There are several possible explanations for that some of which may have more to do with the other station than yours.
    "Oh what a tangled web we weave when first we practice to receive."
    -Otto Watt Sept. 5 1925

  3. #3

    Default

    When you move away from the "design" frequency the length is no longer a half-wave!

    Basically, except for some very specific applications, there is no "magic" length of coaxial cable. Basically, the length from the antenna to the rig plus maybe a couple of feet to allow the rig to be moved is generally optimum!

    The concept of having only specific lengths of coaxial cable is one of those "olde wyves tayles" left over from "CB" operation. When the antenna is properly "matched", then the only thing you have to worry about is the attenuation in the cable and on HF that is generally not a concern except for some really long runs of cable.

    Glen, K9STH

  4. #4
    Join Date
    Feb 2002
    Location
    EM93wa: Ladson - about 18 miles NW of Charleston, SC
    Posts
    1,482

    Default

    Quote Originally Posted by K7MH View Post
    As far as I am concerned, the best length of coaxial cable is the length that gets from the antenna to the rig......
    I have found that usually works best.

  5. #5
    Join Date
    May 2009
    Location
    Toronto
    Posts
    184

    Default

    Thanks for the feedback...

    What about the theory that you need a 1/2 wave length or some multiple of, and what about taking the velocity factor of the coax into play, these don't mean anything?

  6. #6
    Join Date
    Jan 2000
    Location
    Commerce MI (Detroit area)
    Posts
    8,149

    Default

    The halfwave coax idea may have originated in the CB world where antenna theory = voodoo arts. May have something to do with getting a repeated SWR reading at the feedpoint when read at the radio...this only would be true for a very limited range of frequencies ie, one band.//
    If your antenna is trimmed for near 50 ohms with not much reactance at the feed point, ANY length of 50 ohm coax will work, the shorter the length the lower the loss will be.
    SO...the shortest length that reaches from the antenna to the radio is the BEST length.
    For non-resonant antennas, this is a different case alltogether, the length of balanced line can be part of the Z matching system and can be critical.
    73.....JD, FISTS #3853,cc 455,SKCC # 1395,tribune #12,
    Official US Taxpayer

  7. #7
    Join Date
    Apr 2002
    Location
    Rochester, MN
    Posts
    19,940

    Default

    Quote Originally Posted by VE3PCD View Post
    Thanks for the feedback...

    What about the theory that you need a 1/2 wave length or some multiple of, and what about taking the velocity factor of the coax into play, these don't mean anything?
    No, if the antenna is properly matched, the line length doesn't matter. If the SWR is 1:1 at the antenna, it will be 1:1 everywhere on the line.

    Make your coax whatever convenient length you need. If this were not the case, you'd need different lengths of coax for each band.
    EchoLink, IRLP, Allstar and DSTAR linking - adding interest to repeaters worldwide 24X7

  8. #8
    Join Date
    Sep 2009
    Location
    Driffield, East Yorkshire, England
    Posts
    1,665

    Default

    Quote Originally Posted by VE3PCD View Post
    So the question is this, what is the best length of coax that can be use to improve the performance of your antenna system, and I have I missed something here?
    The shortest length to get from the transceiver to the antenna is the correct answer.
    Should I used the shortest 1/2 wave length that I can user?
    No. Only CBers and morons who buy "no groundplanes or counterpoise required" verticals other than vertical dipoles need to use co-ax in 1/4 or 1/2 wavelengths as those types of antenna use the feedline to make up the missing half.

  9. #9

    Default

    Quote Originally Posted by VE3PCD View Post
    What about the theory that you need a 1/2 wave length or some multiple of, and what about taking the velocity factor of the coax into play, these don't mean anything?
    If it is a 50 ohm antenna, for matching purposes, the 50 ohm coax length doesn't matter. What does matter is if the antenna feedpoint impedance is 50 ohms and the characteristic impedance of the transmission line is not 50 ohms.

    For instance, 1/2WL is a good length to choose for 450 ohm ladder-line feeding a 130' (80m) dipole.

    The SWR on a 50 ohm antenna being fed with 50 ohm coax is 1:1 so, for matching purposes, the feedline length and velocity factor doesn't matter.

    The SWR on a 50 ohm antenna being fed with 450 ohm ladder-line is 9:1 so, for matching purposes, the feedline length and velocity factor does matter.
    73, Cecil, www.w5dxp.com
    Can CO2 emissions save us from the coming ice age?

  10. #10

    Default

    As long as the antenna is matched to the coax, the only effect of the length of the line is insertion loss. This is the attenuation of the signal due to the resistance in the cable. It is pretty small in the HF bands if you are using good coax. Make sure the connectors are tight and terminated properly and feel free to give yourself the freedom of a few extra feet of line!

Page 1 of 2 1 2 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •