RG-6 vs RG-58

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N4JWA, Jan 8, 2011.

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  1. KM1H

    KM1H Ham Member QRZ Page

    Maybe not to a nitpicker but a straight edge on the monitor sure looks mighty straight to me over any frequency I have any use for RG-6.

    Well I dont know if it was enjoyable in mid 20's temps and 30 mph winds but I did run that E-W 750' 2 wire Beverage today. That included trimming branches and removing several small trees, mounting on fence insulators, getting 6 ground rods cut (8' down to 4') with the tops tinned, and 10' of #12 copper conecting 2 groups of 3. Tomorrow is pounding in the rods, running out radials, hooking up the end transformers and running a couple of 200' lengths of RG-6 to the relay box. No time to play on the PC tomorrow until after its all done and tested, 12"+ snow coming tomorrow night; have to wind the transformers and drill boxes tonight.

    And "very long" was finally given a real number;)
  2. VK1OD

    VK1OD Ham Member QRZ Page

    Mike, a fairly good model of most practical coax lines at HF and above is that loss(dB)=k1*^f+k2*f.

    The model is a very good fit to most measured data, and for good reason. Because of skin effect, loss due to conductor resistance is proportional to square root of frequency and loss due to dielectric is usually very close to proportional to frequency.

    View attachment 50592

    Above is a plot of the two components of loss for RG213 and LMR400 (a foam dielectric 'equivalent' of RG8). Each of the loss components (the lower four lines) are in fact straight lines on a log-log plot, the different slopes reflecting the different exponent of frequency. (Click on the pic for a larger version.)

    When you add the two components for total loss for each of the cables, you no longer have a straight line, but a curve.

    If a cable uses CCS conductors, the conductor loss line will not be straight, but will curve upwards at lower frequencies due to more of the current flowing deeper in the resistive / magnetic material.

    The graph also explains why LMR400 has lower loss at 1MHz than RG8. It is not the dielectric loss that matters at this frequency, they are both very low. The conductor loss in LMR400 is less because a thicker centre conductor is used with the foam dielectric so as to obtain the same Zo in the same shield / jacket size.

    As frequency is reduced, eventually the loss model is not good because the centre conductor is too small for fully developed skin effect, even with homeneous conductors.

  3. W0BTU

    W0BTU Ham Member QRZ Page

    Thanks, I'll look at this math and plot etc. after I wake up a little more here. :)

    I wanted to ask a question here before I forget it. For people like us that use 75 ohm coax instead of 50 ohm, what would that mean if we bought an MFJ-259B (or similar) antenna analyzer? I thought I read something to the effect that they were "normalized to 50 ohms".
  4. VK1OD

    VK1OD Ham Member QRZ Page

    The impedance measured at the device terminals is displayed in absolute units.

    The displayed VSWR is calculated for a 50 ohm line from that measurement.


    PS: re the graphs, pay close attention to the colours and legend, the legend is not ordered top to bottom like the curves... my presentation mistake, but the graphs are accurate.
  5. W0BTU

    W0BTU Ham Member QRZ Page

    That's what I was afraid of. I don't own an antenna analyzer, but the one I eventually will purchase won't have that "feature".
  6. KA4DQJ

    KA4DQJ Ham Member QRZ Page

    Several years ago I ended up with a ton of the RG-6 too, and use it on everything from verticals, mobiles and NVIS. 75 ohms or not, I don't see any difference between resonance and performance compared to RG-58. I use an antenna analyzer when setting up my antennas, and just set the instrument on 75 instead of 50 ohms. For whatever reason, the transmitters dont seem to know the difference. If the analyzer says resonant... the transmitters agree.
  7. W0BTU

    W0BTU Ham Member QRZ Page

    What make and model of antenna analyzer do you have that is selectable between 75 and 50 ohms? That's exactly what I need.
  8. KA4DQJ

    KA4DQJ Ham Member QRZ Page

    I'm not at home and can't think of the model #, but it's an MFJ-XXXX.
  9. KM1H

    KM1H Ham Member QRZ Page

    I wasnt aware the 259B could be switched to 75 Ohms. I just learned to tolerate the 1.5:1 VSWR reading and mentally shift accordingly.
  10. KD0CAC

    KD0CAC Ham Member QRZ Page

    I wanted to see the model number also , I would need to get the page number of the manual also , I would like to use that adjustment .
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