coax and patch cord lengths

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KA0EIV, Nov 18, 2017.

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  1. KA0EIV

    KA0EIV Ham Member QRZ Page

    As I've stated before I'm getting back into the hobby after sitting on the sidelines for 20yrs or so putting my kids interests first while they were home. Now it's time to get back into this great hobby. In setting everything up I'm going to need several patch cords and somewhere in my foggy grey matter called a brain it seems like" cords and transmission lines should be odd multiplies of 1/2 wavelength according to freq in use." Is this right and do you take into consideration the velocity factor of ur coax? My main interest is 6m, 2m, 222mhz, and 432mhz all modes.
    tnx
    Ron
     
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Hi Ron, welcome back!

    No, feed lines should be no longer than necessary to connect the antenna to the radio.

    73, bill
     
    NE1U likes this.
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    And a few feet extra to allow moving the unit around!

    There are situations where an exact length is needed such as with phasing lines when "stacking" antennas. But, in general, the shortest length possible will have the least attenuation in the feed line.

    Glen, K9STH
     
    NE1U likes this.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Patch cables and transmission lines have no specific lengths that work best, assuming the termination is reasonably well matched to 50 Ohms.

    When terminated in a mismatch (load not 50 Ohms), cable lengths can matter as they become linear conjugate matching transformers.
     
  5. KA0EIV

    KA0EIV Ham Member QRZ Page

    I think I read somewhere too, that transmission line length is the square root of the square of the distance required to reach the antenna. Got it lol
     
    WD0BCT, K2LCK, KC8VWM and 2 others like this.
  6. W0AAT

    W0AAT XML Subscriber QRZ Page

    The old 1/2 wave odd multiple was for the Mirage bricks that had iffy input matching...
     
  7. W1GCI

    W1GCI Ham Member QRZ Page

    It's important to be practical about these things and sometimes when you add up the losses you'll find that - depending on your intended use - fussing over a length of cable isn't significant. Cable, connectors, and devices all contribute loss. PL-259 connectors at vhf and uhf contribute more loss than N type connectors. Having a watt meter in the line also contributes to the over all system loss. But does it matter? If you are doing moon bounce or very weak signal scatter contacts, it may. But if you're doing VHF contesting where signals from the other end are typically 20db out of the noise, the extra 1 db of system loss isn't going to be noticed. I, like most, believe in reducing losses in a practical way, but not to the point of making yourself crazy. If RG-213 cable is .52 db loss at 10 feet long and .26 db at 5 feet, will that extra .26 db of gain make that much difference? If so, use LMR-400, which is about 1/2 the loss of 9913. By the way, if you put PL-259 connectors on it rather than N connectors, you've given back the loss you saved with the shorter cable.

    arnie
     
  8. W2WDX

    W2WDX Subscriber QRZ Page

    Length is not critical.

    However, impedance match is. Even a slight increase in SWR at VHF/UHF increases coaxial transmission line loss significantly. The one application where SWR is truly critical, IMO. The specified losses of cables are always (or should be) listed with at a specified SWR on the line at a specified length.

    For instance at 144.2 MHz at 50' with 100W input at an SWR of 1.8:1:

    RG-58 will have a total loss of 2.9dB. You get about 50W at the antenna.
    RG-213 will have a total loss of 1.5dB. You get about 71W at the antenna.
    RG-8 will have a total loss of .88dB. You get about 80W at the antenna.
    Times LMR-400 will have a total loss of nearly .84dB. You get about 82W at the antenna.

    Things get different at 442 MHz at the same length, same power and SWR:

    RG-58: Loss: 5.4dB Power at ant: 29W
    RG-213: Loss: 2.7dB, Power at ant: 53W
    RG-8: Loss: 1.6dB, Power at ant: 69W
    Times LMR-400: Loss: 1.5dB, Power at ant: 71W

    And this does not include the aforementioned losses from connector type, antenna ground loss, transceiver output roll-back, and other factors. And losses are cumulative. So you can see buying a known brand of LMR-400 makes a difference at VHF/UHF at even a slightly high SWR on the line. Not all LMR-400 (or any type) are the same. Many of the Chinese brands called LMR-400 are simply re-labeled RG-8 and exhibit similar losses to RG-8. However, bear in mind major brand RG-8 may present a good overall value over LMR-400 unless you start working above the 70cm band or have very long lengths (over 100').

    So even if you have a high gain antenna like a high-gain vertical or yagi, you could lose much of that gain before you even get to the antenna in question (excluding the benefits of radiation pattern). And those losses go both ways, transmit and receive. So tune that antenna well and use good coaxial.
     
    Last edited: Nov 27, 2017
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  9. W2WDX

    W2WDX Subscriber QRZ Page

    One last point on connectors. PL-259 losses versus just about every other connector is quite significant; refer to the charts below. The red lines are various brands of UHF connectors. The blue line are everything else; Types N, SMA, etc.

    [​IMG]

    Clearly PL-259 get really "lossy" above 50MHz compared to others like Type-N.
     
    KX4O likes this.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You might want to go back and re-read that article.

    For example on the charts (graphs), the red lines aren't different kinds of PL-259s. The very "lowest" red line is for a PL-259/SO-239 (UHF male/UHF female). That's the line that shows the PL-259 "loss" to be exactly as low as Type N at several frequencies in the VHF-UHF spectrum, like at 280, 340 and 455 MHz -- with the PL-259 actually having slightly lower loss than type N at 455-460 MHz.

    All the other "red lines" aren't different kinds of PL-259s: They are PL-259 double female adapters (often called "barrels" and "bulkhead feedthroughs"), and the longer they are, the worse they are. But the problem isn't the PL-259s, it's the barrel adapters, which other than the mil-spec shortie PL-258 adapter (not a bulkhead, it's just an inline coaxial splice), are almost all terrible.
     
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