Maximum Coax Length

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WB8FUY, Aug 6, 2020.

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  1. WB8FUY

    WB8FUY Ham Member QRZ Page

    I just built a new house and may need to install my vertical in the far back corner. The coax run would be about 200 feet from the shack. Just looking for info on whether the losses would be prohibitive at this length. Also wonder if I can run the coax on top of the ground (it's wooded between the shack and the back field). I am assuming I would need to bury the end of it under the radials. Will this even work or should I be considering going the flagpole route. Thanks for any feedback and/or suggestions
     
  2. NK7Z

    NK7Z Premium Subscriber QRZ Page

    Hi,
    I hope you are using low loss coax, and it is always a good idea to bury coax... I would rent a trencher, and bury a hunk of drainage pipe, then pull coax through that... See:

    https://www.nk7z.net/notes-on-tuning-a-6btv/

    for info on my antenna field.
     
    WB8FUY, KB0MNM and AJ5J like this.
  3. K1VSK

    K1VSK Ham Member QRZ Page

    There are numerous coax loss calculators on the internet. Plug in the type coax, anticipated length and frequency of interest and you will be able to compare losses of various types and sizes of coax. For that length run, also look at the comparable loss by using hard line. It all depends on how much a few dB is worth to you.
    Burying it protects the Feedline from critters and the ubiquitous lawn tractor cutting head.
     
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  4. KB7WG

    KB7WG Ham Member QRZ Page

    200' twin open wire, two 4/1 baluns. At least 10' high. until antenna. A few hours of measurement and a few weeks of operation to decide. Cheap, fast, easy to erect. Fast and easy to replace. Have you ever dug trenches around trees?
     
    AJ5J likes this.
  5. WB5OYP

    WB5OYP Ham Member QRZ Page

    Don’t forget the coax loss calculators are, in general, valid only if the coax is terminated in its characteristic impedance. So if your antenna looks like 50 ohms restive, ok, otherwise your losses will be much higher.
     
    WB8FUY likes this.
  6. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Yup, do the math.
     
    AJ5J likes this.
  7. W4HWD

    W4HWD Platinum Subscriber Platinum Subscriber QRZ Page

    WB8FUY, N0TZU, KL7SG and 2 others like this.
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    As posted above, running decent coax like RG-213, LMR-400 or similar and losses will be meaningless in a 200 foot run to a decently matched HF antenna.

    For example on 20m the matched loss in a 200 foot run of LMR-400 is approximately 0.9 dB and only raises to approximately 1.1 dB at a 2:1 SWR. If that coax run lets you run a better antenna than the flagpole you mention then don't stress over the coax loss but do use good quality coax.
     
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  9. KB0MNM

    KB0MNM Ham Member QRZ Page

    I looked up the address, and can understand why you might want to run coax over the top of the ground. Some parts of Peoria, Az. may have been plowed for agriculture, yet others might require more effort than soil in a Wisconsin valley. As others have stated, you leave the coax vulnerable to lawn mowers and other potential problems if it is not buried. The sun can also degrade common schedule 40 ( white ) drain pipe- a reason to consider schedule 80. There are a few other things to consider- surge suppression ( folks will tell you that lightning will come as no Suprise- pun intended - at your location ); electrical ( in addition to RF ) grounds; structural support for the vertical. You may also want to consider extra diameter in the conduit for other transmission and/or rotor cables, plus the radius of the 'sweeps' ( it is hard to pull around a corner if the angle is tight ). Since you are in the planning stage, consider an extra length of Times Microwave LDF4 compared to the 'softer' coax. There is also the matter of water- it does rain in Arizona, and drainage should be planned for in any buried pipe. All sorts of critters ( snakes, scorpions, etc. ) should not be given easy access. I have no financial interest in Times Microwave, yet mentioned LDF4 as a consideration for potential future use. You may find that there is a buried construction contractor who can give guidance on an approximate cost for a conduit- without a large estimate fee. There are a few fellow amateur radio operators who may still be working at two-way radio shops in the Phoenix area. When/if hamfests are resumed, that would be a good opportunity to meet others who have done what you intend.
     
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  10. WA7F

    WA7F Premium Subscriber QRZ Page

    IMO - It’s best to future proof. It is cheaper to do right once than to upgrade/rework later. Your interests and antenna configurations will probably change in time. Bury pvc conduit that is 2-3 times bigger than you think you will ever need and don’t go cheap on the transmission line.
     
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