What is an acceptable loss for you for your coax on the HF frequencies?

Discussion in 'Ham Radio Discussions' started by KN4DQE, Nov 6, 2018.

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  1. KN4DQE

    KN4DQE Ham Member QRZ Page

    I am presently running barefoot at 100 watts on ~100' run of LMR-400 to the antenna. The highest SWR I am seeing is about 2.5, but mostly it hovers around 1.5 or so. According to one of the coax calculators that should put be at around 89 watts out to the antenna, at 20m. That's not too bad. Up around the 2.5 SWR region drops me down to about 86 watts out.

    Now - I've recently acquired an AL-80B, and would love to make use of the watts that I've paid for. I am going to shoot for 800 watts give or take, when I get it up and running. The loss for that power using the same setup starts to increase quite a bit. At 1.5 SWR I'm down about 85 watts. At the 2.5 SWR I'm down about 109 watts.

    I started playing around with the coax calculator and see that 1/2" heliax gets me down to about a 50 watt loss at the 2.5 SWR. Going to 7/8" heliax gets me to about 33 watts of loss. The Antenna Farm sells some 7/8" Commscope Heliax (I bought the 7/8" for my UHF/VHF station awhile back), at about $2.95 per foot. So - for me to run almost 100' would cost almost $300. I know that folks love to point out that the difference between 30 watt loss vs 109 watt loss would probably be negligible at best - but considering that I paid so much for the amp, why would I not upgrade the cable?

    I'm just curious to know how much of a loss is acceptable in your views?
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    What are your antennas?

    Can they not be tuned for a better match, which would slightly reduce cable loss?

    But to the original question, I always consider "2 dB" the max loss I'd like to have on HF. On VHF-UHF, less than that if possible, as most signals I work (SSB/CW) are close to noise level so a dB actually makes a difference.

    On HF, "QSB" is normally much more than a dB, and we can't control that; but also most signals aren't that close to the noise.

    I run a kW output on HF quite a lot and mid-QSO often just cut it back to 500W and it's pretty rare for the other station to even notice.:p
  3. K7JEM

    K7JEM Ham Member QRZ Page

    Don't look at loss in terms of raw watts, look at it in dB. Your loss could only improve by less than a dB after all of that expenditure, so probably not worth it. No one you are talking to is really going to see a dB of change, on their end that would be like going from S6 to S6.2 on the meter of their RX. IOW, not noticeable.
    K1TGX and K7TRF like this.
  4. KK5JY

    KK5JY Premium Subscriber QRZ Page

    Keep in mind that cable loss is very frequency-dependent. Losses on longer wavelengths -- where power is more helpful for overcoming atmospheric noise -- are quite a bit less than they are at shorter wavelengths.

    And as @K7JEM pointed out, it's dB changes that matter. Even if your coax run burns half of your power, you are only down by ~3dB, or half of a "standard" S-unit.

    The approach I would suggest is to balance the cost of antenna gain (or loss), coax loss, and amplifier gain.
    NL7W and K7JEM like this.
  5. KN4DQE

    KN4DQE Ham Member QRZ Page

    I'm running a Hustler 6BTV (all I can do due to my HOA). I know it's probably only a dB, but just curious to know if it's worth chasing. Unfortunately I can't get the SWR on the antenna lower (I've tried tuning the traps as listed online to not avail). The other thing I was considering is to get an autotuner and place it out by the antenna (MFJ-998RT looks like it may be good, if sealed up in a weatherproof box). But that goes for about $6-700.
  6. K7JEM

    K7JEM Ham Member QRZ Page

    Not worth chasing unless you just have tons of extra money to spend. In electronics, it is possible to see changes that are not really detectable in the real world. This is one of those cases.

    Looking at a dB calculator, going from 100 watts to 85 watts is only .7 dB. That is really nothing.
    N4FZ and KK5JY like this.
  7. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    ^^^ couldn’t agree more^^^
    The cable is the cable... the loss is in the sauce.
    You are running decent cable and the losses are not worth worrying about.
    Ham radio is a cool hobby and not all investment needs to be justified! In this case you could give or take the dB and it wouldn’t amount to much.
    KE5OFJ likes this.
  8. WA4ILH

    WA4ILH Subscriber QRZ Page

    If you atre NOT running high power (>300 watts or so) MFJ makes an auto tuner in the $250.00 range. Of course, if you want the official ICom, Kenwood or Yeasue tuner, you will spend more.
    Tom WA4ILH
  9. WA9SVD

    WA9SVD Ham Member QRZ Page

    A dB is a dB is a dB... Whether you run 100 watts, or a thousand. If you have a SWR that gives you 85 Watts to the antenna, that is 85%, and 85% will apply regardless of the power level in use. Naturally, yo0u would "lose" more power at 1000 Watts than at 100 Watts, but the % should be the same.
    K2XT likes this.
  10. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    Just connect the radio directly to the antenna.

    ...No feedline required, problem solved.
    KE5OFJ, KA9JLM and W5TTW like this.

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