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So for a HF vertical antenna (DX commander Rapide), what is the longest feed line that that I can practically use using high-quality coax? I realize shorter is better, but my HOA (my wife) is raising objections on having the antenna too close to the house. I'm new at this, and would appreciate any advice.

Thanks,

John

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    $\begingroup$ Some of the most competitive contest stations you'll ever see have multiple towers, hundreds of feet from the shack, and then another 100 or 150 feet up the tower to get to the antenna. The loss from that much feedline is a lot less than the gain from having great antennas up high. $\endgroup$ Commented May 9 at 17:20
  • $\begingroup$ You may be asking the wrong question. The correct question may be how close to the house can the antenna be and remain safe. I suggest checking out arrl.org/rf-exposure-calculator $\endgroup$
    – user10489
    Commented May 11 at 12:33
  • $\begingroup$ Isn't it least-obstructive when roof-mounted? No trip hazard, and doesn't spoil the views out the windows. $\endgroup$
    – glen_geek
    Commented May 11 at 17:56
  • $\begingroup$ FYI I've seen feed line that carried >20KW and was half a mile long. It looked like sewer pipe. There is no practical feed line limit or an upper limit to quality, just practical cost limit. $\endgroup$
    – user10489
    Commented May 12 at 11:37

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There's no limit, really. Losses at HF aren't very high, and you're basically just trading cost for low-lossness. I'd go the other way around:

  1. Write down the maximum TX power you want to work at. This simply eliminates any cabling that can't deal with that power.
  2. Set yourself the distance you want to be from the radio. It's not like this is really negotiable. There's probably an acceptable distance for your HOA, and going closer isn't nice.
  3. Set yourself a maximum loss that you can tolerate (don't go "as low-loss as possible", set yourself a maximum loss. Since price and loss are conflicting optimization goals, you'd otherwise would need to introduce an "acceptable loss per $$$" function, and that's going to be hard. You can then get the cheapest cable that still does what you need.)
  4. divide maximum allowable loss by distance, and get the worst-case allowable attenuation-per-length (typically, in dB/100ft or dB/m).
  5. Go to any reputable vendor of cabling (you're in the US, I don't know really who that would be; I'd start with the catalog distributors of the mouser, digikey and element14 kind if I can't find anything better). Go to the right category, select cabling with the right impedance (75 Ω, or maybe 50 Ω if you're not that high-powered); sort by price per meter. Buy cheapest that has loss lower or equal as calculation in 4.

It's very possible that acceptable lengths are in the hundreds of meters, or just a couple meters; it really depends on the factors 1-3.

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You have to look at coax specifications and see what the loss is per foot/meter at the frequency range you intend it for, and see if that's acceptable. I run Belden 9913 to all of my antennas, and my VHF/UHF run is about 100'; one of my HF runs is about 150'...are there better coax cables now? yes; would I notice a difference between better coax and if my coax runs were 25'? ...possibly, but then my antennas wouldn't be as high as they are, and my XYL would probably complain about seeing an antenna farm next to the house. Far more important to me is noise suppression and impedance matching (low VSWR). So the answer is: most things are a compromise, you just have to find the balance that works for you.

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What does "practical" mean to you?

Depending on your definition of "practical" very long. Taking your chosen feed line's loss rating at face value according to this table (reproduced in part below), you can determine practicality:

Coax Cable Signal Loss (Attenuation) in dB per 100ft*

Loss* RG-174 RG-58 RG-8X RG-213 RG-6 RG-11 RF-9914 RF-9913
1MHz 1.9dB 0.4dB 0.5dB 0.2dB 0.2dB 0.2dB 0.3dB 0.2dB
10MHz 3.3dB 1.4dB 1.0dB 0.6dB 0.6dB 0.4dB 0.5dB 0.4dB
50MHz 6.6dB 3.3dB 2.5dB 1.6dB 1.4dB 1.0dB 1.1dB 0.9dB
Imped 50ohm 50ohm 50ohm 50ohm 75ohm 75ohm 50ohm 50ohm

*Note: Coax losses shown above are for 100 feet lengths. Loss is a length multiplier, so a 200 ft length would have twice the loss shown above and a 50 ft length would have half the loss.

Using the above table, if

  • you need 200 ft of feedline to satisfy the requirements of domestic tranquility, and
  • you are fine with, for example, up to 2dB of loss in your line at HF freqs,

that would imply that any feed line with < 1 dB/100ft of loss at 10 MHz would be practical for your application.

There are more feed line options available than are listed in the table.

Other considerations

For receive, you may want to look at installing a LNA at or near the feed point to overcome line loss for weak signals.

Additional items to consider for practicality:

  • locating the transceiver at the feed point and operating it remotely will obviously cut feedline loss to near zero at the cost of figuring out a weather proof enclosure for the radio and how to operate it remotely.
  • adding a transmit amplifier in the shack will enable greater power delivery to the antenna at the cost of additional $$
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    $\begingroup$ Nice answer, and a lot of work! Re: "twice the loss": In dB! (so, if 100ft have 8 dB of loss, then 200 ft have 16 dB; it's an actual squaring of loss every time you double the length, but that's exactly why we use decibel) $\endgroup$ Commented May 9 at 14:30
  • $\begingroup$ Thank you, and Yes! I merely copied the footnote from the linked site for completeness... I should probably update that footnote with a footnote. A feetnote? $\endgroup$
    – webmarc
    Commented May 9 at 15:30
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    $\begingroup$ feedfootnotenote? feednotefoodnote? $\endgroup$ Commented May 9 at 15:37

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