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Problem

I'm planning on mounting a vertical HF multiband antenna on the edge of my roof.

Due to the presence of a paved walkway and some other similar property constraints, I have no room for radials anywhere near the mounting area.

However, I do have a saltwater canal behind my yard.

Antenna/House/Canal Drawing This is a rough and not-to-scale rendering of the situation. The red tapered cylinder represents the vertical antenna. The green part is the yard. Then there is the seawall and the canal itself. The yellow cylinder would be a floating foil-lined tube to maximize surface area and maintain water surface contact as the tides rise and fall.

The base of the antenna will be about 10ft above the surrounding grade and the canal is about 60ft away from the antenna mounting point.

Questions:

  1. Can this idea work to provide a good ground for transmission?
  2. How should I connect the antenna to this somewhat distant ground?

Thoughts:

If I were just to try this without seeking advice, I'd run coax with the center conductor attached to the antenna base ground out to the floating tube, probably in some PVC pipe to protect the cable itself from the soil. Is that a reasonable plan?

I've tried modeling this in CocoaNEC, but the transmission line interface is broken and the project hasn't seen active development for a while.

I've considered mounting the antenna on the sea wall itself, but the surrounding area makes that tricky.

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With the canal 60 feet away it's not of much help.

The objective with a monopole antenna is not just to have any ground connection, but to have a low-loss ground plane under the base of the antenna. The ground plane provides the return current and creates an image antenna. The return current density is highest where it converges at the base of the antenna.

As a rule of thumb, a radius of a quarter-wavelength around the base is where the ground is most important. Small improvements can be realized by extending the ground out as far as two wavelengths. Beyond that, the ground is insignificant.

The canal is so far away, and only on one side of the antenna, so it will have negligible effect on the antenna. The antenna's operation will be dominated by things closer to the base, like the feedline, or the wire you run from the antenna to the canal. You'll essentially have a vertical with one radial.

If at all possible, I'd suggest considering alternate antenna designs. Verticals can be made to work well with as few as two radials, but this involves tuning them to be resonant, and raising them several feet off the ground. This minimizes the current in the soil, thus minimizing losses.

Without deliberately installing radials you will make some contacts, and if that's all you're able to install then by all means go for it. But it's far from ideal.

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  • $\begingroup$ Thank you for the thorough reply. I appreciate it. I could put some very asymmetrically distributed radials along the eaves of the roof. Alternatively I could revisit the idea of mounting the antenna on the dock just past the sea wall, but that would require some significant work and some sort of short tower to get high enough above the banks of the canal. $\endgroup$ – Eric Feb 11 '18 at 15:37
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    $\begingroup$ @Eric Radials on the roof is probably not too bad, even if they aren't symmetrical. $\endgroup$ – Phil Frost - W8II Feb 12 '18 at 2:32
  • $\begingroup$ I agree with Phil. ANY antenna is better than none. If you could make the radials even a TINY bit symmetrical it would help, but sometimes the ideal is not achievable. 2 is the minimum number of radials for decent results. More radials will help the pattern and reduce the effects of the asymmetry, as well as help efficiency. Even 3 radials will be better than 2. In the end the only thing that is valid is to TRY IT! If it doesn't work, go back to the sea wall idea, keeping these comments in mind. $\endgroup$ – Keith Martineau Feb 20 '18 at 23:57
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After many years of quite different antennas, I tried the Cush-Craft R5 vertical, since it's designed to compensate for all the variables. My situation is on Mustang Island, on the Texas Gulf Coast, thus I'm quite well acquainted with salt water and salt air. Salt water grounds need attention regularly to check on efficiency, as well as salt air degradation of antennas.

Gil Gibbs WA5YKK

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    $\begingroup$ This answer does not say anything about whether the proposal in the question will or will not work. $\endgroup$ – Kevin Reid AG6YO Feb 12 '18 at 21:49
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    $\begingroup$ @GilGibbs Welcome to HAM Stack Exchange. I would like to invite you to take the tour and look at the help specifically regarding answering questions. Unlike other "forums" this site is a question/answer site, not really for discussions. Your particular answer has not answered the question, so you have now attracted a negative vote. We would like to encourage you to participate in this site. Keep in mind that this is not a discussion forum. $\endgroup$ – Edwin van Mierlo Feb 13 '18 at 10:55
  • $\begingroup$ I agree with Kevin and Edwin. And if you're saying that the R5 can "compensate for all the variables" here, kindly edit your answer to tell us how. Does it have elevated radials (as Phil said), which is what is needed here? $\endgroup$ – Mike Waters Feb 13 '18 at 17:30
  • $\begingroup$ @EdwinvanMierlo Nice helpful and friendly comment! :-) $\endgroup$ – Mike Waters Feb 13 '18 at 17:34

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