Proper vertical antenna height for different bands

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W3WD, Jun 5, 2021.

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  1. W3WD

    W3WD Ham Member QRZ Page

    Hi guys,

    I'm running that Shakespeare 120-60 kit and it has eight 4-foot sections, for a total of about 32 feet.

    It's connected to an AH-4 at the base.

    I was planning to assemble the full height and run it on both 20M and 40M, but I ran across this post from 2018 in a thread entitled "33 ft vs 43 ft vertical" (see attached, thanks WA7ARK!).

    Is it likely that I'll blow up my tuner trying to run 20M on a 32' vertical? I think I need a refresher on non-resonant vs. anti-resonant, etc. and what total lengths/wavelength fractions are appropriate for which bands.

    Plan B is to assemble the top 4 sections with a height of about 16' and run 20M on that (1/4 wave).

    Please educate me!

    Attached Files:

  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    That antenna is designed and marketed to users who are unlikely to transmit into it at 14MHz. If you give me an idea of your plan for radial wires (how many, how long, how high?), I will find an optimum length for the monopole (less than 32ft) that will allow the AH4 to tune it on as many bands as possible.

    In addition to radials, are you planning to drive a ground rod under the tuner?
    W3WD likes this.
  3. KI8DJ

    KI8DJ Ham Member QRZ Page

    Make it about 24 feet if you can that's a non resonant length that works well on both 20 and 40 meters with a decent racial system. Higher bands should be good too although 6 meters will have a high radiation angle.
    AK5B, KF5KWO and W3WD like this.
  4. W3WD

    W3WD Ham Member QRZ Page

    Thanks guys-- portable setup designed to be configured, used, and then taken down within a few hours or a day (in good weather).

    Pic below-- very short (2") leads top and bottom of the tuner-- bottom one (ground) attached directly to the hex ferrule with the radials. No separate ground rod, etc.

    Got a 2x6 board on the grass at the base of the antenna/tuner for a couple of power cable chokes in series and a coax feedline choke. Add'l chokes at the shack.

    Radials: 30 wires (insulated, stranded 18AWG) in 6 bunches of 5, screwed into the hex ferrule and laid directly on the grass. Length of radials is 5M (~16').

    I don't mind adding/removing antenna sections for optimization-- i.e. I don't need one length for both 20M and 40M. I can reconfigure when I switch frequencies.

    I just don't want to blow up my new equipment.

    IC-7300 with 100W max but I have a long coax run so will incur some losses. I plan to start out at 25-50W or so with full length antenna (32') on 40M and 16' on 20M (1/4 wave for both)-- UNLESS you tell me otherwise!

    Very grateful for all the tech help, much appreciated.


    Attached Files:

  5. KQ0J

    KQ0J XML Subscriber QRZ Page

    I had great results with a 29 Foot monopole with the tuner at the base and 40 radials. Good for 60 -17 meters , OK on 15-10...
    Great DX antenna on 30M .
    W3WD and AK5B like this.
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    I forgot the details of your previous posts... That is why it is better to just append a new question to the previous forum thread, rather than starting a new one.... We lost all of the previous context...

    Your radials are "floating" (insulated, on the surface, not buried), and there is no Galvanic connection to the earth under the antenna (no ground rod). Your radials are also not adjusted to "resonate" the antenna to any specific band, so I will treat this as a not-resonant-on-any-ham-band type of vertical antenna.

    Here is my model:
    I'm using 32 radials because it is easier than custom making exactly 30.

    Here are the Rs and jXs that the tuner sees on various bands if you use the full height on 40, 30, and 17m bands, and shorten the antenna by one 4ft section on 20m and 15m, and further shorten it to 20ft on 10m (L is the length of the monopole in ft):
    The tuner should be happy driving into these impedances.:)

    The reason for shortening it to 28 ft on 20m it to avoid anti-resonance. The reason for shortening it on 15 and 10m is partially to help the tuner, but the main reason is to lower the elevation of peak radiation.

    When you get it working, send me a PM and I can meet you on any band of your choosing where it is likely that you will have propagation to Az... (I just want an excuse to show off my new 40m to 6m beam);)
    Last edited: Jun 5, 2021
    W9XMT and W3WD like this.
  7. W3WD

    W3WD Ham Member QRZ Page


    Thanks very much and I'll likely take you up on that-- thanks for running the numbers too.

    It's a ground stake kit-- so perhaps there is the ground that you mentioned?

    Pic of latest setup attached-- the ground stake (18" or so) is going down in the ground through the hole in the 2x6-- the metal hex ferrule (to which the radials are attached) screws onto the ground stake.

    Attached Files:

  8. W9XMT

    W9XMT Ham Member QRZ Page

    Below is a NEC4.2 analysis of a 32' monopole antenna system based closely on the description given earlier in this thread.

    Its feedpoint Z on 14.2 MHz without a matching network is about 600 +j 750 Ω. The L network shown in the inset on the polar plot will match that to a nominal 50 Ω, using the values shaded there in green.

    W3WD likes this.

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