Example of capacitance hat on 40m wire dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KK4WMX, Jun 1, 2015.

ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: Left-2
ad: Subscribe
ad: MessiPaoloni-1
ad: L-MFJ
  1. KK4WMX

    KK4WMX XML Subscriber QRZ Page

    I put together a fan dipole the other day, just playing around a bit. I took down a 40m OCF (that was working well) and put up a fan dipole with 40m, 20m, 17m and 12m elements built from surplus 12 awg well wire. What I put up plotted out OK on an SWR scan, but the 15m portion sat a bit higher than desirable (the 15m dip was lowest at 22.080 with an SWR of 1.36). I had posted a copy of the SWR scan on another board & got a comment suggesting I look into adding a dangling wire as a capacitance hat on 40m to improve 15m. I did some searching and reading, then followed through with wire and solder & thought I'd share the results.

    This is the first hat I tried. Made of 24" of solid 12 awg and per some of my reading, rather than letting it dangle, I shaped it into two lobes. These were soldered to the 40m elements at about 10' from the feed point:

    cap-clover-hats-pre.jpg

    Here is a SWR scan showing before (fine line) and after (bold line) the cap hat was put in place:

    fan dipole-40-20-17-12-cut3-with hats-1-note.jpg

    As you can see, the cap hat moved only the 15m dip, over to 21.660 @ 1.23.

    Wanting to see what would happen, I made another set of hats and attached them at 90 degrees to the first ones:

    cap-clover-hats-1.jpg

    Here is the scan made after the modification:
    fan dipole-40-20-17-12-cut3-with clover hats-1-note.jpg

    The dip for 15m is now 1.19 at 21.480. The highest SWR in the 15m phone band is less than 1.45:1.

    I am pleased with the way this turned out, with the general class 15m phone band under 1.5:1 SWR, & thought I'd share.

    Nick
     
    Last edited: Jun 1, 2015
    KE0EYJ and WB5YUZ like this.
  2. NH7RO

    NH7RO Ham Member QRZ Page

    That's very neat, Nick; Thank you for sharing this as many 40M dipoles often resonate too high for comfort when it comes to using them for 15M operation---that's an easy way to solve the problem!

    73

    Jeff
     
  3. W9GB

    W9GB Ham Member QRZ Page

    Nick -

    YES, this information is found in the ARRL Antenna Handbooks (and other publications).
    Well known to Novice operators in 1960s and 1970s, who only had 80, 40, & 15 meter HF privileges.

    QST magazine, June 1991
    Antenna - Here is a Dipole
    http://www.arrl.org/files/file/Technology/tis/info/pdf/9106023.pdf

    2005 ARRL HANDBOOK : Chapter 22, Antennas
    Turn to Page 22.13 --- A 40-M and 15-M Dual Band Dipole
    You place the Capacity Hats at the 1/4-wave length for 21.10 MHz from the Center Insulator.
    Tweak the Position and "Gimmick Capacitor" for tuning 15-meter resonance point.

    eHam.net Discussion on this topic (January 2013)
    http://www.eham.net/ehamforum/smf/index.php?topic=87799.0
    ---
    A 40-meter HALF-Wavelength (1/2) Dipole cut for 7.10 MHz is a
    ONE and a HALF-Wavelength (1-1/2 or 3/2) Dipole at 21.3 MHz (actual point is often higher).

    By adding a "Figure 8" capacity hat ("gimmick capacitor") on a 40 meter dipole near the 15-meter point
    will lower the frequency toward 21.2 to 21.1 MHz.
    This is ideal for Digital Mode operators desiring to operate in the lower portion of those bands.
     
    Last edited: Jun 2, 2015
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    Actually, a half-wave dipole resonant at 7.1MHz is 3/2-wave resonant significantly higher in frequency than 21.3 MHz

    There's not a simple 3:1 relationship, as is well illustrated by the "cutting formula" from older editions of the Antenna Book:
    L(ft) = (N-0.05) x 492 / F(MHz)
    where N is the number of half-waves.

    That formula predicts a 7.1MHz half-wave would be 3/2-wave resonant at 22MHz - just as the OP found, and makes the need for the capacitance hats even more acute.

    Steve G3TXQ
     
  5. W9GB

    W9GB Ham Member QRZ Page

    Steve -

    YES, if you run ENZEC ... the 15 meter resonance point will be outside the 15-meter band.

    HOWEVER, it is a Relationship about ODD Multiples of a Half-Wavelength Dipole (and feedpoint Z)
    that should be Learned (or Memorized) by new operators

    In practice, of building these antennas (40-meter, 1/2-wave dipole) since high school (40+ years ago) ---
    the actual resonance point at 15-meters varies considerably based on antenna materials, construction, and height above the ground.

    The other characteristic of a 3/2-wavelength dipole, that is forgotten, is its different pattern (4-leaf clover, with reduced 1/2-wave lobe). Longer wavelength multiples produce more complex radiation patterns.

    image.jpg image.jpg
     
    Last edited: Jun 2, 2015
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Yes, there are many variables - height above ground, wire diameter, insulation/no insulation, ground conditions etc; but a key principle is that the shortening due to "end effect" which leads to the familiar 468/F cutting formula is only present on one half-wave of a 3/2-wave wire; the "middle" two half waves are very close to the free-space value of 492/F. Hence the ratio of the resonant frequencies will always be significantly greater that 3:1.

    It's also worth noting that the feedpoint resistance of a resonant 3/2-wave is typically a lot higher than that of a half-wave. It would be interesting to look at whether the "capacitance hats" affect that at all

    Steve G3TXQ
     
  7. NW2K

    NW2K Ham Member QRZ Page

    AC6LA generated a model for the 40/15 case:

    http://ac6la.com/aecollection2.html#6

    There are a couple of interesting charts there, but not one that shows the complete effect of hats on feedpoint R for the 40/15 combo.
     
  8. W9GB

    W9GB Ham Member QRZ Page

    Dean,

    Thank you for posting the work of AC6LA.

    William J. Lattin, W4JRW received a US Patent, December 1950 for his
    Lattin Multi-band antenna.
    http://www.hamuniverse.com/US2535298.pdf

    He later published his work on this Multi-band antenna in the
    December 1960 and April 1961 issues of QST magazine.
    http://www.hamuniverse.com/lattin5band.html

    William E. "Bill" Fanckboner, W9INN, of Mt Prospect, Illinois (sk, July 2004) would hang wire stubs, for specific fine tuning of the "resonactor coil" used on his wire antennas.
     
  9. KC4YLV

    KC4YLV Ham Member QRZ Page

    I think this is more feasible with a monopole config, playing with it I got things running very nicely in the sim with a small cap hat 1/3 of the way up a 40m vertical - if I recall i just modeled four 0.5m 1/4" spokes, no ring. Didn't take much. But no matter what I did with a hatted dipole, 15m just had too high impedance to make me happy.

    Vertical on 15 definitely has that high lobe, but low angles don't look that bad. Match is good especially if you lengthen a bit and run a series feed cap. Seems like a good 'freebie addon' to a full sized 40m vert to give you backup 15m action.
     
  10. KE0EYJ

    KE0EYJ XML Subscriber QRZ Page

    Okay, I'll bite. My 40 meter linked dipole, at work, resonates too high in the 15 meter band. I'm going to hang some wires down from the 20m links, and see what happens.
     

Share This Page