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Author Topic: 40 Meter Antenna for 15 Meters  (Read 4858 times)
WB4CMB
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« on: January 27, 2013, 09:58:52 PM »

I have read where a 40m antenna could also be used on 15  meters.  Generally, at least at first, it seemed to make sense noting the odd harmonics relationship.  But then, seperately, I've read that a wire of a certain shape would have to be added (maybe at about 13 ft??) as the resonance would be above the 15 meter band or at least at the very top end of 15.
    I've searched and searched and can't find the references to adding the wire.  I can find several references to simply using the 40m antenna for 15 without much explanation.

1.  Is the wire necessary?

2.  If so, what are the details on using the wire?

3.  Are there any drawbacks to doing this?

4.  I'm building a vertical and could simply add another element for 15.

Thanks      Ray
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K2OWK
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« Reply #1 on: January 27, 2013, 10:29:34 PM »

A 40 meter antenna is resonant on the third harmonic of the 15 meter band. I have a 40 meter inverted "V" that works good on 15 meters with a low VSWR. I also have an S9 vertical that does the same. I have not heard of adding any wire to a 40 meter antenna for 15 meter operation.

73s

K2OWK
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K3VAT
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« Reply #2 on: January 28, 2013, 02:04:58 AM »

I have read where a 40m antenna could also be used on 15  meters.  Generally, at least at first, it seemed to make sense noting the odd harmonics relationship.  <snip>
Thanks      Ray

That is correct as K2OWK pointed out.

...  But then, seperately, I've read that a wire of a certain shape would have to be added (maybe at about 13 ft??) as the resonance would be above the 15 meter band or at least at the very top end of 15.
Thanks      Ray

Don't believe everything you read.  Adding that much wire to the 40M doublet will throw off the resonant of 40M and it will not be suitable for operation on 15M.  Specifically a 40M dipole is ~65' long; changing the length to ~78' now resonants the antenna at 6. mHz and the 3rd harmonic at 18. mHz (maybe good for 17M).  Creating some sort of 'bent dipole' doesn't significantly change this.

See the ARRL Antenna Handbook, Chapter 2 which covers the theory.

GL, 73, Rich, K3VAT
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W5DXP
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« Reply #3 on: January 28, 2013, 04:28:02 AM »

I have read where a 40m antenna could also be used on 15  meters.

A 64 ft. dipole fed with ~68 ft. of VF=0.9 ladder-line will be resonant on 40m, 15m, 12m, and part of 10m.
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W1JKA
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« Reply #4 on: January 28, 2013, 05:50:02 AM »

FWIW dept.:My 40m new Carolina (type) windom at 28 ft. coax fed on 15m,1.6:1 bottom to 1.2:1 top of band ,no tuner no problem.
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W4OP
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« Reply #5 on: January 28, 2013, 06:05:21 AM »

A40M dipole when operated on 15M will have its lowest VSWR high in the band.
By adding short stubs at 11' either side of the center insulator you can independently adjust the 15M freq of best VSWR. This will have very close to zero effect on 40M as this is still very close to the 40M current maxima (loop). Something less than 1' is all that is required. Trim as required.
The attachment point for 15M is a voltage loop and therefore hi impedance. You will not even have to make a DC connection- just tightly wind maybe 10 turns of the stub wire around the main radiator and that will be enough to capacity couple the stubs to the main radiator.
The feed resistance of a 1.5 lambda dipole is higher than a half wave dipole, so the VSWR, even once tuned, will likely not be quite as  good as the 40M VSWR, but still very acceptable.

Dale W4OP
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K3VAT
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« Reply #6 on: January 28, 2013, 06:33:52 AM »

A40M dipole when operated on 15M will have its lowest VSWR high in the band.
By adding short stubs at 11' either side of the center insulator you can independently adjust the 15M freq of best VSWR. This will have very close to zero effect on 40M as this is still very close to the 40M current maxima (loop). Something less than 1' is all that is required. Trim as required.
The attachment point for 15M is a voltage loop and therefore hi impedance. You will not even have to make a DC connection- just tightly wind maybe 10 turns of the stub wire around the main radiator and that will be enough to capacity couple the stubs to the main radiator.
The feed resistance of a 1.5 lambda dipole is higher than a half wave dipole, so the VSWR, even once tuned, will likely not be quite as  good as the 40M VSWR, but still very acceptable.

Dale W4OP

Thanks Dale, but now we have separate two antennas - a parallel or fan dipole with a 65' (40M) piece of wire and another wire of 22' (15M).

I believe that the poster "[wants] to use a 40M dipole on 15M".  If a 40M dipole allows one to adequately load up on 15M, perhaps a decent match across most of the band, so why go to the trouble of adding a second piece of wire; what is really gained?  Adding the 2nd wire will produce a very noticeable interaction unless the arrangement is configured as a 90 degree fan dipole.

thanks, 73, Rich, K3VAT
« Last Edit: January 28, 2013, 06:42:10 AM by K3VAT » Logged
W4OP
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« Reply #7 on: January 28, 2013, 08:37:14 AM »

Thanks Dale, but now we have separate two antennas - a parallel or fan dipole with a 65' (40M) piece of wire and another wire of 22' (15M).

I believe that the poster "[wants] to use a 40M dipole on 15M".  If a 40M dipole allows one to adequately load up on 15M, perhaps a decent match across most of the band, so why go to the trouble of adding a second piece of wire; what is really gained?  Adding the 2nd wire will produce a very noticeable interaction unless the arrangement is configured as a 90 degree fan dipole.

thanks, 73, Rich, K3VAT


No Rich that is not the case. You have a 40M dipole with  two 12" wires attached at 11' either side of center. This is not a fan dipole configuration. There is a 66' wire and two 12" (approx) stub wires.

Dale W4OP
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WB6BYU
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« Reply #8 on: January 28, 2013, 08:42:05 AM »

The idea of adding a hanging wire to a 40m dipole was published in QST several years
ago.  This is based on the fact that harmonic resonances in a wire antenna don't exactly
align with harmonics of the operating frequency:  in this case, the 40m wire is 1/4 wave
minus a correction for end effect, while on 15m it is 3/4 wave with only one correction for
end effect, not 3.  As a result, the 15m resonance of a 40m dipole will be somewhat higher
than 3 times the 40m resonant frequency.

The added piece of wire is placed at a voltage maximum (about 1/4 wave out from the
feedpoint, or 1/2 wave in from the end of the wire on 15m) where it has maximum effect
on 15m but relatively little impact on the 40m tuning.  I've used a similar approach for
a 80/40m OCFD to cover SSB at the high end of the bands, because the basic antenna
cut for 3.95 MHz is too short for proper 40m operation.  It's a pretty simple approach
that just adds some capacitance at a high impedance point on the higher band.

B&W used a similar approach with triangular "flags" of wire added to a vertical antenna.


But that doesn't mean it is required to use a 15m dipole on 40m, however.  I've done
that in my portable antenna kit for years, and will depend to some extent on what
frequency you cut the 40m dipole for, how the ends are tied to the insulators.

My recommendation is to put up the 40m dipole and see what the resulting SWR is on
15m.  If it is too high because the antenna appears to be resonant above the band,
then adding the wires and adjust them to center the SWR curve.
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W4OP
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« Reply #9 on: January 28, 2013, 08:47:26 AM »

My thoughts/concept  precisely Dale- you expressed it better.

Dale w4OP
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K3VAT
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« Reply #10 on: January 28, 2013, 09:34:34 AM »


No Rich that is not the case. You have a 40M dipole with  two 12" wires attached at 11' either side of center. This is not a fan dipole configuration. There is a 66' wire and two 12" (approx) stub wires.

Dale W4OP

I stand corrected, didn't understand it without a picture - hi.  thanks.
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W4OP
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« Reply #11 on: January 28, 2013, 10:00:36 AM »

Hi Rich,
My explaination should  have been more clear also.

73,

Dale W4OP
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WB4CMB
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« Reply #12 on: January 28, 2013, 10:47:55 AM »

Perhaps I should have stressed it much more that I'm building a vertical altho I did have that info in my orig post.

The 40 meter p/o the vertical I am building will be 24 ft high (25 ft limitation by City codes with no inspection) with approx a 4ft top hat if that would change anything.  I would think that keeping the number of elements lower would help in minimizing interaction.
Physically, I am putting this vertical on a wood tilt over mast, at 25 ft I'm using 2 2 X 3s (8 ft ea) and additional 2 X 2s bolted together with 2 ft overlap on all junctions.  Why a tilt over for a vertical??  I want to experiment.  Later will try a vertical dipole for 20 and up (with some loading) and may also try an L configuration.
1.  If  use an L configuration, would it make any difference which element (the horizontal or the vertical part) I connected the coax center conductor to?

2.  I have seen a ham vertical (let's hope he can be vertical, LOL) about 2 miles from me, maybe I should introduce myself and see if I could get him to help me do some testing.  Would two miles be too far?

3.  I like the idea of trying the antenna on 15 before I make any changes.  I have a new 259 I want to play with anyway.

Thanks Very Much for the answers!      Ray
Thanks     Ray
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WB6BYU
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« Reply #13 on: January 28, 2013, 11:55:49 AM »

The capacitance hat will have a different effect on 15m than on 40m, though
perhaps not significantly in terms of radiation. 

The B&W antenna used "V" shaped pieces of wire connected to the side of the vertical
(hose clamps would work), with something like 3 at the top of the 40m section for a
top hat and 2 up about 11 to 12' above the base for adjusting the 15m resonance.  That
is a more practical shape for a vertical as it will tend to hold it's shape when using
reasonably stiff wire.  But their antenna was about 30' tall, using rings of large PVC pipe
as spacers to hold 3 parallel elements, each of which worked on 2 bands.  (10+30m,
15+40m, 20+80m IIRC.)  (I don't know if you can get enough capacitance that way
to cover 40m at the shorter height.)

Similarly you can add coils or capacitors in series somewhere in the antenna to shift
one resonance relative to the other:  series elements will have the most effect
at a point of maximum current, and least where current is lowest, so if you design the
antenna to be resonant on 15m you can add the loading at the high impedance point
(the same point as the capacitance wires) and change the 40m resonance without
affecting the 15m resonance significantly.

This is a good case for antenna modelling software:  you can experiment with placement
and size for a number of different options.
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W9GB
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« Reply #14 on: January 28, 2013, 03:12:32 PM »

Quote from: WB4CMB
I have read where a 40m antenna could also be used on 15  meters.  Generally, at least at first, it seemed to make sense noting the odd harmonics relationship.  But then, seperately, I've read that a wire of a certain shape would have to be added (maybe at about 13 feet ??) as the resonance would be above the 15 meter band or at least at the very top end of 15.
CORRECT, for a Half-Wavelength Dipole.

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.

The capacity hats are fabricated from a 2 foot (24 inch) solid (or stiff wire), such as AWG 12 or 14.  
TWIST into a figure 8 and solder or clamp to antenna wire at the 21.1 MHz nodal point (described above).
Quote from: K3VAT
Don't believe everything you read.  Adding that much wire to the 40M doublet will throw off the resonant of 40M and it will not be suitable for operation on 15M.
Read above.  The description and DIY project on proper fabrication (capacity hat for 40/15 dipole) are in other ARRL publications.
« Last Edit: January 28, 2013, 03:27:43 PM by W9GB » Logged
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