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eHam Forums => Antennas and Towers and more => Topic started by: WB4CMB on January 27, 2013, 09:58:52 PM



Title: 40 Meter Antenna for 15 Meters
Post by: WB4CMB 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


Title: RE: 40 Meter Antenna for 15 Meters
Post by: K2OWK 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


Title: RE: 40 Meter Antenna for 15 Meters
Post by: K3VAT 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


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W5DXP 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.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W1JKA 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.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W4OP 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


Title: RE: 40 Meter Antenna for 15 Meters
Post by: K3VAT 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


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W4OP 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


Title: RE: 40 Meter Antenna for 15 Meters
Post by: WB6BYU 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.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W4OP on January 28, 2013, 08:47:26 AM
My thoughts/concept  precisely Dale- you expressed it better.

Dale w4OP


Title: RE: 40 Meter Antenna for 15 Meters
Post by: K3VAT 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.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W4OP on January 28, 2013, 10:00:36 AM
Hi Rich,
My explaination should  have been more clear also.

73,

Dale W4OP


Title: RE: 40 Meter Antenna for 15 Meters
Post by: WB4CMB 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


Title: RE: 40 Meter Antenna for 15 Meters
Post by: WB6BYU 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.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W9GB 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.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: WB6TNB on January 29, 2013, 03:18:02 PM
As a Novice back in the late 60's/early 70's I used a 40 meter dipole on 15 meters with no modifications and worked a lot of DX. Back then the power limit was 75 watts. The SWR wasn't optimum but not bad. I worked many Japanese, Australian and New Zealanders and some Europeans from the West Coast. My Heathkit DX-60 worked just fine with it. Too bad propagation now isn't what it was back then.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: WB4CMB on January 29, 2013, 04:27:15 PM
1.  Now. would I place the wire (effective capacitor) 1/4 wv length (of 15 meters) from the feeed point or fm the tip of vertical?  I have had answers to my post/s stating both.  If I placed the wire 1/4 from the tip, and if I use the cap hats, how would I tell the electrical 1/4 point?

2.  Perhaps I misunderstood, but someone was doubting I could get enough capacitance fm the top hats for 40 with my 24 foot physical length.  I have a book, "The Short Vertical Antenna and Ground Radials" by Jerry Sevick where he used a 4 ft diameter top hat and a 24 ft element for a 40 meter vertical. (Very good, simple, practical book)
He even used top hat loading for elements much shorter than my 24 ft.

3.  In the not too distant future, I would like to build some type of beam antenna.  Thought I had settled on a hex, may yet build one, but thoughts are now on a shortened yagi (wire or tubing) using top hats.
I suspect that Impedance Matching and Bandwidth could be a problem/s altho top hat loading doesn't seem to be as bad as adding coils in reducing bandwidth.  The MA5B uses cap "hats"

Thanks for everyone's participation!     Ray


Title: RE: 40 Meter Antenna for 15 Meters
Post by: WB6BYU on January 29, 2013, 05:19:03 PM
Quote from: WB4CMB


1.  Now. would I place the wire (effective capacitor) 1/4 wv length (of 15 meters) from the feeed point or fm the tip of vertical? 



You want to put it at the point where voltage is maximum on 15m.  But, of course, the
position of the added wire affects the voltage distribution!

Actually, for optimum results you'd place it so that that section below the wire + the wire length
is 1/4 wavelength, and the section above the wire + the wire length is 1/2 wavelength.  Even that
is an estimate, because the length of the wire isn't the same as the effect it has on the resonant
frequency.

So we may need to be pragmatic.  First, measure the antenna as built and see what the SWR
curve looks like across the 15m band.  Then, if it needs some assistance, put the 15m capacitance
up somewhat less than 1/4 wavelength - maybe 11' or so from the bottom.  You can adjust the
capacitance by adding extra wires (forming them in a V and attaching to the vertical in two
places seems like a good approach), bending the 15m wires closer together or further apart (which
requires more than one wire, of course), changing the size of the wires, or sliding them up or down
the antenna somewhat.  It all depends on how much adjustment you need to get a low SWR on 15m.

But the likely location is slightly less than 1/4 wave above the feedpoint.



Quote

2.  Perhaps I misunderstood, but someone was doubting I could get enough capacitance fm the top hats for 40 with my 24 foot physical length.



No, I was doubting whether the type of capacitance hat (the V-shaped wires) used in the B&W
vertical would give you enough capacitance since your antenna is shorter.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: EI2HEB on January 30, 2013, 05:47:10 AM
I was looking at an inverted-V for 40m and 15m, and on another discussion forum (qrz.com) someone posted the following:

A 40 meter dipole will resonate just above the 15 meter band. An easy way to bring the 15 meter resonant point back down to where you need it is to add a couple of stubs (just a couple of short wires). In an inverted vee configuration with about a 90 degree included angle, here is something that should work. For a resonance close to 7.1 MHz, a dipole with 33.6 ft legs, using bare wire. Then on each side go out 6 ft from the feedpoint and add 2.7 ft wires. Just let the ends of the wires hang down. You can prune the length of those 2.7 ft wires to put the resonant point on 15 meters where you want it. Changing the length of the 40 meter dipole will affect the length of the stubs you need for 15 meters.

Now: I have not build this; so I have no experience with it, so use this info as-is.

I am planning to build it in the next couple of weeks.
If this works; it is certainly a neat method to have a dual band inv-V without the use of traps.

73 de EI2HEB - Edwin.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W9GB on January 30, 2013, 06:09:19 AM
Quote
I was looking at an inverted-V for 40m and 15m, and on another discussion forum (QRZ).
HALF-WAVELENGTH (1/2-wave) DIPOLE for 40 Meters, Resonant at 7.100 MHz

This antenna is ALSO a
THREE HALF-WAVELENGTH (3/2-wave) DIPOLE at 15 Meters, Resonant at 21.300 MHz

MATH:  7.1 x 3 = 21.3

For EUROPE, this dipole antenna is suitable or Phone (SSB, AM) allocations, at 7.1 and 21.3

For AMERICA'S, or DIGITAL/CW operators, Capacity Hats permit lowering the 15 meter resonant point to lower portion of band (21.100 MHz).
These operators can also lengthen this antenna (no capacity hat) for: 7.030 MHz, 21.090 MHz


Title: RE: 40 Meter Antenna for 15 Meters
Post by: WB6BYU on January 30, 2013, 07:16:26 AM
Quote from: W9GB

MATH:  7.1 x 3 = 21.3



Except it doesn't work that way in practice.

Using EZNEC, a dipole resonant at 7.1 MHz using #12 wire has a third harmonic
resonance on 21.75 MHz because of the difference in end effects.


The actual position of the 15m capacitance isn't critical:  around 1/4 wave from
the feedpoint (or more technically, 1/2 wave from the free end) will have the
maximum impact, while the further out from the feedpoint the more effect it
will have on the 40m resonance.  For any location between, say, 1/8 and 1/4
wave (on 15m) from the feedpoint there will be a wire length (or capacity hat
size) that will work; similarly, for any wire length (in a reasonable range) you
can vary the location to get a match.  (It has to be long enough, but not
too long.)

So don't get too hung up on the specific details and dimensions:  a bit of
experimentation should come up with a workable solution.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: EI2HEB on January 30, 2013, 08:11:13 AM
Quote from: W9GB

MATH:  7.1 x 3 = 21.3

Except it doesn't work that way in practice.

Using EZNEC, a dipole resonant at 7.1 MHz using #12 wire has a third harmonic
resonance on 21.75 MHz because of the difference in end effects.

I wondered that... because I have a 21.65 MHz harmonic resonance on my "standard" 40m inv-V...
thanks Dale,
Edwin.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W5DXP on January 30, 2013, 10:11:14 AM
Using EZNEC, a dipole resonant at 7.1 MHz using #12 wire has a third harmonic resonance on 21.75 MHz because of the difference in end effects.

I just ran EZNEC on a 66' CF dipole with two 2' foot wires hanging down 11' out from the feedpoint. Resonant frequencies are 7.16 and 21.2 MHz.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: N3WAK on January 30, 2013, 05:52:13 PM
Ray--WB6BYU and others gave you a lot of good advice.  I think WB6BYU said to put up your 40 meter dipole/inverted vee and see how the SWR is on 15 meters before doing anything else.  I completely agree.  Your SWR on 15 might be less than 2:1 without doing anything.  Sure, I know that it's macho and satisfying (and practical in many ways) to have an SWR really, really close to 1:1, but as a practical matter at HF it isn't going to matter at all IMHO.  Should your rig have an internal tuner, it will operate at full output.  Yes, I know you'll have a little more loss in your coax than with a "perfect" SWR, but you probably are never going to notice it. 

I generally subscribe to the "good enough" way of doing things.  If my SWR was 2:1 or less on 15 meters, I'd be very happy.  If the truth were told, based on the coax loss calculations I have run and reading several articles in QST, an SWR below 3:1 is generally fine on HF.  That sort of loss--IMHO--is inconsequential at 160 meters (technically, MF and not HF) but less tolerable at 10 meters, because loss increases with SWR as your frequency increases. 

But with good quality coax, such as RG-213, and a reasonable length of coax out to your antenna--less than 100 feet, lazy hams like me have nothing to worry about at HF with an SWR < 3:1.  A perfectionist or a serious contester will want to get his or her SWR less than that.  But I have no pretensions about being the biggest signal on the band, and I'm a happy go lucky ragchewer. 

Have fun with your antenna experimentation.  But if your SWR on 15 is fairly low even though you're using a 40 meter 1/2 wave dipole, you can completely relax and ignore the "problem" if you're so inclined.  I would. 

73, Tony


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W9GB on December 25, 2015, 12:22:51 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 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.
Ray (and Edwin) -
This is the ARRL article (also found in Antenna section of Handbooks).
QST magazine, June 1991 -- Page 26, Figure 3

Antenna : Here is a Dipole
by Rus Healy, NJ2L
http://www.arrl.org/files/file/Technology/tis/info/pdf/9106023.pdf


Title: RE: 40 Meter Antenna for 15 Meters
Post by: N3DT on December 26, 2015, 05:32:08 PM
The OP is talking about a 1/4 wave vertical?? I've seen where some have close coupled half wave vertical wires to lower band 1/4 wave verticals. http://www.arrl.org/files/file/Technology/tis/info/pdf/9510032.pdf No reason one couldn't couple a half wave 15M wire to a 40M ground plane or whatever it is. I think all this talk about dipoles is off thread, not to be too blunt. You could probably also couple other half wave to the same antenna if you can get the spacing right. I've never tried it, but I may when I get my 40M ground plane up. See chapter 7 of the 20th Edition ARRL handbook under 'coupled resonator' antennas for info.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: KC9ZHR on December 27, 2015, 01:40:47 AM
if your adding the "stub" to a 40m vertical just grab a hose clamp and move it up or down the aluminum vertical until you get it where you want it  ;)


Title: RE: 40 Meter Antenna for 15 Meters
Post by: N3DT on December 27, 2015, 06:29:48 AM
Opps. A half wave 15M wire will not couple properly to a 40M antenna. Nearly anything else should. But the pattern of a 40M GP will be a real cloud warmer on 15M. At least the patterns I've modeled.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: W1VT on December 27, 2015, 02:17:22 PM
You can avoid the cloud warmer pattern on 15M by making the radiator an inverted-L.  The disadvantage is that this drops the 40M impedance, but not so much that a typical transmatch can't handle it.  I did this to my 40M vertical because I wanted to raise the radials well off the ground, while not coming up with a taller support for the vertical wire. I'd start with 17 ft up and 17 feet over and adjust for the best compromise SWR on both bands.

Zack W1VT


Title: RE: 40 Meter Antenna for 15 Meters
Post by: WB6BYU on December 27, 2015, 02:42:31 PM
Quote from: N3DT

...But the pattern of a 40M GP will be a real cloud warmer on 15M. At least the patterns I've modeled.



Try running a quarter wave 15m wire up parallel to the main radiator with the bottom
end grounded at the feedpoint.  With some adjustments that should cancel most of the
15m radiation from the bottom 1/3 of the antenna that is out of phase with the upper
part.


Title: RE: 40 Meter Antenna for 15 Meters
Post by: KX4OM on December 27, 2015, 05:27:13 PM
...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

Are you planning on building a 1/4-wave 40 meter vertical using wire? If so, the length of the wire will be somewhere in the 32 to 33 feet range. Now, for the 15 meter addition, you could build a resonant trap for 15 meters, which can be as simple as a coil in parallel with a capacitor. You'd need to do some research on traps. The trap would be positioned approximately 11 feet above the feed point (in other words, about a 1/4-electrical wavelength for 15 meters.)

On 15 meters, only the portion of the antenna up to the bottom of the trap is active. The trap disconnects the rest of the antenna. On 40 meters, the entire length is active, and the entire length will be shorter than the 32 to 33 foot length of a single band 40 meter vertical. In the trapped antenna, the coil in the trap acts as a loading inductor for 40 meters, reducing the electrical 1/4-wave length. A ground-mounted vertical antenna will need radials to reduce the ground loss.

73, Ted, KX4OM