40 Meter Antenna for 15 Meters

[WB6TNB]

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.

[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?  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

[WB6BYU]

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.

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.

[EI2HEB]

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.

[W9GB]

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

[WB6BYU]

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.

[EI2HEB]

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.

[W5DXP]

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.

[N3WAK]

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