As Jim and Jim have both stated, a low dipole has a high angle of
radiation and no real lobes. Two phased at that height might give you
some f/b, but the directive would be questionable. The phasing would
allow you to maximize the gain and front to back. likely the gain would
be small, but the f/b might be useful. OTOH the two dipoles are likely
to have more of an effect on the 15 meter beam than a single dipole.
With a small lot it take some imagination, ingenuity, and time to work
through trial and error to get multiple antennas to work without
trashing each other, particularly, on harmonically related bands (third
harmonic) such as 40 and 15 . IE. you can use a 40 meter ant on 15.
You might consider a multiband vertical like the Hy-Gain AV640. I have
one mounted on a mast at about 25 feet. Many times it matched or
exceeded the sloping dipoles
On a small lot I don't know if you have room for a sloping dipole, let
alone several, but even one or two at 30 up and 30 out might give you
the coverage you desire and would "likely" have less effect on the 15
meter beam. Depending on the size of the beam it could have a
substantial effect on the 40 meter dipole
A sloping half wave dipole on 40 with the top end at 30 feet with the
bottom only 30 feet out gives you 44 fee for the dipole. With a 40 meter
dipole being roughly 33 feet long that gives you 9 feet of clearance off
the bottom end. That would put the bottom end about 4' above ground 40
feet out would give you another foot or so of clearance. at 30 up and
30 out, you'd need a 60' diameter circle for the 5 element sloper.
With a lower angle of radiation, some gain, and a f/b unavailable for
the level dipole, or even a phased pair. That height would also give
you some coverage of both.
The switched, 5 sloping dipoles as shown in the handbooks would likely
give you more of both The cost would be for the remote antenna switch,
wire, center insulators, and additional coax. Unfortunately I doubt you
have the room.
Most of us are short of something
73
Roger (K8RI)
On 4/29/2016 Friday 6:30 AM, jimlux wrote:
On 4/28/16 5:49 PM, Roger (K8RI) on TT wrote:
Just another thought:
30' pole on the roof supporting 5 center fed, half wave sloping
dipoles. I have used a couple, but if you have the room, use the 5
switched dipoles. I believe it's in either the ARRL handbook, or the
Antenna handbook.. They are simple, seem to work stateside and DX.
Switched as in the handbook, the have a little gain and a bit of a F/B
that really helps.
Even 2 or three will give a bit of directivity. I'm thinking of
switching to 75/40 fan dipoles and experiment with the 5 antennas
instead of just a single monoband on 75 and one on 40 at present. Wire
is cheap and easy to experiment with. I've worked all continents with
just two 40 meter slopers and an AV640 (on 40) at 25 feet.
I would think that two dipoles, with some sort of adjustable phase
combining network might be the optimum strategy. A dipole isn't going
to have good directivity for the main lobe, but can have a pretty good
null to place on an interferer.
There was a clever scheme in one of the antenna compendiums where
there was a adjustable LC, a combiner, and a DPDT polarity reversing
switch.
You adjust the two knobs on the LC to minimize or maximize the signal
as desired, flip the switch and what was maximized, is now minimized.
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73
Roger (K8RI)
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