On 03/18/2016 06:27 AM, Tod wrote:
Paul, what is the point of this speculation? A single antenna
that covers two bands? A rotary antenna for 40 and 30?
Both. I should have specified my goals.
I am a DXer first, contester second. Currently I have a 40 meter
inverted V, apex at 70 feet and 30 meter inverted V, apex at 80
feet. Here is what I would like to achieve:
1. Get the 40 and 30 meter antennas farther from homes and multiple
small sources of RFI which have already been mitigated to the extent
reasonably possible. Some quick tests with inverted V and sloper
antennas in the proposed location would seem to indicate it is quieter.
2. Upgrade to higher performance antennas if possible, but
definitely not lose DX performance compared to what I have now. The
proposed rotatable dipole would be at 105 feet minimum.
3. Reduce the number of ropes tied off around the property by going
to non-wire antennas on these two bands. I'm the one who has to mow
the yard and would like get rid of some tie-off posts.
4. Keep band switching to a minimum. I didn't like my own proposal
in my first post on this criteria, but I have been struggling with
less-than-ideal options for weeks.
The constraints are:
1. Budget. If I weren't on a budget, I would already have multiple
towers with stacks for every band. :)
2. I need to watch how much load I add to the rotator and mast.
Get someone to model for you a dipole, center fed, that has the
length you can put in place.
I could do a full size 40 meter dipole. This was my original plan
before deciding I wanted to see if 30 meters could be included. I
still may end up going back to this.
I thought about full size parallel dipoles for 40 and 30 but the
amount of aluminum became a concern for rotator and mast stresses.
pick a feedline that 'suits' you
A 375 foot run of LDF5-50A 'suits' me -- mostly because it is
already paid for and installed! I had a lot of fun snaking it up the
inside of the tower (my preference) working alone. :)
There is also a 375 foot run of 3/4" 75 ohm cable that could be used.
Feed the station end of feedline using an antenna tuner so your
transmitter will deliver power to the feedline ( and as a result
to the dipole). If the SWR on the feedline is 10:1 or less quit
spending time on the design and go ahead and build and use the
antenna. If you decide to use open wire line for all or part of
the feedline from the station to the antenna don't worry about
feedline losses even if they are greater than 10:1 .
That could be acceptable if the antenna only requires a tuner on 30
meters, since a low power tuner would suffice and band switching
could be as simple as bypassing the tuner. I would prefer not to get
into another set of knobs to be twiddled/set when changing bands.
What I have written above does not produce an optimum antenna. It
should produce one that will work satisfactorily. It also should
take a lot less effort to create and install.
Effort is of no concern. I am used to working harder to achieve
satisfactory results on a budget. :)
Even if only half the power were radiated, the signal will be
only 3 dB lower than if all were radiated. For almost all
operating this will be of no consequence.
If what I have been reading is to be believed, going from inverted V
to dipole and 70/80 feet to 105+ feet should be an upgrade even with
3 dB loss.
73,
Paul
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