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1991-07-04
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4KB
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85 lines
SUBJECT: Part 2, Bird Feeder Antenna
Now, I admit, this design sounds a bit far fetched, but it works. The
vertical dipole, inside the PVC push-up, is invisable. It moves up and
down and can be removed without drawing attention to the fact that it
contains an antenna. The flagpole design has some problems. Not all PUDs,
condos, or apartments allow flagpoles. They represent a permanent fixture.
Not all associations or managers are that patriotic.
Although you can pack a good vertical in a flagpole, there is the problem
of radials. You can run the vertical without radials, but that's another
compromise.
The design for the Bird Feeder Antenna is very simple. Drawing it here is
another problem. Ascii Graphics really do not exist, so a description is
in order.
You can build the vertical dipole without much trouble. The dipole consists
of up to 16 wire 1/4 wave elements. There are eight on each leg of the dipole.
You may use less, it's up to you. You know the bandwidth of a wire dipole and
you know the band spread on 10m. If you are a general or above, you will
want the extra wire elements to give you the bandwidth you need.
First, let's look at the PVC tubular mast / bird feeder support. Most hardware
and home improvement center have PVC tubing. Like the steel counterparts,
the mast will be graduated (large at the bottom and smaller at the top.
I'm not going to recommend any sizes here as availability at your store is
going to dictate what sizes you will need. I will state the the top tube
should be 2" in diameter or better. You will need that size to support the
bird feeder and give the mast some strength.
___
| | <------- Attach upper ring
| | inside
| |
| |
|_| <------- Attach next ring
| | inside
| |
| |
| |
|___| <------- Attach feed ring inside
| |<------- Attach feed ring inside
| |
| |
| |
|_____| <------ Attach feed ring
| | inside
| |
| |
| |
| |
|_______| <------ Attach bottom ring inside
Figure one.
The wire elements are attached in a ring format, evenly spaced, in a 360
degree pattern. Locations are noted in figure one. This allows the dipole
to be folded up when the mast is lowered. More than eight wires on each
leg of the dipole tends toward snags when raising and lowering.
Dipole wire element sizes are calculated by 246 / f Mhz. But I recommend
shorter lengths if eight elements are used.
A BALUN can be inserted, but is not necessary. I feeder coax is needed from
the dipole feed to the base of the mast. A UHF bullet (female to female) to
attach your coax to.
NEXT: How to lay the coax without looking like you're doing it and Bird
Feeder mast install.
NEXT ANTENNA: Mobile resonators and hiding them.
73