Gentlemen, Bridget, (and all other esteemed lurking YL's),
If you will indulge me a partially off-topic question, I feel confident
that some of the best experts in the field of antenna theory and practice
are found on this reflector ( would you like some popcorn with that butter,
sir?)
Some of you know that I am a fireman with the City of Roswell, Georgia. I
am also a computer experimenter and a ham radio operator (obviously). I
have always enjoyed listening to police and fire department radio traffic
on scanners as well. When you throw all of the mental disorders in to a
blender, and press the "frappe" button, you end up with a guy who has put
three streaming audio feeds out on the Internet, so that all the globe can
listen to the Roswell Fire Department, Roswell Police Department, and the
Fulton County Fire Department
(<http://www.roswellfirescanner.com/>www.roswellfirescanner.com,
www.roswellpolicescanner.com, and
<http://www.fultonfirescanner.com/>www.fultonfirescanner.com).
The audio source is three simple hand-held trunking scanners with "rubber
duck" antennae. I am convinced that the RF output from the trunking
transmitters is inconsistent, module to module, because a conversation
"thread" will be all over the map, field strength-wise. One transmission
will be s5+10, the next s3. Since I would probably be wasting my time
pointing this out, and since I don't have any gear to take measurements
that would substantiate my allegations, My only resort is to optimize my
reception.
When I explored using a common yagi antenna for all of the receivers, I
discovered that I would have to acquire (buy) an expensive device that
would shield the units from each other, lest the desensitize each other.
My next idea was, and is, to build a "passive repeater." I would like to
build an 11 element yagi, cut to 855.5MHz, then couple it to a vertical
cut to the same frequency. The odd 11 element number was arrived at
because according DL6WU's program, the total boom length would be 35.36
inches - a yardstick.
Now to show my ignorance:
1. Can I use a yardstick as a boom, or does the boom interact with the
elements such that it needs to be a conductor?
2. I would like to use 1/8" dia. threaded rod as my elements. Ideally, I'd
drill holes at the appropriate places on the yardstick (measuring for them
would be a breeze), then secure them with counterpoised nuts on either side
of the yardstick. Can the 9 directive elements and the lone reflective
element each be one continuous rod, or do the need to be two separate rods
with a gap of some width to isolate them from each other?
3. This antenna design call for the driven element to be a folded
dipole. Bending 1/8" threaded rod into a "U" shape sounds like the
hardest part of this project... and, supposing that feat can be
accomplished, what does the stated value of 6.526" refer to? The distance
from curve apex to curve apex? the total rod length? For a receive-only
antenna, what other choices do I have for the driven element?
4. If I attach a straight wire (like a solid 12 gauge) to the center pin of
a PL-259, and screw it to a SO239 tied to the yagi, what would be better: a
1/4 wave, 1/2 wave, or full wave length vertical? Is there a better idea?
Finally is this scheme a totally hair-brained idea, or does it have
merit? Is it a "level 8" project, not be attempted by beginners?
For those who are curious, here are the statistics generated by the program:
Program - ANTDL6WU.BAS
***** YAGI ANTENNA DESIGN DETAILS *****
DESIGN FREQUENCY 855.51 MHz
LAMBDA (Wavelength) 13.796 ins.
GAIN (With -15 dB side lobes) 12.27 DBD
NUMBER OF ELEMENTS 11
DIAMETER OF ELEMENTS 0.125 ins.
DIAMETER OF BOOM (O.D.) 0 ins.
BOOM LENGTH 34.339 ins.
BOOM WAVELENGTHS 2.49
BOOM CORRECTION FACTOR 0 ins.
TYPICAL BANDWIDTH 11.98 MHz
HORIZONTAL BEAM WIDTH 30 degrees
VERTICAL BEAM WIDTH 34 degrees
HORIZONTAL STACKING DIST. 23.454 ins.
VERTICAL STACKING DIST. 20.694 ins.
DIMENSIONAL TOLERANCE (+/-) 0.041 ins.
For those who might want to jump my case for throwing this "rabbit" into
our discussion, I apologize in advance of your flaming replies. I thought
long and hard about whether to ask this question or to ask if it would be
OK to invite Glenn Baxter (K1MAN) and Herb Schoenbohm (KV4FZ) to join our
group. This question won.
Scott Straw, KB4KBS
Living proof of "The Hiram Percy Maxim".... Scheduling conflicts,
interruptions, and equipment problems will always manage to manifest
themselves most profoundly around amateur radio contest dates.
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