Please excuse the cross post to several lists, but I am trying to
achieve the largest response possible
I am in the design phase of a project, and I think I need a bit of help
and advice. I want to construct a tri-band rotatable dipole for
10-15-20 meters. I am planning to construct it out of PVC pipe (3/4
inch? Not sure yet) supporting wire elements. I wish to use loading
coils to physically shorten the length of the elements. This
construction method is based on the methods used to create the
Homebrewed Buddipole backpack/portable antenna. This antenna however
will be permanently mounted, and as such will be built much tougher (not
using the collapsible whips etc).
At this point I have chosen the following frequency center points:
on 10 meters 28.4 MHz
on 15 meters 21.375 MHz
on 20 meters 14.287 MHz
Using the standard 1/2 Wave dipole formula of 492/ frequency, I have
obtained the following lengths for the dipoles
for 10 meters - 17.32 ft
for 15 meters - 23.02 ft
for 20 meters - 34.44 ft
Calculating the differences in velocity between free space and the
typical conductors, and also capacitance added by insulators on the ends
of a dipole yields the following classic formula 468/frequency, and
returns the following values
for 10 meters - 16.48 feet
for 15 meters - 21.89 feet
for 20 meters - 32.76 feet
I would like this particular antenna to be no longer than the 10 meter
length of 16.48 feet or 16 feet 5 3/4 inches.
My question is how do I achieve this using loading coils, and where
should they be placed.
The construction of the coils is a fairly simple process of winding the
proper amount of wire onto the proper diameter form, and I have done
this in the past and had good success, but in those cases I was always
working from someone else's plans, and I wish in this case to
empirically derive the values that I will need.
I have been unable to find any information on how to calculate the
number of turns of wire of size X on a form of size Y to achieve
resonance on a desired band, and this is what I am searching for, I also
need to know how far out the elements to place the coils and how far
apart they should be from each other.
The problem of droop in the arms has already been solved, in that I will
be using gusset wires to support the outer ends of the elements to
There are two additional questions that I have, again related to my
trying to learn this from the theoretical side.
1. How would one model the coils in MMANA or any other modeling
2. What factors would have to be accounted for or what design changes
would need to be made if said coils were to be used in a Yagi type
I realize that I could easily purchase an antenna of this type, in fact
this antenna is modeled somewhat on the cushcraft D3, however I am
trying to learn some stuff as well as come up with a workable antenna.
At this point I am open to all suggestions and comments. If I have made
an error in my calculations etc, please correct me.
Timothy A. Holmes
W8TAH - Amateur Radio Extra Class Operator
StormNet Chase 9
Certified Advanced Skywarn Spotter
Assistant Emergency Coordinator - Medina County
Web page: http://www.qsl.net/w8tah
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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