I think putting the coils out on the antenna will be well worth the
effort. To get an idea of the advantage, have a look at the section of
the ARRL Antenna Book on mobile whips and loading coils. Note that the
radiation resistance about doubles by center loading as opposed to base
loading. Also remember that the losses in your coil are proportional to
I^2 and the current and that gives you roughly twice the losses in the
I think you will need to figure out a way to tune your antenna, and
fortunately you have a crank up tower. (As an aside, it is the biggest
crank up tower I have ever heard of. I'd like to see photos.) If you
haven't had a look at VE6WZ's yagi for some further ideas. Note: He had
to do extensive tweaking.
On Wed, 2008-03-12 at 12:06 -0700, Dan Hearn wrote:
> I am in the process of building a 2 element 2 band yagi for 30/40m. It will
> use some 20m elements, which I have, extended to full size on 30m. The
> elements will be coil loaded for 40m and the coils will be shorted with vac
> relays, which I have a good supply of, on 30m. Matching will be done using a
> 50/25 ohm balun/xfmr as available from DX engineering. My thought is to use
> coils at the center of each element but I realize the 40m efficiency would
> probably be greater if the coils were further out on the elements, probably
> at the junction between the 17 ft tips and the center tubing section.
> I plan to mount this antenna on the 2 7/8 mast above my 6 el 20m yagi.
> Spacing will probably be 10-12 ft. Height will probably be 142 ft on my 131
> ft crankup.
> I would appreciate any comments on this proposed design. I realize that a
> lot of my questions can be answered by modeling the design but I have no
> experience in that area. I am using the Taper program to come up with the
> element lengths and would rather spend my time building the antenna and
> putting it up, Hi. I will ultimately ask for help in determining the
> inductance values for element loading.
> 1. Would putting the coils out on the elements vs. at the center improve the
> gain enough to be worth the extra effort?
> 2. What would be a good compromize element spacing on the boom?
> 3. What do you think of using the 2:1 xfmr for matching? Seems to work for
> 4. Is there any place I can buy large type 31 ferrite cores, as used in
> K9YC's great balun designs on his web page, in small quanties? I might wind
> the 2:1 balun xfmr myself.
> Thanks, Dan, N5AR
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