I think mounting the coils out on the elements is better, however I would
still keep them near the center where the element is stronger. Relays on
antennas often fail, especially after being hit by lightning, so if possible
make them accessible. You can easily design a two element yagi to have an
impedance of 50 ohms and feed it directly with coax.
GL
John KK9A
To: "T talk" <towertalk@contesting.com>
Subject: [TowerTalk] 30m/40m Yagi Ideas?
From: "Dan Hearn" <dhearn@air-pipe.com>
Date: Wed, 12 Mar 2008 12:06:19 -0700
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
SteppIR.
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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