Topband: Inverted L

[Guy Olinger K2AV]

Elevated radials are also part of the resonance equation.  You did not
mention how long the radials are and whether they are all the same
length and evenly distributed.  If they are short, then your resonance
(really the resonance of the SYSTEM) will be high.  If elevated
radials are not all the same length, you can get strange effects that
are hard to diagnose.  However, if I MODEL your description and ASSUME
equal length radials on your part...

Your L dimensions will deliver an impedance of 1.3 -J7.37 at the end
of the 60' of coax.  Not likely that many tuners know what to do with
a one point three ohm feed.  The low Z is because your low horizontal
wire heavily couples the radials directly underneath producing a feed
of 7.4 -j125, raising the resonance of the antenna, AND the short
piece of coax lowers the feed Z even more.

The resonance (zero reactance, not low SWR) of the antenna without the
coax is around 2.1 Mhz. If you lengthen the horizontal to roughly 105
feet, this will get you to a resistive ten ohm feed more or less, and
then you will need a unun which converts 10 ohms to 50 BEFORE you put
any coax to it.  A 1:4 unun will be fairly close, for example:

 http://www.balundesigns.com/servlet/the-80/unun-12.5-ohms-12/Detail

In practice the unconnected end 30 or 40 feet of that 105 feet can be
routed miscellaneously without much effect on either impedance or
performance.  It must be said, though that with a feed resistance of
seven ohms, that the performance of the radials is critical.
Personally, I think you will be better off with your seven radials all
1/8 wave and an inductor between the radial center and the coax feed.
You can use this inductor to zero the reactance of the feed Z.  You
will still need the unun.

73, Guy.

On Mon, Nov 29, 2010 at 5:11 PM, Michael Kincaid <w7fkf at fast.net> wrote:
> Have built an inverted L based on info derived from this list. The 128' 14ga
> solid copper plastic coated wire is installed on a 45' oak tree. Had to use
> elevated radials at 8' high. Only able to install 7 of them. Feed point is
> also 8' high mounted to the tree. The vertical part is about 42' high and
> bowed from the trunk about 5' hung up in several branches. Horizontal part
> is about 40' high and 86' long. Fed with RG-213 at 60' length to Antenna
> Tuner (Nye-Viking MB-V-A). I can get SWR down to about 1.8:1 with the tuner.
> MFJ 249 shows 2:1 resonance at 2.325Mhz. Is it possible to get the feedpoint
> Z closer to 50 Ohms. I tried adding length to the horizontal section but MFJ
> still showed above 2Mhz. UnUn or variable cap make a difference? The
> receiving difference on 160M using the 265' Carolina Windom vs Inv L is
> sever S units. Inv L is much quieter. Sorry for the BW but after weeks of
> tinkering I'm perplexed.
>
> Mike - W7FKF
>
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