Topband: inverted L antenna match system

[Guy Olinger K2AV]

Lowering Z with more radials is the classic sign of reducing lossy
series resistance in the ground system.  This is a straight power loss
to RF heated dirt which adds to the radiation resistance of the
antenna.  The SYSTEM impedance is the sum of the two.  The bandwidth
narrows and the Z to match lowers as additional radials lower the
ground loss.

The reason for bandwidth narrowing is that the ground loss is quite
flat with frequency, while an efficient radiator will be close to its
theoretical bandwidth as computed in models.

73, Guy.

On Mon, May 31, 2010 at 8:00 AM, sasas asasas <tzitzikas_ee at yahoo.com> wrote:
> thank you all of you for your answers.
>
> So more radials, means lower swr?
>
> Do you think that the variable capacitor is all i need for low swr? If
> i will not achieve swr<1.5 what do you think about to try to use a
> variable coil, which will be connected between the common point (of
> capacitor and 50ohm coax from transmitter) and ground. It will help to
> achieve low swr?
>
> How many uH must be this coil?
>
> i mean something like this http://tzitzikas.webs.com/COUPLER.JPG
>
> (without optional turns which are in series with capacitor).
>
>
> --- On Sun, 5/30/10, Charles Moizeau <w2sh at msn.com> wrote:
>
> From: Charles Moizeau <w2sh at msn.com>
> Subject: Re: Topband: inverted L antenna match system
> To: topband at contesting.com
> Date: Sunday, May 30, 2010, 12:57 PM
>
>
> Here is how I did it, along with my reasoning
> behind the
> design.
>
>
>
> I have a tall ash tree and was able to achieve an 85' vertical run with
> an easy
> bow-and-arrow shot.  I elected to build
> an inverted L antenna
>
>
>
> I knew that the current maximum in any antenna is where maximal
> radiation
> occurs, and that the current maximum in any antenna occurs a
> quarter-wavelength
> back from the wire's open end(-s).  I
> wanted as much radiation as possible to occur from the vertical portion
> of my
> antenna.  With an antenna's sinusoidal
> current distribution, that meant that I should make the length of the
> antenna's
> horizontal portion such that the current maximum would be positioned at
> the
> center of the vertical section.
>
>
>
> I reckoned a quarter wavelength of #12 gauge solid copper insulated
> house wire
> to be about 128 feet.  So with 85' up I
> made the horizontal leg 85' out.  Do the
> arithmetic and you'll see that the current maximum occurs where I wanted
>  it.
>
>
>
> I could point the horizontal leg in almost any direction, but I chose to
>  run it
> towards the northeast.  I did this so
> that the antenna's horizontal radiation would be favored by a dB or so
> from the
> "elbow" of the inverted L towards the southwest.
>
>
>
> I ran my 52-Ohm coax in a buried pvc conduit to the vertical portion's
> base.  With the antenna’s overall length of somewhat greater than 1/4
> wavelength (in fact about 1/3 wavelength), there is an inductive
> reactance at
> the base feedpoint, so all I needed for matching was just some
> capacitance
> between the feedpoint and the 50-Ohm coax.
> I used a husky transmitting variable capacitor.  I
>  haven’t pulled it to measure its value,
> but my eyeball guess is about 300 pF.
>
>
>
> I have thus avoided a lossy inductor anywhere
> within the
> antenna, and at the shack there is no balun or antenna tuner with their
> losses.
>
>
>
> With eight in-ground radials I could adjust the capacitor to give me a
> 1.5:1
> SWR at 1.810 mHz.  I added six more
> radials, readjusted the capacitor to get a 1.35:1 SWR, and then brought
> that
> down to 1.2:1 with 22 radials.
>
>
>
> The antenna performs well on 160m.
> Without any real effort, this past winter I worked several
> European
> countries with five Watts, and no QSOs occurred during contests.
>
>
>
> If I had had only, say, a 40' vertical run, I would have adjusted the
> length of
> the horizontal leg accordingly.  And if
> I were space limited for the horizontal leg, I would have used linear
> loading
> to reduce the needed length by a third or so, or would have made a bend
> in the
> horizontal portion to fit it in the space available; i.e., do whatever
> is
> needed to locate the antenna's current maximum at the vertical leg's
> center.
>
>
>
> I do worry about the emerald ash borer, imported from China in the
> wooden
> packing crates used to pack stuff coming from there.  Nothing
>  has yet been discovered to eradicate this pest that has
> killed many thousands of ash trees in this country (including four of
> mine so
> far).  Also, laying in radials that
> range from 70 to 160 feet in length is very slow and tedious work when
> extracting lots of rocks and going around tree trunks and under their
> roots
> near the surface.
>
> 72,
>
>
>
> Charles, W2SH
> _______________________________________________
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>
>
>
>
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