Wayne,
Try a u-bend of 1/4" diameter tubing about 12" from tip to apex set on
about 6" centers. You can shorten it up as needed from there.
Optimized Yagi designs usually end up with a relatively low feed point
impedance at resonance (~20-25 ohms or so, depending on how aggressive the
design is). With hairpin/beta match, the driver is then shortened enough to
achieve -25j reactance so that the match, which acts like a shorting stub,
tunes out the reactance and raises the impedance to the target impedance
(~50 ohms) in the process.
For example assume a feed point impedance of 20-25j ohms at 21.225 mhz. A
500 ohm shorting stub (match) that is electrically 184mm long will bring it
to 51+0j ohms. The characteristic impedance of the shorting stub does not
matter other than the lower it is, the longer the stub (match) will have to
be to achieve the same result. Although the stub described above should
have a characteristic impedance of 600 ohms, it won't achieve that because
the L/D ratio is relatively short meaning that end effects come into play,
and that lowers the characteristic impedance from theoretical (which is
based on infinite length or long L/D ratio).
Another way to visualize it is that the hairpin acts like a small amount of
inductance. In the above example, 307 nH of pure inductance achieves the
same result as the stub.
What does matter is getting the driver to the correct length and that
depends on how much -j reactance to target. This depends on the antenna's
real impedance without any match. Here's a chart that shows how much
reactive impedance to shoot for to achieve an "optimal" match:
45 ohms real shorted to achieve -15j reactance
40 ohms real needs -20j
35 ohms real needs -24j
17 to 30 ohms real needs -25j (note that -25j covers a lot of territory)
15 ohms needs -23j
Needless to say, set it up at the center frequency of your desired
bandwidth.
Hope this helps and good luck on your project.
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Wayne
Kline
Sent: Tuesday, February 11, 2014 9:10 PM
To: john@kk9a.com; towertalk@contesting.com
Subject: Re: [TowerTalk] TowerTalk] F-12 EF- 515
No not all the F-12 designs at direct feed this is one :(
Wayne W3EA
> Date: Tue, 11 Feb 2014 16:15:54 -0500
> Subject: re: TowerTalk] F-12 EF- 515
> From: john@kk9a.com
> To: towertalk@contesting.com
> CC: w3ea@hotmail.com
>
> Are you sure that the antenna had a hairpin? I thought Force 12 uses
> OWA designs.
>
> John KK9A
>
>
> To: "towertalk@contesting.com" <towertalk@contesting.com>
> Subject: [TowerTalk] F-12 EF- 515
> From: Wayne Kline <w3ea@hotmail.com>
> Date: Tue, 11 Feb 2014 14:01:31 -0500
>
> In preparing for a redo on my 15 meter antenna system. I began rough
> setting
> in place a F-12 EF 5 15 five element monoband yagi the element
> positioning are a no brainer but some how some where the tubular
> hairpin got GONE . I remember when I removed it from the previous
> owners installation A 3/8 tube bent in a elongated hoop.
>
> So dose anyone have any info as the LENGTH of this match ? it's a none
> adjustable !!!
>
> Wayne W3EA
>
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