[TowerTalk] Hex Beam

[i4jmy@iol.it]

You say a 20 Ohm impedance, but from the manufacturer claims on his web
pages it's possible to argue a direct feed and no matching device for 
the Hex antennas.
If the above argued is real (hope not) and your modeling correct, all 
the model gain colud be not confirmed in the practice.
Infact, in the case the feed point of a closed spaced yagi were 50 Ohm, 
but its R only 20 Ohm, the missing 30 Ohm (allowing the direct match) 
are somewhere hidden, but present, in the form of losses, about 3.5 dB.
Losses are also prone, and good, to flatten SWR because they decrease 
the antenna Q (typ. case of some trapped tribanders on 20m).
If you send me a file with the dimension used for your model I'll 
convert it in a form suitable for the modeling software I use. 
In the past I've already done several tries with compact beams (mostly 
those of the Moxon's book "HF antennas for all locations")
I do not like the pure academy so, transforming my early easy example 
models to more complex ones to approach reality and comply with the 
practical needs (reasonable impedance, mechanical strenght, enough 
bandwith/gain/FB, etc.) it came out that most of the compact beams
performances are more theorical than achievable in practice.
Generally speaking not loaded elements do not to rise losses and Q, but
the short spacing in a yagi or phased array, anyhow increase the 
antenna Q and the losses.


73,
Mauri I4JMY


> ---------- Initial message -----------
>
> From    : owner-towertalk@contesting.com
> To      : towertalk@contesting.com
> Cc      :
> Date    : Wed, 5 Apr 2000 15:40:14 -0700
> Subject : [TowerTalk] Hex Beam

> 4.  The feedpoint does not match 50 ohms directly.  The resistive
>     component of the feedpoint impedance when the beam is
>     optimized for F/B is in the mid 20 ohm range.  I used a
>     hairpin to match the antenna up to 50 ohms.
>
> 5.  The three percent bandwidth claim requires some
>     interpretation.  It depends on what you are using for the
>     definition
is not
>     achieved for either gain or F/B.  Surprisingly, even though
>     this antenna is less than full size, three percent (and a bit
>     more) can be achieved with respect to VSWR.
>






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