[TowerTalk] Hex Beam

[Eric Gustafson]

Hi Maury,

I have enjoyed your posts to the reflector on the subject of
antennas.  They are clearly coming from a background of technical
competence in the subject area.


>From: "i4jmy@iol.it"<i4jmy@iol.it>
>Date: Thu,  6 Apr 2000 15:53:35 +0200
>
>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.

I also got that impression when I initially read his web site.
But a further careful reading of the FAQ section reveals that he
isn't claiming that the beam matches 50 ohms.  He just states
that the SWR is 2:1 or better across the band.  I'll have to drop
the hairpin matching element out of the model and see how far the
SWR remains below 2:1


>If the above argued is real (hope not) and your modeling
>correct, all the model gain could be not confirmed in the
>practice.  In fact, 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).

I agree.  But I have had very good agreement between actual
antennas and the EZNEC models they were built from.  I'm betting
that the model could be physically built and it would produce
performance very close to the predicted values.  Take a look at
it using your analysis software and let us know what you find.


>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.

I have already posted this information to the reflector.  I hope
you were able to receive it OK.



>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 strength, enough bandwith/gain/FB, etc.)
>it came out that most of the compact beams performances are more
>theorical than achievable in practice.

This is a simple enough wire antenna that I don't think there are
any really big hidden implementation issue "gotchas" lurking in
there to foul up the modeling.


>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.

Yes.  This is a puzzlement for me too.  The antenna does indeed
seem to be a fairly high Q device in terms of sensitivity to
changes to things like dimensions, material properties, and
relative locations.  But the VSWR bandwidth is pretty large in
spite of this.  Still, I don't think the cause is hidden losses.
If you saw LB Cebik's post on the subject, you know that he
ascribes the same kind of behaviour to other relatively compact
designs where the ends of the elements are fairly closely spaced.
This model was my first experience with one of these unloaded
compact designs.

73, Eric  N7CL


>
>73,
>Mauri I4JMY
>
>
>> ---------- Initial message -----------
>>
>> From    : owner-towertalk@contesting.com
>> To      : towertalk@contesting.com
>> 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.  It 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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