[TenTec] "A low vswr will kill you ! "

[jcox]

When are we going to get off the antenna discussion, this is a Ten Tec
reflector.  A few posts are ok but on and on and on.  I get tired of
deleting post after post.  Please Ten Tec RELATED......
----- Original Message -----
From: "George, W5YR" <w5yr@att.net>
To: "Jim Reid" <kh7m@hsa-kauai.net>
Cc: <geraldj@ames.net>; "Steve Ellington" <n4lq@iglou.com>; "Sherrill
WATKINS" <SEWATKINS@dgs.state.va.us>; <tentec@contesting.com>;
<nh6xk@arrl.net>
Sent: Wednesday, January 24, 2001 13:43 PM
Subject: Re: [TenTec] "A low vswr will kill you ! "


>
> Actually, Jim, a "dipole" antenna can be as short as about 0.1 normal
> size and still be over 90% efficient in radiating the power it is fed.
> (Those are very approximate values.)
>
> The idea that an antenna must be "resonant" in order to radiate
> efficiently is one of the unfortunate urban legends of amateur radio.
> Broadcast stations, of course, are an excellent example of shortened
> antenna structures that are efficient enough to be licensed and cover
> their service areas. Aircraft HF antennas are another example of
> efficient shortened designs. They also make short work of the notion
> that a "ground" connection is always necessary for an antenna system to
> operate efficiently.
>
> The difficulty with any short antenna is the fact that its driving point
> impedance is one of low resistance and high capacitive reactance. This
> makes for a difficult matching situation, usually leading to significant
> loss in the matching network, primarily in the required matching
> inductance(s). But, the loss is *there* and not in the ability of the
> antenna to radiate efficiently.
>
> Any antenna will radiate all the power it receives less any that it
> dissipates in its own or in nearby structures.
>
> The notion of "resonance" being either desirable or necessary or both is
> one essentially of convenience in matching: a resonant antenna has a
> purely resistive driving-point impedance. Usually, this is an easier
> condition to match than the more general case of a complex impedance.
>
> But, therein can lie a fallacy, in that away from the magic resonant
> frequency, the impedance presented to the transmission line by the
> antenna will become reactive and cause the antenna to fail to accept
> power as readily unless a change is made in the matching system.
>
> This is most widely seen on the 160 and 80 meter bands where the
> percentage bandwidth is quite large. Normally, special steps have to be
> taken on those bands to obtain efficient antenna *system* operation over
> the entire band. This notion is quantified in the "SWR bandwidth" spec
> that we see for antenna systems, such as the 2:1 limits. A common
> solution to this problem is the use of tuned feeders to obtain a
> resonant antenna *system*. In this solution, matching is done in the
> shack with a convenient matching network instead of at the antenna. The
> tradeoff for this convenience is added loss in the feeders.
>
> All this business also wraps up into the myth of "low SWR" on the
> feedline as being necessary for an antenna to radiate. While it is true
> that an SWR larger than 1:1 will always introduce additional feedline
> loss, the amount of such loss is usually overstated, especially with
> open-wire or ladderline types. And apart from decreasing the power
> actually delivered to the antenna, an SWR larger than 1:1 has no effect
> whatsoever on the ability of the actual antenna structure to radiate the
> power it does receive.
>
> So, we are not bound always to use "resonant" antennas if shorter ones
> will do the job, or must do the job. The problem ultimately becomes one
> of delivering power to the antenna and that is a matching task. Here is
> a rich area for Smith Chart activity to both design a matching network
> and to understand what it is doing and how. Lots of examples in the
> books . . .
>
> 72/73, George   W5YR - the Yellow Rose of Texas      NETXQRP 6
> Fairview, TX   30 mi NE Dallas in Collin county      QRP-L 1373
> Amateur Radio W5YR, in the 55th year and it just keeps getting better!
> Icom IC-756 PRO #02121 (9/00) Kachina #91900556 (12/99) IC-765 (6/90)
>
>
>
> Jim Reid wrote:
> >
> > In H. Hertz's paper of   1889*,  he did not require that
> > the radiating element be "resonant".  In fact,  that idea
> > did not then exist!
> >
> > *1. Hertz H., "On the relation between light and electricity"
> > Gesammelte Werk, 1, 340 (1889).
> >
> > 2. Hertz H.,  "The force of electrical oscillations treated with
> > the Maxwell theory"**, Ann. Phys. 36, 1 (1889).
> >
> > **  Maxwell, J.C., "On physical lines of force: Part 1. The
> > theory of molecular vortices applied to magnetic phenomena;
> > Part 11. The theory of molecular vortices applied to electric
> > currents."  Phil. Mag. 21, 161, 281, (1861).
> >
> > Hertz's work was completely depended upon the work of both
> > Maxwell and Lamor.  Lamor, who in 1897 was able,  at last,
> > to write down the expressions for the fact that radiation
> > occurred only form Accelerated Point Charges,  Phil. Mag. 44,
> > 503 (1897).
> >
> > Perhaps of interest to some.  But, certainly to me!
> >
> > 73,  Jim,  KH7M
>
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>


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