So, all we are doing is using more sofisticated methods to arrive closer to
what some think is the correct. (The reason that I say such is that I have
read discussions on the correctness and refining of the various modelling
programs.)
Now, I would like to be able to do all the modelling and am trying to move
in that direction. However, the formula certainly gives a very good
approximation, especially if one wants to find out if there is enough room
to put up the atnenna.
Chris opr VE7HCB
At 10:40 PM 2002-06-25 -0400, Guy Olinger, K2AV wrote:
>This is just so folks know what the dipole length = 468/f business is
>about.
>
>Note below the wild variation in resonant frequency, impedance, and
>SWR limits, just by changing height.
>
>The formula just doesn't come real close except in a few spots,
>notably 30 feet up, maybe the height of the infamous Newington alley
>dipole? The table is why ARRL quit publishing the formula.
>
>Pick 40 meters. 468/7.150 is 65.4545 feet. EZNEC 3, High accuracy
>ground. #12 wire. 15 segments. Use fixed font to view table.
>
>Height Z @ 7.15 SWR50 SWR75 Z @ F RES 1.8:1 SWR @ Z=75
>
>20' 50.6 - j 6.2 1.13 1.50 51.4 @ 7.177 7.052-7.309
>30' 74.8 - j 9.1 1.53 1.13 75.9 @ 7.191 7.003-7.398
>40' 87.0 - j 24.9 1.94 1.41 91.5 @ 7.267 7.132-7.355
>50' 84.8 - j 41.7 2.25 1.70 90.4 @ 7.352 7.129-7.575
>60' 73.1 - j 50.0 2.42 1.94 77.2 @ 7.386 7.178-7.602
>70' 60.4 - j 47.4 2.35 2.06 63.7 @ 7.361 7.193-7.544
>80' 55.4 - j 37.2 2.01 1.90 59.2 @ 7.310 7.167-7.480
>90' 59.5 - j 27.8 1.70 1.60 63.7 @ 7.270 7.116-7.459
>100' 63.4 - j 25.4 1.70 1.44 73.1 @ 7.265 7.081-7.478
>
>I am a bit mystified that some are apologetic for this formula
>(468/f).
>
>It is only accurate at one height, 30'. The resonant center of the
>antenna is either out of band or near the band edge above that height.
>The 75 ohm SWR varies from near unity to over 2:1 at the design
>frequency. The range of impedances at resonance is almost two to one.
>
>If a 40 meter Force 12 antenna had a resonance center at 7361, a SWR
>of 2 at the design center, and N6BT tried to defend it, you'd be after
>him with tar and feathers. I've seen hugely critical posts for a
>fraction of this variance.
>
>This should show the danger of extrapolating small or single sets of
>old data. Or for that matter trying to simplify something naturally
>complex. It really takes a height table to do a cut/solder/string it
>up dipole.
>
>And for how long did 468/f rest resplendent in the books,
>unquestioned, until Joe Common Ham got his mits on antenna modeling
>software?
>
>Arrival angles, guy wire anti-resonance lengths, beware.
>
>73, Guy.
>
>----- Original Message -----
>From: "Joe Reisert" <jreisert@jlc.net>
>To: "Guy Olinger, K2AV" <k2av@contesting.com>; "Bill Hider (N3RR)"
><n3rr@erols.com>; "tongaloa" <tongaloa@alltel.net>;
><towertalk@contesting.com>
>Sent: Tuesday, June 25, 2002 11:18 AM
>Subject: Re: [Towertalk] Guy wire resonance. Break it up or use stubs?
>
>
> > Guy,
> >
> > I think if you look back a few years (perhaps 20-25), the whole
>theory of
> > guy resonances was first mentioned and explored by the late Jim
>Lawson,
> > W2PV, and I think he provided the info that is now in the ARRL
>references.
> > Furthermore, none of the so called WARC Bands (12, 17 and 30 meters)
>were
> > in existence at that time.
> >
> > As for the 468/F formula, that is for a half wave wire with
>insulators on
> > the each end. the assumption here is that the insulators shorten the
>wire
> > by 5%. No magic but a loose approximation for the simplest cases as
>we all
> > now know.
> >
>
>
>
>_______________________________________________
>Towertalk mailing list
>Towertalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
|