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.
>
|