What ever happened to the old
method of standing it up vertical
with the reflector closest to the
ground ?
Is that method out of favor now ??
73, Dick, W1KSZ
On Fri, May 27, 2016 at 2:25 PM, Jim Thomson <jim.thom@telus.net> wrote:
> Date: Fri, 27 May 2016 08:40:28 -0700
> From: jimlux <jimlux@earthlink.net>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Change in Frequency with Height above Ground
>
> On 5/27/16 8:05 AM, Jim Thomson wrote:
> > Date: Thu, 26 May 2016 20:37:09 -0700
> > From: Jim Brown <jim@audiosystemsgroup.com>
> > To: towertalk@contesting.com
> > Subject: Re: [TowerTalk] Change in Frequency with Height above Ground
> >
> > On Thu,5/26/2016 8:24 PM, Don W7WLL wrote:
> >> Lowering the height of an antenna above ground, such as a beam, will
> cause the resonant freq to decrease, increase?
> >
> > Decrease. But not always -- I have a 10M beam at about 15 ft that goes
> > up! The variable here is mutual coupling between elements and between
> > elements and ground.
> >
> > Another error in using SWR as the indicator of resonance is that
> > feedpoint Z changes with height, and in a very complicated way (and
> > because of mutual coupling to ground). In general, low dipoles (as a
> > fraction of a wavelength) are lower Z, high dipoles are higher. AND --
> > feedpoint Z is also affected by the quality of the ground.
> >
> > SO -- we may THINK the resonance is affected by the height as indicated
> > by the low value for SWR, but it's actually the feedpoint Z changing!
> >
> > 73, Jim K9YC
> >
> > ## so why does eznec depict RESONANCE, moving all over the 20m
> band, as height is varied
> > from 0-100 feet ?? ( this also depicts 3 x different ground condx).
> http://s33.postimg.org/69rm0qscf/resonate2.png
> >
> > ## f12 sez to NOT use a MFJ-259B to measure resonance, but to
> instead use the freq of lowest swr.
> > When tweaking 40m REF + DIR..and also DE, no matching coil is used.
> Ele tips ( or loading) is tweaked, till
> > the REF / DE /DIR is resonated at whatever freq the manual /software
> sez it should resonate. And that?s done
> > by measuring each ele, one at a time, by measuring the freq of lowest
> swr. Meanwhile the other els that are not being tweaked
> > will temp have their centers opened off. IE: shorting strap removed
> if a REF or DIR, matching coil + coax + balun removed if a DE.
> >
>
>
> Resonance is where the reactive component of the feedpoint Z is zero.
> Lowest SWR could be at some point where the the impedance happens to
> work out closest.
>
> @@ That’s what I thought. Resonance = zero reactance. Perhaps it
> had something to
> do with the length of coax from choke balun at feedpoint ..... to
> MFJ-259B ?? Unless
> the coax jumper to MFJ-259B had zero length, Z and R could easily shift
> about, depending
> on band, etc. What might /should work is a double male connector
> between balun and MFJ ?
>
> @@ I went through this with F12 years ago, and they were adamant about
> using the lowest swr freq
> as the resonance point. Some of the junk baluns out at the time, would
> easily shift the resonance point
> by 100 khz on 40m...... IE: van gorden ‘baluns’. The wire leads from
> balun to feedpoint will also lower the
> resonance point.... but that is moot, since most folks know the leads are
> a part of the ant.
>
> @@ so what is the correct method then ?
>
> Jim VE7RF
>
>
>
>
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