You will get somewhat broader range of low SWR by having the antenna
present zero reactance and using a flat matching device like an
autotransformer to get 50 ohms.
The reason is that the change in reactance (delta X) for a given
change in frequency (delta F) increases as you move away from
self-resonance.
Delta X/delta F is at its smallest near current maxima feed and at its
greatest near voltage maxima feeds.
This also puts the current maxima at the ground which improves low
angle performance, assuming a good radial field and no low angle
clutter which would block the low angles anyway.
The counter to this is that the autotransformer has to work at 1.8
MHz, and saturation and high current issues are constraining. Takes a
BIG powdered iron torroid to handle 1500 watts. To handle a 50 to 25
step-down means that there will be almost 8 amps RF in the low
impedance winding, more as the SWR rises.
Lengthening the antenna and using a single series high current rated
cap is a simpler solution and reduces losses in marginal ground
systems.
----- Original Message -----
From: <n4kg@juno.com>
To: <TOWERTALK@contesting.com>; <knason@emeraldis.com>
Sent: Sunday, August 11, 2002 6:58 AM
Subject: Re: [Towertalk] 160m Inverted L
> There are an infinite number of solutions.
>
> The most popular are to tune the length for resonance
> where the is ZERO reactance and then match the
> (low) real impedance to 50 ohms, OR, to adjust the
> length to provide 50 Ohms plus some inductance and
> use a single capacitor to cancel out the inductive reactance.
>
> Tom N4KG
>
>
>
> On Sat, 10 Aug 2002 20:17:23 -0000 "Kevan Nason"
<knason@emeraldis.com>
> writes:
> >
> > After a couple years of 'listening' I've finally joined.
> > Thank you all for your help -- even if you didn't know you
> > were giving it. I'm still learning some of the basics
> > and thought y'all might help out again. I'm new to using
> > an antenna analyzer. Used to simply use the SWR bridge.
> >
> > I'm constructing an inverted L for 160m and am unsure of how
> > to proceed. My MFJ269 tells me the following:
> > Freq Rad. Resis XL SWR
> > 1.9 45 155 13.5
> > 1.7 32 28 2.3
> >
> > I haven't measured my ground characteristics, but judging by
> > the general documentation available for this area and the
> > large amount of clay around I figure its poor.
> >
> > My question is this. If antenna efficiency is:
> >
> > eff = Rrad/(Rrad+Rloss)
> >
> > and I don't change Rloss by running more radials or some
> > such (I ran out of wire and want to play in the NAQP next
> > weekend); would I be better off leaving the length as is and
> > cancel the inductive reactance with a capacitor, or should I
> > keep on trimming the antenna. If keep trimming, how far?
> > Until Xl is zero and then match the antenna using
> > a transformer or antenna tuner?
> >
> > Kevan
> > NS4T
> >
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>
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