I was always under the impression that the coil added inductance to
cancel out the Xc due to the shortened antenna, Then they tapped up
from the ground end to get R=50. Never thought about it increasing
the electrical length to 3/4λ (resonance). I will have to read up on
this one, for some reason my old mind doesn't grasp the 3/4λ part..
Fred
On Wed, 2010-11-24 at 13:03 -0500, Mike Hyder -N4NT- wrote:
> I was always intrigued with the 148 MHz automobile antennas which were 5/8
> λ. They had a coil at the base which increased the electrical length to 3/4
> λ.
>
> Mike
>
> ----- Original Message -----
> From: "Dr. Gerald N. Johnson" <geraldj@weather.net>
> To: <tentec@contesting.com>
> Sent: Wednesday, November 24, 2010 11:10 AM
> Subject: Re: [TenTec] (no subject)
>
>
> > When you KNOW you have a low impedance antenna at low frequency you can
> > improve life for the tuner by adding a transformer. Not a balun, a
> > conventional ferrite cored transformer. There have been such designs in
> > QST and ARRL Handbooks for feeding short verticals on 80 and 160. At
> > least 20 years ago, maybe longer. Though the conventional 1:4 balun
> > operated with the high impedance side to the tuner and the low impedance
> > side to the antenna can be a benefit operated as a unun which may
> > require unhooking the ground from the center tap of the high impedance
> > side.
> >
> > Or if its the usual case causing a low impedance, a short vertical with
> > no loading coil, you can raise the feed impedance by adding a loading
> > coil, not making the tuner be the loading coil too.
> >
> > Without digging out a design, I'd suggest a start for a ferrite
> > transformer would involve a F200 core, with 12 to 15 turns on the
> > primary and a 5 turn secondary tapped every turn. Pick the connections
> > that give the lowest SWR without the tuner.
> >
> > Or a loading coil that resonates the short antenna which still make end
> > up with a low resistance but cuts the reactance problem which is part of
> > the problem that fries a tuner, and then add the ferrite cored
> > transformer which was the emphasis on those short verticals in QST.
> >
> > 73, Jerry, K0CQ
> >
> > On 11/24/2010 7:05 AM, kf6e@mail.com wrote:
> >> I would strongly recommend the MFJ-998, unless your antenna is very close
> >> to 1:1 SWR anyway.
> >>
> >>
> >> They rate tuners by the range of impedances they can match, and by the
> >> power they can handle. But it's "or," not "and." If a tuner is rated at
> >> 6 to 1600 ohms matching capability and 300 watts, it will handle 300
> >> watts only around 50 ohms load. You must decrease power when the
> >> impedance varies greatly from the 50-ohm nominal load, especially at the
> >> low impedance end. I blew up an LDG AT-600 Pro (rated at 600 watts) with
> >> 175 watts on CW, running into a low impedance antenna. LDG repaired it
> >> for free, but I can't use that power on that frequency on that antenna
> >> with that tuner.
> >>
> >>
> >> I've been very pleased with my MFJ-998. I've run up to 1000 watts
> >> through it over a wide range of antenna impedances with no problems.
> >>
> >>
> >> 73,
> >> Frank
> >> KF6E
> >>
> >>
> > _______________________________________________
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> > TenTec@contesting.com
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>
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