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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