There is more than one way to look at it. The antenna shorter than
quarter wave is capacitive. The antenna longer than quarter wave and
shorter than half wave is inductive so a series capacitor brings it to
resonance. Then longer than half wave is a lot like a short than quarter
wave, except for the radiation pattern. There are standing waves on the
antenna so input impedances repeat at half wave intervals much like a
transmission line.
The 5/8 wave antenna isn't so much shortened as lengthened from a half
wave which is a pain to feed because of the high impedance.
73, Jerry, K0CQ
On 11/24/2010 1:07 PM, Fred Moore wrote:
> 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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>>
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>
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