Measuring SWR (Long)

[Bob McGraw - K4TAX]

In many instances using a balun, not for balanced to unbalanced
applications, but for impedance transformation application is
suggested.   Use a 4:1 as a 1:4 in order to get the low Z to a higher
value.  In most tuners, the low impedance range has more loss than the
high impedance range.   I recall that Elecraft makes just this item.

73

Bob, K4TAX


On 4/11/2019 8:37 AM, Richard Corfield wrote:

> I'm experimenting with portable antennas at the moment for my KX3 and,
> rather than cut the 80m legs of my link dipole to get an antenna that
> resonates on 60m, I've experimented with running it off centre. I've found
> what looks like resonance at the right frequency range, based on low return
> loss.
>
> http://m0rjc.me.uk/screenshot_20190411-132334/
>
> The problem is, that impedance is low. I guess it's being transformed down
> by the 10m of feed line, as normal wisdom for an off-centre feed is high
> impedance due to the lower currents and higher voltages towards the ends of
> a half wave antenna. Considering a velocity factor of 0.66 that feed line
> is about 1/4 wavelength long!
>
> Presumably the KXAT-3 will tune this, but do I lose efficiency by having
> what would be higher currents in its circuitry?
>
>   - Richard
>
> On Wed, 10 Apr 2019 at 22:56, Fred Jensen <[hidden email]> wrote:
>
>> It was a feeble attempt at humor, given the nearly countable infinity of
>> posts here on baluns over the last few years including but not limited
>> to what they are, what they aren't, how they work, do they even work?,
>> and do they even exist at all.  Perhaps my attempt was more feeble than
>> I thought. [:-) And, yes I know that "near infinity" is meaningless.
>>
>> 73,
>> Fred ["Skip"] K6DGW
>> Sparks NV DM09dn
>> Washoe County
>>
>> On 4/10/2019 11:13 AM, Bob McGraw K4TAX wrote:
>>> Hummmm.......see: http://www.dj0ip.de/balun-stuff/
>>>
>>> I seem to think this fellow has a good grasp about the "alien origin"
>>> of baluns.
>>>
>>> 73
>>> Bob, K4TAX
>>>
>>>
>>> On 4/10/2019 12:23 PM, Fred Jensen wrote:
>>>> It also spawned the "balun," a mysterious device that may be of alien
>>>> origin since no one seems to know exactly what it does or how it does
>>>> it.
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[Don Wilhelm]

Richard,

More specifics are needed for a good answer.
The easy way to do it is to measure the impedance at the shack end of
the feedline with an antenna analyzer.
Yes, you moved the feedpoint away from center (but did not say by how
much) and the voltages will be higher and the current will be lower at
the frequency where the radiator is resonant, but it is an unknown at
other frequencies.
The quarter wave feedline -- at what frequency is it a 1/4 wave?
It will certainly act as a matching transormer, but more detail is needed.

As I indicated, measure it with an antenna analyzer to see what the real
situation is on 60 meters.

The theoretical loss will because of currents through the inductors will
increase, but the practical consideration for the KXAT3 will be minimal
as long as the SWR does not exceed 10:1.

73,
Don W3FPR


On 4/11/2019 9:37 AM, Richard Corfield wrote:

> I'm experimenting with portable antennas at the moment for my KX3 and,
> rather than cut the 80m legs of my link dipole to get an antenna that
> resonates on 60m, I've experimented with running it off centre. I've found
> what looks like resonance at the right frequency range, based on low return
> loss.
>
> http://m0rjc.me.uk/screenshot_20190411-132334/
>
> The problem is, that impedance is low. I guess it's being transformed down
> by the 10m of feed line, as normal wisdom for an off-centre feed is high
> impedance due to the lower currents and higher voltages towards the ends of
> a half wave antenna. Considering a velocity factor of 0.66 that feed line
> is about 1/4 wavelength long!
>
> Presumably the KXAT-3 will tune this, but do I lose efficiency by having
> what would be higher currents in its circuitry?

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Post: mailto:[hidden email]

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