[TenTec] OCF antennas evolution

[Robert Mcgraw]

The 600 ohm or 400 ohm or 300 ohm line has nothing to do with the
matching relating to the tuner.  The tuner nor its internal balun, to my
knowledge, is not designed for any specific impedance. A 4:1 balun is not
"designed" for anything other than as a balanced device with a
transformer ratio.

FYI  - 300 ohm open wire line will have less loss at a given load
impedance which is less than the characteristic impedance of said line.  
Thus 600 ohm line due to the fact the SWR on the line will most likely be
less on 300 ohm than 600 ohm.  Example: If the feed Z of the antenna is
50 ohms then the SWR on the 600 ohm line will be 12:1 and for the same
antenna Z with 300 ohm line the SWR on the line will be 6:1.  Thus added
loss due to SWR will be less at 6:1 than 12:1.  In reality the loss
values are negligible.  Of course 300 ohm open wire is lots of trouble to
keep straight with wind and such as compared to the wider spaced 600 ohm
line.

The line in general will simply act as some form of a line section
transformer based on the electrical length per frequency.  The line, if
at 1/2 wave electrically, will repeat the center impedance of the
antenna.  Or the line being something other than 1/2 wavelength per
frequency will act to transform the antenna impedance to some other value
based on line Z and electrical length.

You shouldn't have any issues with the 600 ohm line or 400 ohm line.  If
you do encounter some matching issues, just increase or decrease the line
length a few feet.

73
Bob, K4TAX




> Gang,
>
> I'll have to jump in here, too.
>
> I remember reading in one of the antenna books that some guy, to prove a
> point, worked DXCC using a light bulb as an antenna.
>
> But...
>
> Several years ago I homebrewed a OCF dipole with a 4:1 balun and stuck it
> up a tree. I've had nothing be outstanding results without a tuner. I
> won't cover 30 meters, but what the hey.
>
> There is currently a large antenna/tower project in the final stages at
> this qth. One of the antennas I plan on installing is a 260 foot doublet
> feed with 600 ohm open line feeders
>
> Now, I wonder how much of a problem my 238 tuner will have with the 600
> ohm line instead of the 400 ohm the tuner had been designed for?
>
>
> Mike Bryce, WB8VGE
>  the heathkit shop
> SunLight energy systems
> J e e p
> o|||||||o
>
>
>
>
>
>
> On Jul 11, 2013, at 11:29 AM, Carter wrote:
>
>> On 7/10/2013 11:33 PM, Michael Tortorella wrote:
>>
>> > I feel compelled to weigh in here....back in the late '70s I used an
>> > OCFD design from Ham Radio magazine which was pretty straightforward.
>> > It was 135' long, fed with 44' of 300-ohm open wire to a 4:1 balun
>> > and then a run of RG-11 into the shack. It was superb. Never had any
>> > stray current problems. Worked about 150 countries using a Johnson
>> > Navigator, 40 watts input. Would like to reproduce that next time
>> > out. Don't pooh-pooh the OCFD design, be careful and it works well.
>> > 73 Mike W2IY
>>
>> I smile when I see everybody describing their favorite wire antenna, so
>> let me jump in with my comments (asbestos suit on). <grin>
>>
>> Mike, not to deny your success nor picking on you or anyone else
>> describing their favorite wire antenna...but with ANY antenna, the RF
>> has to go somewhere.
>>
>> I'm using a dipole about 130 feet long and maybe 25 feet off the ground
>> fed entirely with ladder line (and a Johnson Matchbox) and worked a LOT
>> of DX  including Indonesia on 80, the VK0IR dxpedition and even North
>> Korea on RTTY with that antenna.
>>
>> Heck, Kurt N. Sterba wrote in 'World Radio' magazine that he used a
>> SHOPPING CART as an antenna during a DX contest and worked 17
>> countries! Again, the RF has gotta go somewhere.
>>
>> Bottom line: DX performance seems more attributable to the Propagation
>> gods rather than what flavor of wire antenna is being used.
>>
>> 73,
>> Carter   K8VT
>>
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>
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