Mike:
The application of using a balun or wide band transformer {that's a balun
function} at each end of a run of 450 ohm line makes for a very efficient
means to get RF from one place to another one. Under these conditions,
total system loss at HF frequencies is in order of a tenth {0.10dB} of a dB.
As to frequencies, the limit in this application is basically the core
material type that is used for the transformers. The line radiation issue
becomes important but only when the spacing between the conductors becomes a
significant part of a wavelength. In general, open wire, 450 ohm line,
typical 1" spacing with air dielectric is good to upwards of 1000 MHz. For
balanced line conductor spacing of 4 to 6 inches the upper limit is around
250 MHz or lower. I seem to recall the guideline is conductor spacing which
equals 1/16 wavelength or less.
In practice, for a 50 ohm application, I would suggest using a 9:1
balun/transformer on each end. This transformers 50 ohms to 450 and then
450 ohms back to 50 ohms thus given all factors, a correctly matched
condition. The ferrite today is available for operation between 1 MHz and 50
MHz without any concerns. Solid wire of #10 AWG, preferable with Teflon
insulation, makes for an excellent starting point for legal limit
applications. Ferrite cores about the size of a donut are needed for legal
limit applications
Someone asked earlier why use a 9:1 balun and why not use a 4:1 balun. If
the input Z is to be 50 ohms and the output Z is to be 50 ohms then the line
Z using a 4:1 balun at each end should be 200 ohms for correct match.
Should one choose to use 450 ohm line then the SWR on the line is about
2.25:1. Another asked then where did the 4:1 balun concept originate? Best
I can figure it came from the use of the folded dipole where both conductors
were of equal diameter and was fed in the center by 300 ohm TV line. This
provided a correct 75 ohm Z for the transmitter. In those days,
transmitters had Pi output networks and were very happy operating at 75
ohms. Typically a folded dipole will have a 300 ohm Z at resonance but as
it is brought in proximity to ground, the center Z will decrease to
something in the order of 200 ohms. In this case the 300 ohm line will be
operating with a SWR or 1.5:1. Even today with the solid state transmitters
that would produce a 1.5:1 SWR, not a bad value at all.
73
Bob, K4TAX
----- Original Message -----
From: "Mike Gorniak" <mgorniak@genesiswireless.us>
To: <tentec@contesting.com>
Sent: Wednesday, October 05, 2005 1:10 PM
Subject: Re: [TenTec] Openwire Feed and Transformer
Hey Lee,
Out of curiosity, would you happen to know what frequency bands your
friend uses, and what the design particulars are for the broad band
transformers?
That could be good information to have!
Thanks!
Mike
NM7X
----- Original Message -----
From: "Lee Crocker" <w9oy@yahoo.com>
To: <tentec@contesting.com>
Sent: Tuesday, October 04, 2005 7:41 AM Subject: [TenTec]
For completeness of this thread, I checked with a
friend of mine who runs 1000ft of openwire line from
his house out to his antenna farm. The line is
terminated by broad band trandformers at each end. He
uses this as a low loss condiut rather than trying to
run a thousand feet of coax with its inherent loss,
and he uses it to feed his beams etc. He does not
twist, and notices no problem with balance. The line
has very good isolation.
Just another data point for the hopper
73 W9OY
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