[Elecraft] 4:1 balun
Jim Dunstan
jdunstan at tbaytel.net
Sat Dec 10 16:35:34 EST 2011
At 09:12 AM 12/10/2011 -0800, you wrote:
>The down side of the 300 ohm twin lead folded dipole is that 300 ohm twin
>lead usually is made from small wire and not mechanically very strong, so
>it is not durable for fixed stations with varying weather conditions and
>the wire is too small for anything but low power. With either the 300 ohm
>twin lead or a folded dipole fabricated from #14 or heavier conductors a
>4:1 balun is needed to tune well with a 50 ohm output or to transition to
>coax. Then it is about as good as a well made dipole.
>
>
>Willis 'Cookie' Cooke
>K5EWJ & Trustee N5BPS, USS Cavalla, USS Stewart
Hi,
No problem using good quality 300 TV twinlead with high power. Remember
current is much reduced when feeding into a 300 ohm load vs a 75 ohm
load. As I mentioned, I consistently used such an arrangement with a 4:1
balun feeding the folded dipole at 70 ft running with 300watts and fully
plate modulated AM. In addition since the twinlead is operating with no
standing waves there is no chance of magnified voltages and currents as a
result of phase shifting. Never had a problem with RF power. Now to be
fair TV twinlead was commonly available in at least 3 grades ... heavy,
medium, or light. I am not sure what is available today in what is called
TV twinlead. I understand it is possible to buy 300 ohm twinlead that is
manufactured for power handling .... I don't know if anyone uses it for
making folded dipoles (I suspect not). If one were to construct such a
folded dipole for any HF band I can guarantee they will not have any power
handling problems to today's legal power limit.
I have also used TV twinlead to feed non-resonant doublets (100 inverted
vee is a favourite) and used a 100watt AM transmitter (DX-100) with a
balanced transmatch .... again, never had a power problem with the feed
line. This arrangement is a bit tricky. Choosing a useable feed line
length solves any power problems.
However, you are correct about mechanical reliability. I did have to fix
the antenna several times a year.
At one time the cable manufacturers also made 75 ohm twinlead in both a
lightweight and heavy duty version. The 75 ohm versions were more
susceptible to current/heat damage due to the much greater current feeding
an antenna with with a 75 ohm resonant impedance. The light weight version
was only good for 25 to 50 watts but great for portable work as it was very
light. It used to be the standard configuration for commercial portable
radio service used by northern trappers es miners. Their radios only ran 3
to 5 watts. I used the heavy duty 75 ohm twinlead to feed a cubical quad
for years ... again 100watts AM.
Jim, VE3CI
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