[Elecraft] 4:1 balun

[WILLIS COOKE]

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
 

________________________________
 From: Paul Christensen <w9ac at arrl.net>
To: elecraft at mailman.qth.net 
Sent: Saturday, December 10, 2011 8:58 AM
Subject: Re: [Elecraft] 4:1 balun
 
> It seems that many Hams considered the 4:1 ratio some sort of magic 
> number,
> when all it was intended to do was provide a good transfer between 300 
> ohms
> and 50 ohms.

I just modeled a typical 20m folded twin-lead dipole using 4Nec2 and TLD 
software.  Had never tried that one before.

The antenna feed-point Z at a half-wave height (33 FT) above average ground 
computes to 291-j7.7.  That's surprisingly close to 300 ohms resistive. 
With TLD software, I then coupled the feed-point with 300-ohm twin-lead 
line.  Finally, I varied line length between 0 and 1/2 electrical wavelength 
and watched the resulting Z at the line input.  Z always stays between 290 
and 310 ohms with very little reactance.  Total system loss never exceeds 
0.25 dB.  The 300-ohm VSWR stays near 1.05:1 and not surprisingly, the 
50-ohm VSWR stays near 6:1.  While a 4:1 balun can probably result in 
efficient transfer of power into the line from a modern 50-ohm output Z 
transceiver, I think a 6:1 ratio current balun would be a better choice 
under these circumstances, assuming one wanted to avoid an ATU at (or 
within) the rig altogether.

No wonder this antenna was so popular.  The antenna and line all use the 
same inexpensive 300-ohm twin lead material.  The match at the antenna is so 
good that line VSWR (300) and loss is negligible.

Paul, W9AC

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