Wednesday, May 27, 2015

W8JI Revisited

Regarding a multi-band dipole...what is the best balun for maximum efficiencies across all HF bands?

The best balun ratio for a dipole depends on a combination of factors, including frequency, feed line length and characteristic impedance, antenna length, and height above ground.  

If I really wanted to get the best results using a balun, and I had a particular balun in place, I would examine the impedances across the bands of interest on the coax side of the balun.  If I had high impedances and was using a 1:1, I would try a 4:1 balun, and vice versa.....or change the feed line/antenna length.

You may remember I did the latter when designing the 202/60 antenna. Note that Antennas less than 80% of a half wave can have low impedances, so caution is needed on short antennas.

In the past I have tried 4:1 and 1:1 baluns.  At the moment, I am using a 1:1, but I am mostly just on 80m.  I am able to tune all bands with my auto tuner, but that does not mean it is efficient on all bands.  This antenna was really designed for 160, 80, and 40, and sort of for 60m.

If you are really serious about this, You could go with an elegant approach and use a balanced tuner, such as the Johnson matchbox.  It does not require a balun.  The addition of one outboard capacitor makes the Johnson boxes usable on 80-10m, including WARC bands.

Further,  my tests show that the optimum length for "W8JI" for 160m and up is somewhere between 202' and 208', not 220'; and that is also using 300 or 450 ohm ladder line, 60' long, with a 1:1 current balun, antenna at 55' height.


No comments: