A 2-Element Vertical Beam for 12m (Cheap Price!)

The 12m band is great for working far eastern countries from here in north Wales.  From early to mid-morning during the spring and autumn propagation periods, all sorts of distant and disparate areas come to life on the PSK screen.

But, I've never had a dedicated 12m antenna, so relied on forcing my 20m delta loop to radiate at a frequency quite unnatural to it.  OK it worked, but the radiation pattern is very distorted and obviously not an ideal way to work.

So, inspired by my friend 2W0MTD doing very well indeed into the east with his homebrew doublet, and the 12m band livening up a lot in autumn 2013, I grabbed two 7m fishing poles (I recommend you use 8m versions) previously destined to be part of a Moxon beam, and cut some wire for a vertical dipole for the 12m band.  A number of thin cable ties keeps the wire snugly attached to the poles.

The 2-element 12m beam looking towards the far east.

I fed the radiator with 300 Ohm twin, led into a homebrew 4:1 current balun outside the shack wall.  With no cutting, this came in at an SWR of about 1.7 at the bottom of the 12m band.  Because I'm using twin feed, the losses due to SWR, which tends to rise as the weather gets drier, are minuscule and utterly inconsequential. I cut the reflector element at about 5% longer.

Pounding (with a bad back!) some half round fence posts into the ground as tried-and-tested supports, I simply cable tied the poles to them, led the twin wire at a decent angle away from the radiator, and retired to the shack.

The result?  Excellent!  A noticeable improvement was that calling CQ almost always brought an immediate reply.  I started working all over Russia, out to the far east and even a QRP station running 5W into a dipole somewhere on the line between European and Asiatic Russia.  The gain is pretty respectable at anything between 6 and 7.4dbi, depending on which ground conditions you use; it's also DX-useful low angle radiation.  This is the only antenna I have which regularly yields better (objective, JT65A-based) outgoing signal reports than incoming.

Standing behind the reflector, the RF meter reads zero, showing excellent F/B.

I took out the homebrew RF meter to get a fell for the radiation pattern.  Remarkably, this extremely sensitive meter, even at its maximum setting, gave a zero reading directly behind the reflector.  The model suggests there ought to be a reasonable amount of radiation, as the F/B is computed to be a modest -6dB or so.  In reality, the F/B appears to be more like 10-12dB.

Here's the pattern for the vertical beam as computed for extremely good ground, bottom of the antenna at about 40cm off the ground.  The -3dB beamwidth is, for a manually-steerable beam, a usefully-wide 120 degrees.

How MMANA-GAL computes the 2-element 12m beam.  The F/B is, in reality, much better than this.

I've modelled the antenna spun 90 degrees to one side, as a horizontal beam at half a wavelength up (about 6m.)  The gain below 10 degrees is in fact better for the vertical, even though the horizontal has a better peak gain of 11.7dBi, but which doesn't occur until 27 degrees.

So, the vertical is better at rejecting higher angle signals, having a peak gain of 7.6dBi at 12 degrees, and this was evident in on-air tests.  By a combination of grey line working and this modest beam, I've managed to log several African stations in the space of just a few days, after a long period of being a bit tough in the southerly direction.  Given the sparcity of African stations, that's pretty good going!

I've added three more fence posts to enable the reflector to be moved around the radiator (at a separation of about 2m), making a manually-steerable beam. I'm now using 40mm drain pipe clips as an easy way to locate and relocate the reflector, but I did need to rotary-tool sand away the ring on the base of the screw butt-end of the fibreglass pole to make it all go smoothly.

An unexpected bonus is that this beam works more than decently on 10m as well, allowing nice contacts into KQ2H-R when the propagation is running.  

Happy vertical radiating!