[Elecraft] 6 meter beam - OT

[Lew Phelps K6LMP]

If you're looking for a relatively compact six meter beam antenna, consider a three element quad.

Dimensions are available at http://www2.mmae.ucf.edu/~ssd/ham/quadcalc.html, which has a Java calculator for various quads, based on formulae developed by the legendary antenna guru,  L. B. Cebik, W4RNL, SK.

I think the three-element quad has by far the most bang for the buck (gain vs. size) for a single six meter antenna.  Free-space gain on Cebik's "high gain 3 element" design is 9.34 dB on a boom of approximately 8 feet, or 0.4 WL. It has impedance at resonance of 55 ohms, which means it's a piece of cake to match to the transmitter. Since the quad is a balanced antenna, you do need a choke balun to restrict current flow in the coax shield. (I use the Elecraft balun kit.)

For comparison, a 2 element quad, with a boom length of about one meter, has only 7.07 dB gain, and although the four element quad gives you an additional 0.8 dB gain with the additional element, it comes at a high price -- the boom is almost 30 feet long!

For a Yagi to achieve gain similar to a three-element quad, you need to double the boom length.  If you look at the marvelous spreadsheet that VE7BQH posted a bit earlier, you'll see that single Yagis of about the same boom length as the three-element quad offer free space gain of only about 5.5 to 6.5 dB, and Yagis with about 9.5dB have boom lengths of roughly 1.1 wavelengths, more thn twice as long as needed for a comparable gain quad. 

A quad can be configured for either horizontal polarization (for SSB) or vertical (for FM), depending on whether you feed it on the bottom or side. For ease in mounting the feed point balun and cable, I position my quad as a "diamond" with the spreaders pointed vertical and horizontal, and the wires at 45 degree angles. With the feed point at the bottom, this antenna has horizontal polarization, which is the setup I use for weak signal SSB work. (My computer modeling shows that at antenna heights of roughly 6 meters above ground, the "diamond" configuration very slightly improves antenna performance relative to having the spreaders at 445 degree angle and the elements aligned horizontally and vertically.

I have sightly modified Cebik's design and created a 3 element six meter quad with slightly more gain (9.56 db) and even better SWR of 1.05:1 (51.2 ohms +j 2.2) with the antenna cut for 50.130 mHz resonant frequency to center on the frequencies most used in VHF contesting. Here's are key measurments for my 50.130 mHz quad antenna:

Dimensions:  	Radiator spreader 1.082 meters, position 0.00
		Reflector spreader 1.16 meters, position -1.02 meters
		Director spreader  1.05 meters, position +1.50 meters
		total boom length 2.52 meters (8.25 feet) total wire needed about 75 feet. 

Real-world (not free-space) performance is outstanding. Positioning the antenna 6 meters or more above ground is ideal, and provides a maximum gain of about 14.5 to 15 dB at a takeoff angle of 10 to 30 degrees, depending on height (the higher, the better). 	

Construction methods are readily available online from many sources, or you can email me offline if you'd like details on how i built mine. You can make the boom from any material, but the spreaders must be non-conductive for RF (which eliminates metal unless you use insulators to connect shorter segments) and also eliminates carbon fiber shafts. DI was surprised to learn, when researching this antenna, that carbon fiber is basically an insulator for DC, but a good conductor of RF.) 

I also have a web page under construction on this antenna, located at: http://web.me.com/lewphelps/K6LMP/6_meter_quad.html.

BTW, if you're really serious about 6 meter work, you'll want to invest in Elecraft's  PA-6 6 meter pre-amp, since the receiver design for the K3 is great for lower bands, but has less than optimal gain on 6 meters. You'll also want to get the transverter/receive antenna module for easy attachment of the PA-6. Definitely worthwhile.   But a really good antenna is essential, too. 
 
Lew K6LMP

On Mar 17, 2010, at 11:18 PM, Gary Smith wrote:

> I find I have a place for a 6 meter beam. I don't have one but would like to make one if feasible. Does anyone know of good plans for a 6 meter beam that would be realistic to make? 
>