Saturday, November 13, 2010

2304 Yagi

Being I got some power out of the transmitter, I had an idea a couple of days ago, and thought about building a yagi antenna for 2304.

I went with the DL6WU design, but with a twist.  Instead of doing the folded dipole and balun, I went instead with the WA5VJB unbalanced half - folded driven element.  Everything else about the yagi is in accordance to DL6WU's calculations.

I have 10 elements built so far.  The longest element is about 60 mm long, and the boom length is 12 inches (305 mm).  The entire antenna is made from hobby brass, the boom is 1/4 inch square brass.  The elements are 1/16 inch diameter brass rod.

Here is a photo of the driven element.  The driven element is insulated from the boom. At first I could not get the SWR down on the driven element.  Some adjusting of the driven element length, shape and spacing to the first director, I got the return loss down to -30 dB or more.  I cannot measure below that level of reflected.

As you can see, the antenna is quite small.  The antenna appears to have decent directivity.  Although I have not yet completely tested it with a weak signal source, placing my hand in front of the antenna causes wild fluctuations in the reflected power, but moving my hand around the sides and to the rear of the antenna shows very little change in reflected power.  This kind of tells me that the RF is being radiated off the front of the antenna, which is what we are looking for.

The antenna will be extended to about 2 feet.  I wanted to try to see if I could make this work with 10 elements before I waste the additional brass (and time) with a longer antenna.

I used the VK5DJ Yagi Calculator program to design this antenna.  The driven element was designed by me, using the principle that WA5VJB used on his 1296 & lower antenna.  The length was arrived at empirically, trimming the length and spacing between the DE and 1D.  The odd shape of the hairpin does not seem to affect anything, its just how I happened to bend the DE.

I still need to package up the transverter and actually finish it.  The antenna experiment was a little bit of a distraction to break some of the monotony of the rest of the project, besides, I had some ideas after talking to N6NB, Wayne at the Packrats VHF conference regarding his Quagi design.  Since his designs were done on an antenna range, with a limited number of elements, I posed the question "How do you think your principles would work on a long boom design, such as the DL6WU design - using his director calculations, but replacing the reflector and driven element with your quad elements".  His reply was that it should work.  Although I did not go with the quad elements on this design, I  did think that the WA5VJB driven element should work with this antenna, which apparently it does.

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