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.

Monday, November 8, 2010

2304 MHz Transverter - Photos

As I promised, here are some photos of the nearly completed 2304 rig.

 This is the complete rig

 One of the modified RF amplifiers with the cover off.

 The other amplifier.

 Mixer and IF Attenuator

 Bandpass filter

 Unused amplifier module.
Modified MMDS unit.

So these are the pictures of how this thing looks right now.  The amplifiers are not bolted down to the aluminum panel yet.  I have yet to install a T/R switch and I need to come up with some kind of power distribution.  This unit takes +20V, -20V and +12V to make it all work.

Sunday, November 7, 2010

2304 MHz Transverter - Its almost done!

I know I promised a posting describing the filter and other modules that I'm using in this project, but I thought I would wait until I had gotten some of it actually working.  Since I am now getting a useable amount of power out, I felt it was time to write a little.  Today I built an IF attenuator to drop the 144 MHz IF level from 250 mW down to 1 mW to feed the mixer.

I then started hooking up the modules that I had built up.  The LO was fed through a commercial low level amplifier module that was part of an amplifier chain which I will describe in a couple of paragraphs down this entry.  Measured LO was about +16 dBm.  The mixer wants between +10 and +20 dBm, so this level is perfect.  From there, the output goes into the LO port of the mixer.  The IF port goes to the 24 dB attenuator that I just built, and from there goes to the FT-817.

The RF port is connected to the band pass filter.  This is a 3 pole inline resonator filter that had to be modified slightly to get it to tune 2.4 GHz.  This filter does have a cool feature that is worth talking about.  It has 2 notch filters incorporated in it.  I tuned one notch to the LO frequency, the other to the image.  This should be one clean transverter!  The LO and image look to be down better than 80 dB because of these notch filters.  The tuning of the bandpass filter is quite sharp as well.

From the band pass filter, the RF goes through my 2- mmic preamp I built from Down East.  I'm only seeing about 20 dB of gain, however, so I'm not sure if there is a problem or not.  I'm getting 0 dBm out of the preamplifier.

WA3TTS gave me some surplus 1.85 - 2 GHz hardware, including a filter and an amplifier chain.  The amplifier chain consists of 5 modules.  I'm not certain how much input power is required to drive the entire chain to its rated 5 watts, but my guess is about a milliwatt or 2.  The modules all needed to be retuned to 2.4 GHz.  The modules are built one stage per module, and are built on ceramic circuit board material and have etched tuning and matching circuitry with no tuning adjustments.  The modules that I've tuned so far have been tuned with my Dremel tuning tool - carefully grind away printed capacitors and tuning lines.  This is opposite of most microwave commercial hardware, as most surplus is designed to operate above the amateur bands, such as the MMDS unit that I added material to its internal PC filters to get it to cover the ham band.  Because these units operate below the band, the opposite has to be done - copper has to be removed.  Because the copper is either vacuum deposited or etched on ceramic, an X-Acto knife was pretty much useless.  I then attempted grinding away the traces carefully with a fine grinding bit on the dremel tool.  With the unit powered up and the covers off the modules, I connected the power meter to the output of the first module through the attenuator, and applied drive from the IF.   I then began grinding the input network in the module while watching the power meter.  As the power came up, I continued until I saw no more increase.  I did the same with the output network.  I'm seeing about 10 dB of gain thru each of 2 modules I have done so far.  Currently I'm getting +23 dBm out of the last amplifier.

Well, thats where I am with the project right now.  I still have a 5 watt PA to tune.  All of the modules have to be mounted down to the chassis, and the T/R switching still has to be built.  The receiver is working, and the transmit is working at the 200 mW level, and the next thing I think I'm going to do is install the T/R relay and try to make a short range contact with what I have so far.  The 5 watt PA will be more difficult to tune, as it consists of a pair of devices which will have to be balanced somehow.  I'm not entirely certain how I'll retune it, but once I figure out a gameplan on tackling it, you can bet I'll be posting a procedure right here.  Check back soon for the info!
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