Parallel Dipole for the 6 meter band

 My 3 element gamma-fed yagi crashed about a year ago.  I hadn't done anything with it, and its mangled carcass has been lurking behind the shed.  Recently my club built a cage dipole for 40m, which got me thinking.  Why couldn't I just make a cage or parallel dipole for 6 meters?  It should work.

So over lunch I took the old antenna apart, and tossed the broken pieces.  There were enough good parts to make my experimental parallel dipole.

The pieces I had left over were just a tad short, so it does cover the whole band at 1.5:1 at top of the band, with the ideal resonance somewhere around 51Mhz.

Mobilinkd TNC4 and Alinco DJ-F1

 Yes, the Mobilinkd TNC4 will work with the Alinco DJ-F1 handheld transceiver.  The PTT needs to be set to Multiplex.

The required cable is not shown on the website, since this Alinco radio is fairly old.  It does work well, however.

AEA Iso Loop 10-30Mhz antenna

We were given this AEA Iso Loop which covers 10 to 30 Mhz.  The date codes go back to 1982, and for the age its pretty good.  It was really quite difficult to keep in tune.  The mechanism was sloppy and needed some tender care and improvement.

There are a couple things to look for when restoring one of these.  The axial play must be eliminated to keep the capacitor in a precise location.  Also any gear lash must be eliminated.  Watch the video to see what was done to repair this antenna.

Here's the detail of where the additional screw was placed for the motor bracket.

Beam Me Up! Star Trek TOS Communicator for 70cm

 I've been a life long Star Trek original series fan.  Yes, it is pretty nerdy I know.  I was talking with my friend Ed KA3LJL about what it might take to create a working communicator.  We tossed around a few ideas and I settled on the Chip Carter toy that came with a nice illustrated book.  For the price I couldn't go wrong, or could I?  When it finally arrived just before Christmas, I discovered that this toy is actually 2/3 the size of the original series prop from the show.  Ok, so now I have a real challenge in miniaturization.

The heart of the working amateur radio 70cm band communicator is the SA818 module.  This little wonder costs less than the communicator toy did and will put out 1 watt on high power and 1/2 watt on low power.  To maximize battery life, low power is really all that is needed.  The purpose of this build is to create a small HT that I can use with my AllStarLink node around the house.  And I could also use it at hamfests to impress my fellow nerdy friends.

So the first thing to do was take the toy apart and see how much room was in there.

The SA818 runs easily from a single Lithium Ion battery.  I had a very small battery from an old GPS that still held a nice charge.  This fit PERFECTLY into the toy.  The sound board and internal speaker would have to go.  There just wasn't enough room to maintain the original sounds.  (Maybe I can program AllStarLink to make the sounds?)

There were a lot of supports in the plastic case that had to go as well.  I made a hot wire plastic cutter and went to town slicing up the internals of the case.  The original coin cell battery door would be used to hold a pin header for frequency and tone programming.

The swirly "speaker" would also have to go and be replaced with a real speaker.  The module doesn't produce enough audio for a loudspeaker by itself.  I had to get a little amplifier board to get sufficient audio output.  Despite the SA818 application datasheet showing that a mic could drive the input without a pre-amplifier, this was also not the case.  I had to create a preamp board for an electret mic element.  It was probably the toughest part of the build.  I made it out of surface mount components except for the low gain transistor.  the circuit works a little too well, and the gain had to be dialed back with a few resistor modifications.  In the final version I removed the mic gain adjustment and just left it at a fixed value.  The element was then soldered directly to the board.

The whole system was mocked up on a breadboard just to test the functionality and audio quality.  I didn't want to start building and packaging the final product without a good understanding of how the whole device performed.  The next part of the build took weeks of planning with only a few components added at a time.

One big design challenge was to come up with a functional antenna.  I was just going to use a dummy load resistor to keep the module happy, but that just wasn't practical.  I may want to take this down through the neighborhood on a walk, so I wanted a real antenna.  I tried a magnetic loop for 70cm.  It worked, it was very narrow and it was efficient.  The problem was that it would not fit in the case.  Ed and I kicked around antenna ideas for a couple weeks.  The flip-up grill was supposed to be the antenna, so we decided to make it the antenna.  Some strips of adhesive-backed copper tape zig-zagged back and forth across the grill made a perfect antenna.  I was able to use my NanoVNA to get everything trimmed just right.  The antenna would eventually be fed by a small low pass filter board that I bought on eBay.  It is actually pretty large, so I'm glad that it fit alright after I took off the SMA connectors.  It was impractical to feed the antenna grill with a piece of wire or coax.  It had to be able to move and twist as the grill was opened.  I created a sliding joint using the copper tape that mated to the case and was fed by a small chunk of RG-174 coax.  This design is frankly brilliant if I do say so myself.  It functions perfectly and affords a wide enough bandwidth and is probably better SWR than most rubber ducky antennas.

With the antenna design proven, I moved back to the main construction.  A main perf board would hold the components.  The battery sits underneath everything else.  The RF board and filter were joined together using pieces of copper strip material to provide a mechanical stiffness.  The RF board was then secured to the perf board by just soldering the tabs.

The preamp and audio output boards would just mount to the main perf board.  The mic would use the grill in the front part of the case.  I glued the original rubber grommet to the case.  This was salvaged from the old cordless phone that I stole the mic from.

One requirement was to have it charge from USB.  I had a USB mini jack from the GPS unit that worked perfectly.  The case was notched out to allow access to the USB port.  The battery has an on-board charging circuit, so I just put a single diode to drop things down from 5 volts to be nice to the charging circuit.

Here we see how the board fits in with the USB charging jack mounted.

The programming port is tucked into the old battery compartment.

The header allows the communicator to connect to a 5vdc FTDI TTL adapter.  Top pin is TX then RX and finally ground.

As things got closer parts were fitting exactly where they needed to be.  A potentiometer with a tiny shaft would stick through one button hole to allow the audio to be adjusted.  A Push On / Push Off switch would serve as the power switch.

Things are very tight in the case.  All the interconnecting wires had to be in just the right spots for the case to close.

The most fiddly part was the PTT button (not pictured).  I was going to cut a hole in the side of the case and just mount it off the main perf board.  This would have been easier, but it wouldn't have been as cool as using the existing holes.  I ended up mounting a tiny tactile switch under the leftmost jewel lamp.  There had to be a lot of precision filing done to make everything sandwich together perfectly.  The other fiddly part was the LED for receive audio.  It put an ultra-blue LED under the rightmost jewel lamp.  It was way too big and had to be filed down to fit as well.  After filing an LED lens it will be cloudy.  Just slather some petroleum jelly over it and it will turn clear again.  (This is just one of those tricks I learned watching MacGyver.)

I'm genuinely please with how this turned out.  It is really slick.  I can stick it in my shirt pocket and flip it open when I get a call on AllStarLink.  I bought enough parts to build two, but I won't do it again with the 2/3 size toy.

Beam Me Up.

73 DE N3FIX

Hi-Par 'Saturn 6' Halo overhaul

 I was fortunate enough to receive this old Saturn 6 Halo antenna from a friend.  It had been stored for the last 25 years.  The aluminum condition was good, but just needed a good polishing.

The fasteners were even in decent shape.  I only replaced the ones at the feed point.

It came without the original matching network.  After reading this eHam article, I learned that the impedance required a 4:1 transformer.  The details of the construction were provided.

I first tried the 25 ohm coax (parallel RG-58) @ 37".  I wasn't quite satisfied with this.  The coax tail was cumbersome and the match wasn't quite as good as it might be.

I then wound the wire transformer.

8 turns of 14 gauge around a 9/16" form spread 2" long.

One end connects to one feed point on the antenna.  Two turns in it connects to the coax ground.  6 turns in it connects to the other feed point of the antenna.  The 8th turn connects to the center of the coax connector.

The whole thing fit in a nice plastic enclosure (formerly from a wall wart).  A little squeeze here and there and the transformer functions well with a much better ideal impedance at resonance.  The ability to massage the transformer made it superior to the coax matching method in my mind.

We are ready for Field Day.

HAMVOIP - email on a Raspberry Pi pin status change

 I wanted to have the clubhouse send me an email if the alarm system went off.  The plan is to have a contact closure bring GPIO 0 (Pin 11) logic low on alarm.

The GPIO example didn't include a way to then detect when the pin went high again, so I had to come up with a slight modification.

The first step is to set up email:

https://www.hamvoip.org/howto/Allstar_email_howto.pdf

Then I wrote some clever scripts to perform an endless round-robbin task of monitoring the GPIO pin in question.

The script /home/email_alarm.sh is started from /etc/rc.local

----------------
home/email_alarm.sh
----------------

#!/bin/bash
# Required button hold down seconds
# set mode to in and pullup pin 0 (Physical pin 11, GPIO17)
PIN="0"
gpio mode $PIN in
gpio mode $PIN up
bash home/alarm_wait_active.sh &
exit

-------------
home/alarm_wait_active.sh
-------------

#!/bin/bash
# Required button hold down seconds
HOLDTIME=5
#define pin to watch
PIN="0"
TIME1=1
while [ 1=1 ]
 # wait in interrupt for pin to go low
 gpio wfi $PIN falling
 do
 # Got the low now poll to see if it stays low for holdtime
 while [ `gpio read $PIN` -eq "0" ];
 do
 sleep 1
 let TIME1+=1
 if [ $TIME1 -gt $HOLDTIME ]; then
 # if greater than holdtime then exit past done
 break 3;
 fi
 done
 TIME1=1
 continue
done
# code to execute when holdtime is exceeded goes here
#echo -e "Subject: Pi alarm active" | sendmail -v email@gmail.com
echo "alarm active"
bash /home/alarm_wait_clear.sh &
exit

---------------
/home/alarm_wait_clear.sh
---------------

#!/bin/bash
# Required button hold down seconds
HOLDTIME=5
#define pin to watch
PIN="0"
TIME1=1
while [ 1=1 ]
 # wait in interrupt for pin to go high
 gpio wfi $PIN rising
 do
 # Got the low now poll to see if it stays high for holdtime
 while [ `gpio read $PIN` -eq "1" ];
 do
 sleep 1
 let TIME1+=1
 if [ $TIME1 -gt $HOLDTIME ]; then
 # if greater than holdtime then exit past done
 break 3;
 fi
 done
 TIME1=1
 continue
done
# code to execute when holdtime is exceeded goes here
#echo -s "Pi alarm cleared" | sendmail -v email@gmail.com
echo "alarm cleared"
bash ./alarm_wait_active.sh
exit

Spot Island-style Copper Clad board prototyping

I can't claim to have invented this style of prototype board layout, I'm sure I've seen this somewhere before.  "Dead bug" style prototyping where there is no printed circuit board layout is available is also popular.  The components are glued to a copper clad board in "dead bug" style while the leads are thrust in the air above the board like legs of a dead insect.  This is a messy way to try to attach components.

I prefer this spot island style method, which is similar to Manhattan-style.  Copper clad board is great for prototyping with either through-hole components or even surface mount.  This method isn't appropriate for DIP packages, but for discrete components it works just fine.  A perf board for DIP packages can be used above the level of this method to transition the circuit into IC chips as needed.

We've all got broken drill bits laying around (unless you've thrown them away).  The broken drill bit is still a valuable commodity.  I turned a couple into hole saws for making spot islands on copper clad boards.  First put the bit in your drill or drill press and use a grinding wheel to flatten it back out past the broken point.  Then put it in the vice and cut the center out with a Dremel tool grinding disc leaving only the edges.  This creates two fly cutter blades to act as a hole saw.  Cutting through the thin layer of copper into the circuit board is all that is needed.

Don't try to hold the board in your hand.  The drill press WILL drag it out of your hand the slice up your precious work.  First drill mounting holes in the board and use some screws to hold it to a block of wood.  The wood can be fastened safely into the vice of the drill press allowing for precision machining.  Lower the bit into the board until the copper is machined away leaving an island of copper and a recessed circle of board substrate.  Don't go the whole way through the board.

Once the board is laid out with islands it can be populated using good radio frequency layout practices.  Component leads can be cut short.  In the example below the board is cut with a strip at the top to feed Vcc power to components, where the rest of the board is left for a ground plane.

Here we see the DIP package of an Op Amp floating above the copper clad.  This is done by making "stilts" of components leads.  Once the DIP part of the component is finished, it can be incorporated into the copper-clad board.

When working at VHF frequencies this construction method works very well to keep leads short.  Islands can be drilled and later bridged with short jumps of clipped leads.  I have created two islanding bits.  The larger one can accommodate a hole drilled in the center if needed, while the small bit is good for just a few component leads that must be joined together.

Don't be stifled by a lack of printed circuit boards while experimenting with electronics.  There are many useful circuits that can be manufactured in this way.  If additional RF shielding is required, card stock can be sprayed with contact cement to facilitate the adhesion of ordinary kitchen aluminum foil.  In the photo I sacrificed an old Dilbert comic strip printed on cardstock with Omega advertisements on the opposite side.

The card stock is easily folded up to provide the RF shielding that may be required.  In this project photo below, the entire circuit board is RF shielded leaving only the inductor adjustment slug exposed for alignment tuning.