Category Archives: RF Design

40 Meter Antenna: Thought Process

I picked up some hardware to connect the antenna bits together into one cohesive unit. Sometimes with hardware you have to just go to the store and look around for solutions to a specific problem or situation.

20150426_122414With this project I wanted to keep things as simple as possible and use off-the-shelf materials that are easily found and relatively cheap. One of the pieces I wanted was a simple piece of hardware I have seen many times. I have even bought it and used it, a three or four-inch long nut.  We used to use them for connecting CB antennas to rigid mounts with a nylon spacer block on the bottom and a bolt. The same basic format used to connect the spring mount base for mobile use. The intended use in this project is to connect the steel spike at the top of the antenna to the fiberglass body.

I remember buying these at Orchards Supply and Ace Hardware. I never had a problem finding one back in the 80s. Yesterday I went to OSH and Home Depot and the employees looked at me like I was from mars. They had these little 1-1.5 inch versions for connecting all-thread, which is what I am looking for, just longer. No clue. Nothing registered on their faces, even with the smaller version in my hand.

20150426_122018Anyway, the next closest thing was turnbuckle bodies. The “solid” bodies were aluminum and the open body type looked like nickel plate and galvanized. I ended up buying an open body style, but it really brought about several issues. First and foremost are the mechanical issues. The contact points on both ends are fairly small, less than half an inch. Even with the fiberglass rod and the steel spike screwed down all the way and touching there is still only a half inch of contact on either end.  These things are intended for stresses in tension not in bending which s what they will be exposed to in this application.

The next issue is the left-hand thread of one end of the turnbuckle. I was willing to deal with this I even started looking for a LH die to thread the fiberglass and this is where I decided to take a step back, reevaluate, and take stock of my options.

20150426_121813At this point the base end has been worked out. The right-angle conduit box is the base and will have 12 inches of half-inch PVC conduit inserted in the port that is parallel to the box. The wound fiberglass rod will be inserted through the PVC pipe into the base. I will cut off all but an eight-inch of the other port which will be the outside housing of the SO-239 which will be inserted from the inside of the box.

Once the antenna coil is soldered to the SO-239 on the inside of the box I will fill the box and the PVC pipe with polyester resin making the base a single solid weatherproof piece. The reason for the PVC pipe being 12 inches long  is to provide a solid place for mounting standoffs to be attached. All done with off the shelf hardware and materials with only one minor modification and no special tools.

Now back to the top end and mounting the steel spike to the fiberglass rod. The most direct method is using the all-thread nut I originally wanted to use. Since it is not available, and a sufficient replacement in size, complexity, and structural strength does not appear to be an off the shelf item, at least around here, I guess I need to make one.

I thought of welding nuts together and other welded configurations but they all require more work than what is really the simplest solution. Drill a piece of half-inch bar stock and tap it for 5/16″ 18 threads per inch. I already picked up the die, now I just need the tap and drill.

This solution does require a couple of tools, tap and die along with their respective handles, the right size drill (an F), and a little machining. This operation isn’t really a big deal. If you have a bench top drill press it will make things easier, but it can be done by hand with a bench vice.

The remaining decision is material for the threaded block. I know I have aluminum and I think I have both steel and brass that are the right size, or at least useable.  I am leaning towards the steel or brass. Aluminum and steel don’t play well together when there is the potential for moisture penetration between the surfaces making galvanic corrosion an issue as well as heat dissipation and mechanical contact issues.

More to come on this subject soon.

73,
~Jon KK6GXG

All Aboard the Project Train

Sometimes it is indeed a project train, not just a project; occasionally a train wreck, but it’s early so we’ll hope for the best.

Now that I have built several receivers and a transmitter for 40 meters, I probably should have an antenna that does more than pick up all the RF noise around the shack. The antenna, and I use the word loosely, is nothing more than a dipole of 26 AWG insulated wire strung around the house. While it might work well if the wire was straightened out on a hill top, or even in a backyard, elevated 15 feet or more above ground, in its current incarnation, in a word, it sucks.

Next up on the project block, an antenna, sort of…

I have been looking at several small footprint antenna options like spiral coils that look like those God’s Eye yarn and popsicle stick things you may have made as a kid,  the one in the movies from the 40s when the resistance spy transmitting Nazi secrets and the SS storms in. It looks like a X and has thin wire wound around it.
288And Small Loop antennas that look more like old direction finding antennas on aircraft. And by the way, according to the formulas and general information, for 40 meters, not so small, 10′ diameter.

Now there are lots of variations and tons of experimental configurations to run with, and I do want to play with many of the possible configurations, but I need a stable, small, portable option for the immediate future. Something I could mount on the car, throw up on a pole, or stand up on the back porch, all without much effort. I also need to keep the cost down.

Scan from the 1975 ARRL Handbook, page 606.
Scan from the 1975 ARRL Handbook, page 606.

Enter the Helically-Wound Short Vertical. I ordered some 14 AWG lacquered copper magnet wire and a 72″ long 5/8″ diameter fiberglass rod, I need to learn to make these so I also ordered a How-To book on making composite fishing rods, and I’m in $20 so far. I have all of the hardware items including the 12″ spike for the top. What I don’t have is the variable inductor, or a tuner.

Whooooo Whooooo! Here comes the train.

RollspuleLets start with the one I haven’t done any research on yet, the variable inductor. The mechanics are fairly simple, rotate the coil and the follower moves up and down the coil creating a tap point. As long as the coil is perfectly cylindrical, the follower makes good contact, and smoothly moves down its axis rod, no sweat. he he he, no sweat I only need the one variable inductor according to the diagram so I really need to find out what value I need. I already have several small value iron-core variable inductors so I may not need to make this one, but if I do, it will be a homebrew. These things are a high-dollar item otherwise.

Moving on to the antenna matching unit. I have plans for one that is fairly simple and straightforward, I even have most of the parts on hand. What I don’t have is the variable capacitors. Sensing a theme here? Variable…

cav-167-205-71_lgBA funny coinky-dink on capacitors; while I was doing research on the Small Loop antennas I came across several websites that went into some lengthy discussions on making variable caps. Turns out, they aren’t that hard to make, I even have nearly all of the materials to make several. Not only are they fairly easy to make, but the formulas for calculating the capacitance based on the area of the plates, number of plates, and the distance between the plates is also fairly straight forward.

So lets see where we are now…
I have three receiver and one transmitter for 40 meters. There is an antenna for them, but it sucks particularly in this location. My next project is to build another transmitter, But I will need a better antenna first…

  1. Antenna
  2. Resonator for antenna (variable inductor)
  3. Antenna tuner
  4. Variable capacitors for the tuner

Now, after these are done I can move on to the new transmitter. As a side note, with the next transmitter project I wanted to pump up the power all the way to a couple of watts, not just miliwatting it. I also want to include SSB (Single SideBand, voice) as well as CW so I can incorporate digital modes into the shack. For all of this the antenna really needs to be better than what I am using.

That’s all for now, 73,
~Jon KK6GXG

40 Meter Loose End

20150410_115049Today just started off good. The post came early and an expected package came a few days early, my QSL card order from Vistprint.

I’m not quite sure when I will need to send any out, but now I have them and I am very happy with how they came out. I’m also very proud of them, I spent the better part of a day working on the layout and confirming the text-safe areas around the card. The finish product is just what I was hoping for.

20150408_154753A couple of days ago I got a package in the post from another ham, Brad  (AA1IP) and oh my, what a box it was. A medium sized  USPS Priority Mail box filled to the top with stuff.

I started sorting it out that evening and last night and again this morning. Its all sorted into like or similar components. There is still a lot of sorting to do, particularly with resistors and capacitors. Among the jumble there were a few packets of resistors already sorted by value and those are my project for tonight. Get them all stowed away.

20150408_162644The sorting process will take a while and I’m in no rush, so bag by bag over time it will all find homes. Once I get everything stowed I may ask for another. Mmmm, a masochist, I am *in my best Yoda voice*.

A small follow up project got completed over lunch today. I finally finished the power cable for the 40 meter receiver I built. The original plan was to build the receiver and a follow up transmitter, package them together, and power them both with a solar charger / battery pack. Also in the original plan was to build out the cable from scratch.

Funny thing about plans… I dont have any good multi-conductor cable anymore. The closest I have is some cat 5 and some twisted-pair for telco work. Nether is appropriate for this project. I was waiting to go to the Electronics Flea Market @ DeAnza this weekend but I also have plans to go to a radio builders club meeting on Sunday and check things out there. I was encouraged to bring my project along, and since the build was complete I thought I should power it as planned.

20150410_132528Long story short, I know, too late, I decided to pull an extra USB printer cable out of my computer junk box and use an existing cable, clip an end, and install the power plug. Easy peasy.  The plug isn’t quite big enough for the cable but I adapted it and heat shrunk that end for some extra stability. Thoroughly checked it for shorts and opens twisting it about and we are good to go.

The radio can now be powered by a computer, USB car adapter, or the solar power supply I had originally intended to use. Any standard USB source with the appropriate 5 volts will do.

I’m sure I’ll have more to post about after the Flea and the club meeting.

Until then, 73,
~Jon KK6GXG

 

Fresh Prespective

Over the last few days I have been working on a direct conversion receiver project I found in the book Experimental Methods of RF Design last month. I had been planning and even started a very similar design back in February. I was having problems with that project so I picked up the EMRFD book to help figure out what the issue was.

EMRFD-40m-ReceiverThere in the first chapter was this design and I decided to scrap the other project and started working on this one.  It wasn’t like “oooo, a new shiney…” because they are very similar designs and the goal was to build a direct conversion receiver for 40 meters, not specifically that receiver.

Now that I have finished unnecessarily defending my position on hitting the reset button on the project, we can continue.

This time I decided to go with the “ugly” method of prototyping. I was not aware of the induction issues with the plastic pinboards or the other problems that can arise with high-speed or precision circuits. I had seen a few things on ugly construction and comments on its superiority over the plastic breadboards, along with other similar prototyping methods like “Manhattan.”

Since I had never done ugly before I was a bit nervous about it. I have used perfboard successfully on other projects and while perfboard does not have the same negatives as pinboards, it doesn’t have a ground plane ether. This can be problematic in RF circuits.

Any hoo. Yesterday I finished soldering the board and started mounting stuff. The project is screaming along and I have been pulling back regularly to make sure I don’t rush to finish. I have been going back an checking the new additions for physical problems, shorts, cold soldering, and the like. So far things have gone nicely.

20150401_193337This morning I sat down and looked at the partially mounted project box and some things crossed my mind. If I only put an antenna port on the back, instead of including a power port, I could actually have an internal battery pack. I could even add the power port and include a charging circuit for the internal battery…

I nixed this idea on this build because I intended to use a battery pack / solar charger that I already have so the power possibilities will be saved for later.

I do plan on adding a transmitter to this case. This brought my attention to the front panel. I am installing a power switch and headphone jack for the receiver, but I would need a jack for the cw key and a TX/RX switch. I could fit both of these components in the front panel integrated with the receiver parts without a problem. I could even install them now to prepare for the inevitable. I even have the parts in stock.

What I didn’t think about last night was the tuning of the transmitter. I don’t have the room for the tuning components on the front panel. I don’t know if I could modify this oscillator/mixer to work in conjunction with a transmitter thus converting the whole thing into a transceiver. I will have to get back to the book and read on.

In all honesty there are a couple of other things too. I would want a beat oscillator which would need a momentary switch and some other logistical elements to think about as well.

I spent a lot of thought on keeping this board small. I didn’t want to crowd things too much, but I wanted a small planform. I got what I wanted and I learned a few things about experimental design.

Planning ahead is not just about where the rest of this circuit is going. It includes thinking about the likely evolution of the project. Would I convert this project to a transceiver? Probably not. I would more likely build a new project ether from a different design already established, or take a lot of time and build modular circuit clusters testing them as individual units and working out the logistics of placement, interfaces, and housing configurations as the project developed.

I am actually surprised at how much I have learned from building this little DC RX, and I’m not even finished with it yet!

73,
~Jon KK6GXG