Antennas


As Radio Amateurs who like to build, anything that we come across could be inspiration for a project or upgrade. I was Elmered into the hobby by  Dave Carter, KA1HDG, during a one week class at Great Lakes Adventist Academy. GLAA is an ARRL “The Big Project” School. With part of their funding they set up a satellite station… the inspiration.

Satellite Station at K4LKL Field Day 2009

Satellite Station at K4LKL Field Day 2009

This is the top of my satellite station. I made a 100 point field day contact with AO51 at 10 degrees over the horizon. My first soldered component of my first kit was inspired by that satellite station at GLAA. Needing computer control of the AZ/EL rotors I purchased the PIC16F688 Satellite Tracker Interface: ST1 from FOX DELTA. I wanted USB control so that required soldering a small FT232R chip.

My first soldered IC chip

My first soldered IC chip

This chip was the first thing I put on a board. I figured that if it didn’t work, why solder the rest.

If I can solder a chip anyone can. Heat up the iron and follow your inspiration. 73

Robin

I’ve more or less completed my small loop antenna to my satisfaction. I’ve also updated the pictures and added a good edge on picture of the completed capacitor.

Reception is great, transmission is not as good as I had hoped, but not too bad.

I’m still experimenting with the height — it seems to work well at any height above about 4 ft, but has better local range up higher and less noise down lower. I need to do some directionality experiments.

I’ve gotten use to the tuning; it would be nice if I had better control of the speed.
Perhaps a good next project would be to make a pulse width modulation controller for the tuning, or make an interface to my ic706 to handle auto-tuning via the external tuner interface.

It’s hard for me to tell if anyone cares, so this will probably be my last post about this antenna unless people express more interest (via the net, comments, email, etc.).  Unless I get around to building either the PWM controller or the automatic tuner interface, which isn’t likely in the next month or so.

As I have mentioned on the net, I’m working on building an HF small loop antenna, as described in the ARRL Antenna book, chapter 5.  The target is 80m, but the more bands I can cover the better. The antenna I intend to build should be close to this:

Diameter: 6 ft
Circumference: 20 ft
Tuning capacitor value: 28 – 380 pf (for 30m-80m coverage)
Tuning capacitor voltage: 7kv – 10kv for 100w xmit

Since the trickiest part of this antenna will be the tuning capacitor, I’ve started there. I have not seen any air variable butterfly capacitors for a while (I saw one 2 years ago at hamcation, but I didn’t know what it was at the time), and vacuum variable versions cost $150-$500 on ebay (I saw a broadcast rated one for $3k), so I’ve decided to try building one.

The parts I’ve found at skycraft are:

  • some brass 2″ bolts
  • 1 mm plastic sheets (for dielectric between plates)
  • 0.7mm fiberglass single sided PCB
  • 1.3 mm plastic washers for spacers
  • 4mm brass nuts

I’m cutting the PCB into alternating stator and rotor plates, layered until I have enough capacitance.  The rotors are spaced by the 4mm brass nut (which I solder to the rotor and then tighten to the brass bolt).  One unit of the assembly looks like  stator – dielectric – rotor / plastic spacer – dielectric.

The nut passes between the stators and through both dielectrics and touches the fiberglass side of the next rotor.  So, each nut/rotor is 4.7mm (nut + pcb).  The stator side is 0.7 + 1 + 1.3 + 1 = 4mm leaving about 0.7 mm of air gap around the rotor for it to turn in.

I’ve drawn a nice diagram at http://www2.mmae.ucf.edu/~ssd/capacitor/ which also has a log of measured capacitance which I will update as I add plates.

Currently, I’m up to 13 plates for about 280 pf.

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