20 Minute 2 Meter Ugly Antenna

I bought a home brew 2 meter J-Pole antenna for $15 last summer.  I have tried it with multiple radios with no luck.  I could receive but not transmit.  After much trial and error, 20 minutes before our club tech net call this past Sunday, I decided maybe my problem was the antenna and not the radios (I spent most of my time navigating the mysteries of the 2 meter rig menus and wondering maybe I had not set something correctly).

I had mounted the J-Pole on a 9' wooden pole.  I took it down, did some back of the envelope calculations and cut four 20" lengths of antenna wire.  I stapled one of them to the 9' pole for the radiating element and soldered it to the center of the PL239 connector.  I put ring lugs at one end of the other three wires and bolted them to the holes on the connector and let them hang down as return radials.  

I was on the air in 20 minutes and participating in the tech net.

Conclusions:

• Thee is close to 6' of really nice copper tubing in the J-Pole antenna.  In these inflationary times, $15 is a good deal.  I will cut it down and build a proper 2 meter vertical with four return radials with correct down sloping angle.
• I now know why our club president does not like these antennas.

Here is a picture of my 20 minute, 2 meter, ugly antenna.

A 500 Watt HF Power Amplifier: Part 5 - Controller

Caution: This project is in the design and simulation stage.  Changes will be made as prototyping, building, testing, evaluation and commissioning proceeds.

The controller is based on the Arduino Due module.  I have chosen it for two reasons (other than I don't have any in my junk box).  One is that it has high I/O pin count and will obviate the need for adding an I/O expander.  The other is is that it has a 12 bit A/D convertor (weather I need it or not is a different story).

I am powering the Arduino from the 12 volt supply and using its 3.3 volts output to power the controller board.

These are the connectors on the controller module, pin count, connecting module, and function.

1. Connector to the back panel (12 pin) picking up the signals from the Ten Tec transceiver.
2. Connector to the directional coupler (2x6 pin) for forward and reflected signal power levels and hardware fault status as well as sending the transmit and reset signals.
3. Connector to the power stage module (2x6 pin) bringing back the supervisory monitoring signals and sending signals to turn off power and bias.
4. Connector to the low pass filter (2x6 pin), one line for each of the filter banks for a total of 3 lines (software note - they are exclusive of each other).
5. Connector to the front panel (2x6 pin) for the LEDs and enable and reset switch
6. Connector to the fan module (2x6 pin) to set the fan speed with PWM signal
7. Serial link to the Raspberry Pi (2x2 pin) running the Stationmaster software.
8. 12 Volt power input connector (2x2 pin).

All connectors (except the front panel and back panel Ten Tec connectors) have interleaved ground pins.

The transmit signal from the Ten Tec (it is 13.5 volts high) through an opto isolator before presentation to Arduino.

The power detection and thresholding of the forward and reflected voltage are done on the directional coupler (see Part 4) and directly input to the Arduino.  From the simulation data, I did not see any reason to peak detect the Idd monitoring voltage from the Hall Effect device on the power stage module.  So, the Arduino directly reads it.  For quick action hardware protection, a comparator similar to the one used by Jeff, K6JCA is used.  The forward and reflected voltage comparators are on the directional coupler board.

The hardware protection logic is exactly the same as Jeff's except it is implemented with NAND logic (a habit from my youth).  All logic signals that matter (e.g. turning on power and bias to the output stage) are low true and pulled up at the destination to disable the function in the absence of stable input signal.  All input signals that have a risk of exceeding 3.3 volts are protected by a 3 volt zener.  The source resistance of the signal will limit the current through the zener.

Here is the schematic for the controller.

A 500 Watt HF Power Amplifier: Part 1 - Overview

A 500 Watt HF Power Amplifier: Part 2 - Power and Supervisory Circuits

A 500 Watt HF Power Amplifier: Part 3 - Low Pass Filter

A 500 Watt HF Power Amplifier: Part 4 - Directional Coupler

A 500 Watt HF Power Amplifier: Part 5 - Controller

A 500 Watt HF Power Amplifier: Part 4 - Directional Coupler

Caution: This project is in the design and simulation stage.  Changes will be made as prototyping, building, testing, evaluation and commissioning proceeds.

This circuit is a copy of Jeff Anderson, K6JCA's directional coupler with some changes.  I have chosen to use the Analog Devices ADL5904 for processing the forward and reflected voltages.  It is specifically designed for protecting the inputs and outputs of RF amplifiers (among other things).  This choice necessitates changing the attenuator circuit values.  Here is my logic:

• The input range of the ADL5904 is 45 dB from -30 dBm to +15 dBm
• The highest power level in my various simulations without the need to trip the hardware circuit is about 550 watts.
• I will design the attenuation chain to to set the  ADL5904 input to 12 dBm for about 630 watts or 58 dBm of power.
• This requires 46 dB attenuation from the output to the forward voltage port.  
• The directional coupler itself provides 28 dB of attenuation.
• That leaves 18 dB of attenuation that I will implement in two pi attenuators, similar to the K6JCAs design.

Jeff, K6JCA, in his analysis calculates the maximum reflected power  to be 56 watts with a 2:1 SWR or 75 volts peak into a 50 ohm load.  This is exactly the number I get from LT Spice simulations across various loads.

I will keep the attenuation the same for the forward and reflected paths so once the power level is read into the Arduino A/D convertors, the level of mathematical gymnastics (and the associated errors and debugging time) is minimized.  I may have to change my mind, we will see.

The ADL5904 has an absolute maximum at the RF input pin of +25 dBm.  So, there is 13 dB of headroom between the design point and the absolute maximum rating.

I will set the hardware trip point for forward power at 550 watts or 57 dBm (which makes it closer to 500 watts but this can be tweaked during testing).  After 46 dB of attenuation that is 11 dBm or 1.12 volts peak into 50 ohms.  This is also consistent with simulation results (1.15 volts into 50 ohms).

I will set the hardware trip point for the reflected power at about 50 watts of reflected power or 47 dBm.  After 46 dB attenuation, this is 1 dBm of power into the RF input of the chip and again, consistent with simulation.

So the forward and reflected power trip points are 11 dBm and 1 dBm respectively at the ADL5904 RF input. 

Consulting the ADL5904 data sheet for setting the comparator threshold voltage without calibration (I will see if that is good enough during the testing or I will have to do some level of calibration);

• For 1 dBm in the 10 to 30 MHz range: 269 and 266 mV.  I will set it to 266 mV.
• For 14 dBm in the 10 to 30 MHz range: 858 to 844 mV.  I will set it to 851 mV.

The two ADL5904s are on the directional coupler sub assembly so there will be no need to ship RF around the box any more than necessary.  The cost of this is a 3.3 volt regulator.

The schematic of the directional coupler and the signal processing circuits following it is below.

A 500 Watt HF Power Amplifier: Part 1 - Overview

A 500 Watt HF Power Amplifier: Part 2 - Power and Supervisory Circuits

A 500 Watt HF Power Amplifier: Part 3 - Low Pass Filter

A 500 Watt HF Power Amplifier: Part 4 - Directional Coupler

A 500 Watt HF Power Amplifier: Part 5 - Controller