Preliminary Tuning of Little Tarheel II Antenna
This is Part 10 of Outfitting a Jeep JK Unlimited for Ham Radio. In this part, I will walk through a preliminary tuning of a mobile HF antenna.
Amateur Radio Equipment to Install
Tools and Equipment Used
- MFJ-9213 SWR/Watt Meter
At this point in the installation of my portable ham radio station, I have all of the major components installed. I am almost ready to move into eliminating alternator noise from the system, grounding (which isn’t the same thing), and maximizing performance of the radio station itself.
With the screwdriver antenna and controller installed, now is a good time to perform a preliminary tuning of the antenna to measure performance across several amateur bands. I also want to know approximate values for the antenna controller when I put the station on the air.
Step One – Program Radio and Lower Transmitter Power
When I operate portable, everything will take place in the phone sections of the amateur bands.
For this test, I loaded the Emergency Center of Activity frequencies using my radio programming software. I used the official Yaesu software by RT Systems.
Using the menu controls on the radio, lower power to 5 watts. Not only do you not want to cause any interference during the tuning process, you don’t want to damage any equipment by running 100 watts through the system.
Step Two – Insert SWR/Watt Meter
The key to this experiment is measuring reflected power in the antenna. To do this properly, you will need an SWR/Watt Meter that switches between Forward and Reflected power measurements. Since we’re tuning with only five watts, a QRP Watt Meter will do the job.
Step Three – Adjust Screwdriver for Loudest Atmospheric Noise
Once you have the Watt Meter between the transceiver and antenna, use the antenna controller to raise the antenna to the point where you hear the loudest atmospheric noise. This process will put you in the ball park of the correct antenna length.
Step Four – Turn On Beacon Function or Key Transmitter
Next, you will apply power by keying the transmitter. If you do not have a Morse Code key, you can use the programmable beacon feature of the FT-857D. I chose this later option rater than make a new wire for my key. Make sure you place a few “T’s” at the end of your beacon so you can have a good reading on the meter. You can also slow the code speed down to 10 words-per-minute to help.
If you’re not a Morse Code operator, here is the beacon text I used:
VVV VVV VVV DE N4AE N4AE N4AE T T T T T
Step Five – Calibrate Forward Power on Watt Meter
While the beacon is transmitting, adjust your watt meter so the Dah peaks at five watts. If your watt meter is not as sensitive, use the closet power level to gauge 5 watts.
If the beacon stops transmitting before you calibrate for forward power, wait for the next beacon.
Step Six – Switch to Reverse Power on Watt Meter
At the next beacon cycle, switch your watt meter to measure reverse power. This is the power reflected back to the transmitter when it transmits. You want the LOWEST reflected power reading possible. Anything higher than 1 watt is too much. Our target is 0.5 watts or less.
Step Seven – Adjust Screwdriver for Lowest Reflected Power
Again, while the beacon transmits, use the antenna controller to raise or lower the screwdriver antenna until you find the point of lowest reflected power. This setting is where you will configure your antenna when you want to transmit.
Later, when we perform our final tune of the antenna, we will test several frequencies in order to measure the bandwidth of the antenna system. In a mobile configuration, it isn’t good to assume that one good tune in the center of the dial will suit us elsewhere on the dial. But this is a project for later.
Step Eight – Turn Off Beacon and Record Observations
From the radio, turn off the beacon and record your observations. In the chart below, I recorded frequency, forward power, reflected power, and the TuneMatic setting.
Step Nine – Change Bands
Change bands on your radio. Go up one band from where you last measured.
Step Ten – Repeat Steps Three through Nine
Rinse and repeat as long as your watt meter will work.
Here are my preliminary observations. Since my SWR meter does not work on 50 MHz, I can’t get an accuracy tune on 6 Meters. Once I ground the antenna to the chassis, I will measure performance again, next time on at least four frequencies per band. I want to see how much bandwidth the antenna has in this configuration.