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Hints and Tips
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Simple antenna tuning
Every ham should know the formula for cutting a half wave dipole:
468/FMhz = Length (in feet)
Many hams install a dipole or Inverted Vee and find that the above formula does not seem to work all that well. This is because the formula is based on the antenna being mounted in free space, without interaction with other objects. Few hams have the luxury of such an installation.
In this situation, a little algebra will help you find the proper dimensions. To start out, cut your antenna a couple of feet longer than the above formula indicates. I go by the axiom that it is easier to cut a wire than splice a wire. Next, fold back the wire so that the total length is what you calculated in the above formula. Install the antenna in the desired location and measure the SWR at the calculated frequency. Move down a few Kilohertz and note the SWR. If the SWR improves, then your antenna length is too long. Conversely, if the SWR goes up, then your antenna is too short. Continue moving the frequency in the direction of lower SWR until you reach the minimum value. Note this frequency for the next step.
Let's call the calculated frequency of your antenna Fcalc and the lowest value of SWR frequency Fnew. If your antenna was too long (I.e. the SWR was better below the calculated frequency), then we need to lower the constant 468 by the percent difference of Fcalc and Fnew. So in this case, you would calculate the constant as follows:
Fnew/Fcalc x 468 = New Constant
If you find the that your antenna was too short (I.e. the SWR was better above the calculated frequency), then we need to raise the constant like this:
Fcalc/Fnew x 468 = New Constant
Example: We cut a dipole for a frequency of 7.090 MHz. Calculating the length gives us about 66 feet. We put the antenna up and find that the antenna has the lowest SWR at 7.020 MHz. This means that our antenna is too long, so we use the first formula to calculate a new constant:
7.020/7.090 x 468 = 463.4
Plugging in the new constant, we now calculate the proper length of our antenna with this formula:
463.4/7.090 = 65.36, or about 65' 4" for a dipole on a frequency of 7.090 MHz
For this example, we need to remove about eight inches of wire from the total length of the antenna (I.e. four inches from each end) to make the SWR the lowest at our desired frequency.
In my experience, this formula will scale to other frequencies as well. For example, if you wish to put up a 20 meter dipole in the same location as the above example, you would use the same constant to calculate the total length. While it is not always perfect, this method will get you closer than using the standard formula for the majority of the time.
Most modern rigs require 13.8 VDC for proper operation. In many cases, a reduction of as little as a volt will degrade the rigs operation. This is why it is important to check the voltage at the rig under load to verify that it is getting enough voltage. This is one if the first things to check if you are having weird problems with your rig.
Proper microphone gain settings varies from rig to rig, but as a general rule, you want to adjust the level for minimal Automatic Level Control (ALC). Also known as Automatic Load Control, the ALC automatically adjusts the peak power out of your rig as you speak into the microphone. Under no circumstances should you use the power meter as a gauge of voice output power! The reason for this is because few, if any, power meters read peak power.
When running constant carrier modes such as RTTY, PSK31, SSTV, etc., you need to adjust the rig for NO ALC reading. Be sure to turn off any speech processing as well. For all intents and purposes, peak power for these modes is the same as the average power, so you can use the microphone gain for a sort of "power out" control.
A very popular mode on the amateur bands these days is PSK-31 (Phase Shift Keying, 31 Baud/31 Hz bandwidth). All one needs to operated this mode is a computer with a sound card, and some free software. However, not every transmitter will work under every condition, which is why it is important to set your rig up properly.
PSK31 works by sending a phase shifted audio signal into the audio input of an SSB transmitter. The sideband used is not important when using the most popular mode (BPSK31), but using USB makes the math somewhat easier. In this case, you can calculate your center frequency by just adding the dial frequency to what audio frequency your PSK31 program is outputting.
It is very important to set the proper audio transmit levels. Under no circumstances should you ever increase the audio levels to where your ALC starts to engage. At this point, your IMD (splatter) will start to rise, while your power output stays the same. ALC is meant for voice signals, which has an average to peak power level that varies quite a bit. PSK-31 is a constant carrier mode, so the average power does not vary, and ALC is not needed or desired.
A point that even some of the more experienced PSK-31 operators miss is setting the correct audio output frequency. Most PSK-31 programs have the ability to send (and receive) audio tones with a frequency of between 500 and 2500 Hz. Unfortunately, most SSB transmitters have a rather steep frequency rolloff at these extremes. Even operating close to the these end points can cause distortion of your signal. The ideal output frequency would be the center of your SSB filter, or something close to 1000-1500 Hz. If you need to QSY outside of these frequencies, then you may be better off resetting your VFO, rather than resetting the PSK-31 program output frequency.