Trimming a Dipole Antenna
You’ve got that shiny new HF transceiver out of the box and on the shelf in your shack. You have a nice DC power supply ready to provide 13.8 volts. You’ve even got the rig components properly grounded! Now, what do you need to do with that dipole antenna to get on the air?
This is a common scenario for many new aspiring HF operators. As Bob KØNR mentions in his article, Your First Dipole Antenna, erecting an antenna for HF operations is perhaps the most challenging aspect of establishing a basic HF station. The horizontal wire, half-wave dipole antenna is one of the simplest HF antennas to set up, it offers very good performance, and that makes it a very popular choice for hams. Let’s see how trimming a dipole antenna, and following a few other guidelines, can make it glimmer like an RF gem!
To get the best performance from your dipole you’ll want to follow a few simple guidelines.
Try to keep the dipole away from other conductors, especially long, linear ones like household rain gutters, or at least try to avoid aligning the dipole parallel with such conductors.
A dipole will provide low take-off angles for good over-the-horizon skip propagation when it is approximately one-half wavelength above the ground. At lower heights the radiation pattern will become more vertically directed and more omnidirectional.
The strongest signals radiate broadside to the antenna, or at right angles to the orientation of the dipole’s wire, and you may want to establish your dipole so that those strongest signals are pointed in desired propagation directions.
Be sure to seal up any connectors that will be exposed to the elements to avoid water penetration into your coaxial feed line.
Finally, trimming your dipole antenna for the band and frequency range you intend to operate on is critical!
Trimming a dipole antenna refers to the adjustment of antenna length to operating frequency. The total length of the dipole should be just under one-half wavelength for the operating band. When the dipole is properly trimmed for an operating frequency the antenna feed point will present an impedance that is closely matched to the feed line impedance. When feed line and antenna feed point impedances match, your antenna system will have effective power transfer and will radiate efficiently. If the trim is poor for the operating frequency the impedances will not be well matched and some of your transmitter’s power will be reflected back down the feed line instead of being radiated as RF energy.
SWR: Nearly all antenna systems will have at least a little power reflection due to mild impedance mismatch at the antenna feed point. The standing wave ratio (SWR) is a comparison of the forward power in your antenna system with the reflected power. A low SWR indicates little power reflection and efficient power transfer to the antenna, while higher SWR values indicate greater reflection and less efficient power transfer. Generally, you should strive for a low SWR in your antenna system. You can judge the proper trim for your dipole by measuring the SWR as you adjust the antenna length.
Measurement Instruments: How do you measure SWR in your new dipole? You’ll need a measurement instrument. Two very popular instruments for trimming a dipole antenna are the SWR meter and the antenna analyzer. These two instruments work differently, so let’s briefly review the functioning of each.
The SWR meter is positioned into the feed line between the transmitter and antenna. Most hams will place the SWR meter into the feed line immediately after the transmitter so the readings are viewable in the shack while transmitting. The SWR meter evaluates feed line voltages in the forward and reflected directions and displays the SWR computation for the operator. So, you have to actually transmit a signal for the SWR meter to take a reading, and you must read the SWR value during the transmission.
The antenna analyzer requires the feed line to be connected to it, but no connection to the transmitter is needed. The analyzer generates its own signals for the antenna system, computes SWR, and displays it to the user alongside frequency. It is very common for an antenna analyzer to allow the user to dial through a range of frequencies while observing the SWR readout, and some will display a graph of SWR across a frequency range, similar to the SWR curve plots depicted below. This way the user can see where along the frequency range the SWR value dips to its minimum value, and thereby see the precise frequency range for which the antenna is currently trimmed.
Step-by-Step: With those measurement devices in mind, let’s consider the big picture practical steps of trimming a dipole antenna:
Determine the band and frequency range for which you desire the antenna trimmed. For example, you may want to trim a 20-meter band dipole for the General Class phone frequencies of 14.225 MHz to 14.350 MHz.
Compute the approximate antenna length for the center frequency of the range for which you are trimming. In our example that would be a trim for about 14.287 MHz, or a dipole length of about 32.75 feet (32 feet, 9 inches).
Cut the dipole wire to be a little longer than the computed length – it’s easier to cut wire than to extend it. So, perhaps you would cut your 20-meter dipole length to be about 34 feet long, with each of the two segments at about 17 feet. (17 x 2 = 34)
If possible, erect the dipole into the desired position to make SWR measurements. You might accomplish this by anchoring the center point of the dipole in its intended elevated position, while using lengths of cord to temporarily “pull up” the ends near their intended permanent anchor points. The specific methods used will depend on your dipole configuration (flattop, inverted V, or sloper) and its height above ground, as well as the type of anchor points being used.
Note: Getting the dipole into its approximate operating position and height above ground will provide the most accurate SWR measurements, especially if other unavoidable conductors are within a wavelength of the dipole’s operating position.
Note: If you cannot erect the dipole near its final operating position, approximate it as closely as possible and elevate the antenna above the ground to the extent possible for measurements.
Use one of the measurement instruments to determine the frequency at which the lowest SWR is achieved. (See SWR Measurement Techniques that follow.)
Given the extra-long length of wire left on the dipole segments, the SWR should bottom out at a frequency below the desired operating frequency. In our example let’s suppose you measured a minimum SWR of 1.2:1 at 14.100 MHz.
To raise the frequency of minimum SWR, trim the antenna shorter. Cut each of the dipole’s segments by equal amounts so that the two halves maintain equivalent lengths.
If the minimum SWR is minimized at a higher frequency than desired, you must lengthen the wire segments. This is usually a very rare circumstance, but to avoid it you should trim carefully and trim often rather than taking only a couple of giant chunks of dipole length at once!
Physically trim the wires shorter – lower the antenna ends to accomplish this if you erected the dipole near its operating position. You may trim in one of two ways: Either cut the wire or wrap the wire back along itself toward the center feed point. Be sure the wire is routed through the insulating anchor before wrapping, and you may wish to use a combination of cutting and wrapping to carefully trim into just the right frequency without having an excessive wire wrap.
Reposition the dipole and make another SWR measurement to see what effect your trim has had. Likely you’ll find the frequency of lowest SWR has been raised closer to your desired center point frequency, but not yet there.
Repeat the trim action in small adjustments until you achieve lowest SWR near the desired frequency.
Once you have your antenna trimmed satisfactorily for your desired operations, tie it up permanently and get on the air! It’s a good idea with dipoles to provide a little strain relief for the wire, and a little slack or droop in the wires will not impact performance significantly. Especially if you are using trees as anchor points, be sure to provide some slack and strain relief to avoid snapping a wire when the trees move around with wind. Some operators prefer to hang a weight over a pulley or over a tree limb with the cord attached to the horizontal dipole wire. When the tree moves the cord and weight will keep the wire taught without over-straining it.
SWR Measurement Techniques: Before we wrap up, let’s chat about SWR measurement techniques. We’ll start with the antenna analyzer, since it is usually more convenient than the SWR meter.
It is easy to dial across frequencies to find the lowest SWR with an analyzer. You can measure, adjust the trim, and measure again in quick cycles. However, you may want to plot an SWR curve rather than just identifying the lowest SWR frequency. The SWR will be lowest at just one frequency position, and it will rise gradually for frequencies above and below this center point. An SWR curve is typically a U-shaped or V-shaped curve with frequency plotted horizontally and SWR plotted vertically. Such a curve tells you more about your dipole’s performance across the frequency band on which you are operating. A common metric of antenna performance is SWR bandwidth, and this is often defined as the bandwidth for which the SWR is at a value of 2:1 or less. At SWR values greater than 2:1, most modern transmitters will begin to automatically reduce transmit power to avoid high power reflections returning into the transmitter circuits.
An SWR curve is pretty easy to plot with an antenna analyzer. Simply record the SWR readings every few thousand kilohertz as your dial across the frequencies with the analyzer. Then, plot the SWR values against frequency with graph paper or using a spreadsheet utility on a computer. As noted above, some analyzers will display an SWR plot directly, reducing the necessity to record individual frequency-SWR pairs for manual plotting.
Plotting an SWR curve using an SWR meter requires slightly more effort. As noted, the SWR meter is read while transmitting with the meter inserted between the transmitter and the feed line/antenna system. You must change your transmitter frequency and take multiple SWR readings across the frequency band. Again, tune your transmitter in steps across the band and record the SWR readings with each transmission, and then plot your results as described above. Be sure that you do not transmit in sub-bands for which you do not have privileges! Stay within your license class sub-bands.
You can “move” your SWR curve up or down the frequency band by changing the length of your dipole. Your performance with your dipole should be quite satisfactory within the 2:1 SWR bandwidth that you measure with these techniques, and with an antenna tuner you will probably get somewhat reduced, but still quite operable, performance well outside of your 2:1 bandwidth.
Multi-band Dipoles: And remember, there are several different varieties of dipole antennas, some of which can help you get onto multiple bands with a single antenna and feed line. The fan dipole, or multi-element dipole, is a good choice for the amateur who wants to have access to three, four, or even more HF bands with a single antenna. The trap dipole offers similar multi-band performance. See our General License Class book for more about these options.
I hope this helps you get off to a great start with a dipole antenna on the HF bands. I’ve used one for years, stealthily positioned in high pine trees near my home. Good luck, 73.