A bowtie antenna is a type of antenna that reputedly provides higher gain at lower radiation angles than a center-fed dipole antenna at heights considerably less than 1/2 wavelength above ground. Bowtie antennas have been around since the “spark gap” days of radio. I was curious about HF bowties so I decided to learn more about this type of antenna.
Phase I – Antenna Modeling
I selected a simple form of the bowtie antenna for modeling purposes. Figure-1 shows the target bowtie configuration.
Figure-1. 40 meter bowtie antenna configuration
How long should the radiators (L) be and how far apart (W) should they be at the ends? I started with a commonly used formula for computing the radiator lengths of a 1/2 wavelength dipole (468 / freq. MHz).
L = 468 / 7.15 MHz = 65.45’; 65.45 / 2 = ~32.72’ = 33’ (rounded up)
For W, I decided to start with 20% of the length of L. Why 20%? I had to start somewhere and I knew that I could easily adjust the value of W in the model.
W = 33’ * .20 = ~6.6' = 7’ (rounded up)
The antenna was specified to be in a horizontal plane 25’ feet above real ground. The radiators were defined as #14 AWG stranded insulated wire. The antenna was assumed to be center-fed with 450 ohm balanced line using a 4:1 balun at the source for impedance matching purposes.
I plugged in the starting values of L & W and adjusted the model to get the best predicted SWR curve. After some experimentation, I found that a value of 6’ for W and radiator lengths of ~31.25’ produced the best predicted SWR curve.
The EZNEC models used in this project can be downloaded from the links below.
Predicted radiation patterns: Figure-2 shows the EZNEC predicted elevation radiation pattern for a 40 meter center-fed dipole at a height of 25’.
Figure-2. 40 meter center-fed dipole predicted elevation radiation pattern
Figure-3 shows the EZNEC predicted elevation radiation pattern for the target 40 meter bowtie antenna at a height of 25’.
Figure-3. 40 meter bowtie predicted elevation radiation pattern
Predicted SWR curves: Figure-4 shows the EZNEC 40 meter center-fed dipole predicted SWR curve.
Figure-5 shows the EZNEC predicted SWR curve for the target 40 meter bowtie antenna.
Figure-5. 40 meter bowtie predicted SWR curve
Before you get concerned about the predicted bowtie antenna SWR curve, remember that balanced line will be used for the transmission line. High quality balanced line provides low transmission loss even in the presence of relatively high SWR values. What we really care about is antenna efficiency which is a measure of the effective power delivered to the antenna feed point. We will examine the estimated efficiency of the completed antenna later in this article when the antenna analyzer measurements are available.
Comparison of center-fed dipole and bowtie models: As far as predicted radiation patterns go, the center-fed dipole is a “cloud warmer” with negative gain at radiation angles less than 45 degrees. On the other hand, the bowtie exhibits modest gain at radiation angles of 25 degrees and higher. The bowtie antenna predicted SWR curve is flatter than the predicted SWR curve for the center-fed dipole. According to the model predictions, the bowtie antenna should provide good local/regional coverage. Encouraged by the predictions, I decided to build the bowtie antenna.
Phase II – Construction
The characteristics of the wire used and environmental conditions present will affect the physical lengths of the radiators needed to achieve the best SWR curve. If you build the antenna, be sure to allow extra radiator length to compensate for wire characteristics and factors such as ground quality and “capacitance” caused by proximity to trees or buildings. The extra wire can be left dangling at the ends of each radiator and trimmed as needed to achieve the best SWR curve.
TIP: If you don’t have 65’ feet of horizontal space for the antenna, let the radiators dangle at the far ends so you can fit the antenna in the space you have. Keep the lengths of the dangling portions of the radiators to 20% or less of the radiator length (L) and the antenna will still perform well.
End spreaders: The spreaders used to maintain the end spacing were made from 2 X 2 X 8 treated wood furring strips. To facilitate attaching the radiators and support ropes, I inserted 1/4 X 20 eyebolts at the ends of the spreaders. Figure-6 shows a diagram of the spreader bar.
Figure-6. diagram of spreader bar
TIP: If you use wood furring strip spreaders, select the strips carefully to find two that aren’t full of knots, split, bowed, or dripping with preservative. After you insert the eyebolts, spray paint the spreaders and bright metal hardware with dull finish camouflage paint to reduce visibility.
Figure-7 shows one of the completed end spreaders.
Figure-7. completed spreader bar
Feed point connector and balanced line support: The feed point connector was made by inserting an eyebolt in the center of a plastic dog bone insulator. The balanced line support was made from rigid composite landscape edging. The support “sandwiches” the balanced line between two pieces of edging. Nylon bolts (1/4 X 20) and wing nuts were used to fasten the two halves of the support. The support is attached to the feed point connector with a plastic zip tie. Ring terminals were crimped on the ends of the radiators and balanced line. The radiators were connected to the balanced line with 3/4” long #8 pan head screws, flat washers, lock washers, and wing nuts. Figure-8 shows the completed feed point connector and balanced line support.
Figure-8. feed point connector and balanced line support
Phase III – Analysis
The completed bowtie antenna was hoisted and analyzed with an AIM-4170C. Figure-9 shows the measured SWR curve after the antenna was tuned. Notice the measured SWR curve is lower than the model predicted SWR curve and is relatively flat across the 40 meter band.
Figure-9. 40 meter bowtie analyzer measured SWR curve
40 meter bowtie antenna efficiency estimate: We can use the AIM-4170C impedance readings and the free online transmission line loss calculator (TLLC) provided by VK1OD to estimate the efficiency of the completed bowtie antenna. The parameters for dry Wireman 554 balanced line were used in the calculations. Figure-10 is a table showing antenna efficiency estimates at different frequencies across the 40 meter band.
Figure-10. antenna efficiency at different frequencies
Based on the estimates, the completed bowtie appears to be reasonably efficient for a 40 meter antenna at a height of 25’ (~.28 wavelength). Most autotuners should have no problem matching the 40 meter band impedances presented by the antenna.
Phase IV – Operational Test
Operational test configuration: The operational test configuration for the bowtie antenna consisted of a IC-706 MKIIG transceiver and a Palstar AT-500 manual antenna tuner. The Wireman 554 balanced line TL is connected to an LDG RBA-4 4:1 balun. The RBA-4 is connected directly to the AT-500 using a double-ended PL-259 connector. The AT-500 is connected to the IC-706MKIIG with a short coax jumper cable. Figure-11 shows the operational test configuration.
Figure-11. operational test configuration
Bowtie operational test results: September 17, 2013. QTH Cary, North Carolina. 100 watts.
I’m a member of the Navy Amateur Radio Club (K4NAR.org) that operates a 40 meter net on 7.245 MHz (0800-1000) daily. I was able to check in with the duty net control located in Georgia with a 59 signal report. The net routinely has check-ins from all over the eastern half of the United States. I was able to clearly hear check-ins from New York to Florida and as far west as Ohio. Check-ins from North Carolina were also clearly heard. I checked in with the daily Communications Ontario Net (Chatham, Ontario, Canada) on 7.153 MHz with a 59 signal report. I was able to hear numerous net check-ins from all over the Eastern half of the country on ECARS (7.255), MIDCARS (7.258), and SOUTH CARS (7.251).
CONCLUSION: The bowtie antenna performs well as a local/regional antenna capable of covering the Eastern half of the United States and lower Canada. The bowtie antenna would make a good Field Day antenna that could easily be suspended between trees or a couple of portable masts. To me, the most attractive features of the antenna are the good distance coverage it provides at a low height and the flat SWR curve. Flat SWR curves are nice when using a manual antenna tuner. All you have to do is tune once and use the same settings for the entire band.