LAB NOTES: The Magnetic Hairpin Monopole
Back in April 1989, antenneX published an excellent article on this subject entitled “The Hairpin Monopole Antenna” written by Wilfred Caron (SK) who at the time was working as a scientist at the Los Alamos Research Laboratories in NM, USA. The “Hairpin” showed great promise as a form of compact antenna. During the decade that has passed since that article appeared, the demand for compact antennas has increased even more. As a consequence, we felt it was time to revisit and further investigate this unique antenna concept as it certainly seems to possess a great deal of potential—and mystery! Now, our science investigator, Joel Hungerford begins his experiments with the “Hairpin” and his initial findings are very, very interesting indeed!—Jack L. Stone, Publisher
his month I redirected my efforts to look in more detail at magnetic antennas. Another loop-like magnetic antenna is called the “magnetic hairpin monopole antenna”. This unique antenna is a vertical made of a section of 2-wire transmission line shorted at the top and fed at the bottom. One side of the line is grounded at the bottom, the other side is tuned with a parallel capacitor (“Cp”) and fed with a series capacitor (“Cs”). Figure 1 shows the configuration. It looks like a loop squashed horizontally and squeezed out vertically!

It is well described in an excellent little book Antennas and Transmission Lines by John A Kuecken published in 1969 by Howard W. Sams. I found the hairpin was easily built out of 1-inch copper water pipe. It is stable, understandable, and easily tuned-and there is lots of room for experimenting!
The hairpin acts like a very high Q loop. It has a very narrow bandwidth and a low radiation resistance. Therefore, the experimenting is along the lines of widening the bandwidth and simplifying the tuning from band to band. It radiates magnetically by high currents induced unsymmetrically in the transmission line. These high currents imply very high voltages at the feed point. (The radiated power can be considered dissipated in the radiation resistance R and is expressed as I^2*R or V^2/R. When R is on the order of 1 ohm or less, both I^2 and V^2 have to be very large to radiate useful power.)
The first step in understanding the hairpin antenna is to devise a way to predict the tuning curve. antenneX originally published an article, The Hairpin Monopole Antenna, in its April 1989 paper issue written by Wilfred Caron (SK) based on information derived from Kuecken’s book. This interesting antenneX article by Wilfred contained a tuning curve for a 14 foot hairpin, which operated from 2 MHz to 30 MHz. I measured the values of capacitance from this tuning curve, and compared them with the values computed. Table 1 shows the results.

The method used consists of the following steps:
- Measure or compute the length in wavelengths from the short to the feed point at the bottom of the antenna
- Using the Smith Chart, move around the outer circumference of the chart from the zero impedance point of the chart toward the generator (clockwise) to the value found in (1).
- Read the reactance at the point in (2) of the shorted transmission line seen from the feed point.
- This reactance will be expressed in units of the characteristic impedance Zo of the transmission line. (Zo of the line is given by the equation below). Zo is around 240 ohms, and with these short antennas the reactance is between .1 and .9*Zo. This reactance increases with frequency, so the antenna is inductive.
- Compute the size of the capacitor that has the same size reactance (but opposite sign). This is the value that resonates the antenna.
The impedance of a 2 wire transmission line is:
Zo = 276 log(2*s/d) s = center to center spacing of the wires
d = diameter of the wire
log = log to the base 10
reference is the ARRL Electronics Data book by Doug Demaw W1FB
The impedance of a capacitor is:
Xc = 1/(j*2*pi*f*C) f = frequency in cycles/sec C = capacitance in farads

The next step was to compute the capacitance change I might expect to tune a 2-foot hairpin across the 20 meter band. Table 3 shows the result: it only takes about 12pF! The 2-foot hairpin is tricky to tune!
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.


Table 4a shows the calculations for the 53 cm hairpin #2 along with the measured values that resonated at the frequencies listed. The frequencies were the natural frequencies achieved using a collection of fixed value transmitting mica capacitors. This was an effort to standardize the tuning at each frequency. The data shows that the calculations give the sum of the parallel and the series capacitors. This makes sense since the so called “series capacitor ” is really in parallel with the so-called “parallel capacitor” with the ground end connected through the 50-ohm cable impedance. Table 4b shows the Q of hairpin #2 at the test frequencies. No attempt was made to find out why the Q was low at 11 and 21 MHz.

I found that the measurement of these mica capacitors, which ranged in value from 100pF to 2000pF, was very sensitive to lead length. These values all exhibited a series resonance using the lead inductance at some frequency in the HF band. The result was a capacitance that varied with frequency when measured with the MFJ antenna analyzer. This is because as the resonance is approached, the cancellation of L and C reactance that is complete at resonance is not complete elsewhere. Below resonance, the C component is being more and more cancelled as frequency rises. Smaller residual capacitive reactance computes to higher values of apparent C as the frequency approaches the resonant frequency. The leads are too long for the test frequency if the measured value changes with frequency.
Table 5 shows the composite of several experiments with hairpin #1 tuned to 14.1 MHz and driven by the MFJ-249 antenna analyzer, which gave SWR and the exact frequency digitally. Hairpin #2 was tuned with a variable capacitor and moved around in the vicinity of hairpin #1. The radiation was sensed with a small antenna outside, 55 feet away. Three conditions of the two hairpins are tested: the radiation curve and SWR for hairpin #1 alone, the radiation curve and SWR for hairpin #2 tuned to the exact same place as hairpin #1 and located 75 cm from hairpin #1, and the same conditions with the spacing between hairpins of 18 cm. The three conditions are listed on the same frequency scale to show how the radiation response of the conditions compare.

The first column in Table 5 is the frequency in MHz. The steps in frequency are small in the vicinity of a pattern peak, and larger elsewhere. The yellow background indicates the frequency and value of a radiation peak. The rust color background shows the values of the radiation null that appears when the coupling between the antennas is increased.
The second and third columns show the radiation detected and the SWR for hairpin #1 alone. The data shows that the radiation versus frequency for the hairpin is maximum when the SWR is 1:1 and appears to have approximately a (sin x)/x shape-a typical resonant peak.
The 4th and 5th columns show what happens when the hairpins are very strongly coupled. A very sharp null appears in the center of the radiation peak as the coupling between antennas is increased. At a spacing of 18 inches the null is over 50 dB deep and the radiation peak splits into two peaks separated by 917 kHz. The two peaks are between 4 and 5 dB weaker than the single antenna radiation. This loss probably is due to the additional losses in the second hairpin, and seems to be independent of coupling. The good SWR of 1.35:1 stayed with the higher frequency peak, and the lower frequency peak had an SWR of 8:1.
The 6th and 7th columns show the effect of coupling at 75 cm spacing. The SWR went from 1:1 to 1.3:1 at hairpin #1 and a radiation null was observed as hairpin #2 was tuned through the radiation from #1. At 75 cm spacing, the null is only about 5 dB deep. A setting was found in the tuning of hairpin #2 that flattened the top of the radiation curve as shown in columns 6 and 7. After producing Table 5, it seemed useful to show the skirts of the radiation pattern in columns 6 and 7. By then, I had set up the 20-meter loop. I took that antenna back down and set up the hairpins again and tuned as before. The tricky tuning meant that I could only get within a few kHz of the previous setting. So the skirt of the radiation pattern is shown in column 8 with a similar color to column 7. The new column indicates the data is not part of the same test as column 7 but is a similar set up at a different time.
Since the hairpins are basically magnetic loops, they can be rotated around the long axis to change the coupling. The 18 cm spacing’s null could be completely eliminated by putting the planes of the loops at 90 degrees to each other. Thus, a larger ensemble of hairpins might be more easily adjusted if they were mounted on individual pieces of copper attached to rotate about the long axis.
The previous hairpin articles referred to described a procedure, where a short part way up, the 14-foot hairpin was switched on in order to tune the antenna to a higher frequency. At, say 14 MHz, the 14-foot antenna is almost a quarter wave long–not an electrically short magnetic antenna!
Originally posted on the AntennaX Online Magazine by Joel C. Hungerford, KB1EGI
Last Updated : 30th May 2024