A small 500 kHz vertical antenna

A small 500 kHz vertical antenna.

This antenna, used for my 500kHz beacon, is a small base loaded vertical. It is very small and consequently very inefficient but nevertheless the slow CW or QRSS signals from the beacon reach 100miles radius most of the time and this range extends after dusk. So far the best 'DX' report has been from John, GM4SLV in Shetland over a distance of 984km. - All this from apx 5mW ERP.

The antenna specification changes slightly as I experiment with different configurations. What I've described here is the antenna used for the measurements below. Here are a few basic facts about the beacon:

The antenna is a 10m high vertical made from 2m sections of aluminium tube held together with hose clips. The antenna has a small 3m top loading wire which comes down at a 45 degree angle. The base is insulated from ground by having the vertical radiator standing on a plastic insulator. The feed point is a solder tag bolted onto the base of the vertical.

Transmitter output. 7.3 Watts PEP         Antenna Current. 280mA          Loading coil Inductance. 898uH         Calculated antenna capacitance 117pF

Calculated antenna performance
Total loss resistance ( Ground + coil + radiation resistance) from P= I^2 R   93 Ohms
Electrical length of 10m antenna at 503kHz   5.4 degrees
Antenna bandwidth at 2:1 SWR    15kHz

Matching: Any length of vertical that is less than a quarter wavelength will be capacitive and can be brought to resonance by adding a loading coil in series with the radiator. As my vertical is around 5.4 degrees long at 504kHz it needs a large amount of inductance. This a approximately 900 micro Henrys. My loading coil is actually 3 coils in series. The lower coil has one end connected to ground or earth and the 50 ohm feed from the transmitter is tapped onto the coil 15 turns up from ground. the 'top' coil is a variometer so that I can adjust the resonant frequency of the antenna and the middle coil just adds some more inductance so resonance is within range of the variometer.

If all this sounds a little Heath Robinson, then I've described it quite well. Have a look at some pictures and then the plots from an N2PK network analyser as built by G3WOE. - (Thanks for spending time on a rainy evening making the measurements Michael.)

Matching network VSWR of the antenna

Click on the small image on the left for the full size picture of the matching network. Note the croc clip connections to the tap on the first of the loading coils from inner of the coax and the braid connection which connects the lower end of the coil to a ground radial wire and a wire to a 1m copper ground rod.

Vector Network Analyser plot of the small vertical antenna from 300kHz to 800kHz.

So, what's on this Smith chart? The Smith chart above has a central horizontal line which marks points that are purely resistive. Zero ohms is at the left, while infinite resistance is on the right. The normalised resistance of 50 ohms is in the middle. The 'hemisphere' above the central horizontal line is inductive while the area below the line is capacitive. What the VNA does is to measure and calculate individual values for resistance and reactance. After making many measurements across a specified range of frequencies, the individual points are plotted and joined up as the curved green trace on the chart.

The Smith chart above shows the impedance presented to the VNA from the antenna its matching network and 15m of RG213 coax.
The red marker is at 505kHz Resonance is shown at the points where the plot crosses the central horizontal line of the chart. Any point on the line is purely resistive and therefore resonant. The plot shows 3 resonant frequencies. As you can see, 505kHz is just above resonance with the load shown as 37.1 Ohms with 2.6 Ohms of inductive reactance. The plot below looks more closely at the frequencies near 501 to 504.

Vector Network Analyser plot of the small vertical antenna from 490kHz to 550kHz.

This plot looks at the antennas impedance from 490kHz to 550kHz with a marker at 503.20kHz. Looking at the plot, 490kHz is the start of the plot in the lower (capacitive) section of the Smith chart. It then travels up and clockwise as frequency increases. This software for the N2PK VNA was written by GM3SEK and it can continually plot the antennas characteristics as you make changes to the matching network. From the results above I can resonate the antenna correctly anywhere within 501 to 504 kHz with the variometer. However the point where it crosses the resistive line on the Smith chart is always around 38 Ohms. This indicates that the tap on the lower coil from the transmitter is incorrect and should probably be further 'up' the coil. Notice that 503.20 is slightly under resonance and is in the capacitive area of the chart. This capacitance is indicated by the -2.4 ohms of reactance whereas an inductive reactance is +ve

Vector Network Analyser plot of Impedance and phase from 300kHz to 800kHz.

In the graphs above, the phase is shown in blue while the impedance is shown in yellow. Frequency is plotted from 300k to 800k on the horizontal axis.
Resonant frequencies are shown where phase passes across the center of the screen. If you look at the marker around 505 kHz you can see resonance as indicated by phase crossing zero degrees and you can read off the corresponding value of resistance. In this case 37 Ohms. The other resonant frequencies are 445kHz and 715kHz. However, at these frequencies the resistance is a long way from the 50 ohms we need to match to the transmitter. At 445kHz the resistance is 1225 Ohms, while at 715kHz the resistance is even worse at 5687 Ohms. ( For the enthusiastic....If you look back at the first Smith chart, you can predict that the 3 points that cross the resistive line on the smith chart must be at 445kHz 503kHz and 715kHz)

I would like to thank Michael White G3WOE for making the plots of antenna with his VNA and for the inspiration to turn my dust covered box of components marked N2PK VNA into one of those excellent pieces of test equipment.