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.
7.3 Watts PEP Antenna Current.
280mA Loading coil Inductance.
898uH Calculated antenna capacitance
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.)
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
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.
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.