SSB Radio

While SSB is by many regarded as out of fashion, it still represent a good way of talking to other yachts while out on the open sea (VHF has a short range). Not to mention all the weather and safety transmissions that can be received. Adding to this is all the broadcasts you can listen to while being far out from any DAB or FM transmissions. With enough persistence and will power it's also possible to send and receive (short) emails. As with all radio communications the antenna is an essential part !

Installing email software

Installing email software on Raspberry Pi is possible, winlink is not fully supported for Raspberry Pi. While RPis are popular for rig control not all software development have shifted to Raspberry and it's associated set of Linux operating systems. I have installed the latest Pat software which is a client offered for RPi. There are a some good instructional videos available, but I like text instructions with cut & paste commands that I can just paste. My github repo have instructions and some helper files. My radio is a Xiegu G90 which I also have written some scripts for, like setting the dual frequency MF/HF maritime working channels.

Backstay antenna - installation and testing

Testing the raw backstay

First one need to measure the impedance for the various frequencies. The Z is a complex impedance with resistive and capacitive/inductive components, but only the magnitude (length, a real number in ohms) is used hereafter. A lot of magic is hidden behind the scenes, like the fact that if imaginary part is positive there is a net inductance, while a negative is an indication of a net capacitance.

A SARK-100 antenna analyzer from China is used to measure the antenna (manual). As with all end fed antennas it show high impedance for large parts of the band. The backstay is a type of random wire antenna. For this antenna the reactive part is over the frequency range mostly inductive.

Secondly, how should this match the antenna tuner, most tuners struggle with this high impedance. A transformer (unbalanced to unbalance - or an autotransformer) could bring the impedance down within the tuner's range. A 1:9 transformer would bring 450 down to 50 ohm, while still keep the lowest valleys within the tuner's range.

Tesing the unbalanced to unbalanced transformer

A small unbalanced to unbalanced transformer is a autotransformer with a common ground connection. The signal is fed into a part of the coil while the antenna is connected at the top of the coil. The impedance change is a function of difference in the numbers of windings.

The impedance transformer, Unbalanced to Unbalanced (unun) shows some impdence variance over the MF/HF band. Using a 450 ohm resistor as a dummy antenna load the impedance at the input should be found to be around 50 ohm, the plot show acceptable variance below 20 MHz, with good fit at 8 MHz.

Without information about the torioid material it's not easy to predict the behavior at 25W power either. A DIY is probably a better solution, where all parts are well defined and documented.

Testing the backstay and transformer together.

Installing the 9:1 unbalanced to unbalanced transformer provide a solution ready to be connected to the ATU-100 antenna tuner.

The antenna seen from the low impedance side of the the transformer is a nice fit for the antenna tuner. The ATU-100 can tackle 15-800 ohm impedance (it can choose inductance from 0.05 μH to 8.53 μH or capacitance from 10 pF to 1869 pF). Even without the tuner the SWR is not prohibitive high. However, an SWR above 5 is not regarded as a good fit. However, the whole band from 1.6 to 30 MHz is within the range of the ATU-100's capabilities.

The antenna ground is AC grounded using 5 x 100 nF 1kV capacitors, which effectively DC isolate the negative from the hull. Isolation is important with a metal (Aluminium) hull.

High voltage cable spaced using garden hose and strips. Isolator high enough not to be reached when holding onto backstay («your wife want curly hair?» someone once remarked).