The diagrams included in this article were modelled with a program called MMANA. If you are inclined to give this a go, its a free download and I have produced training films on YouTube.
NOTE: I have intentionally modelled the vertical antenna with a bad ground to replicate the findings of some fibreglass yacht owners who run a 15 to 20 foot copper ground strap to Dynaplates. I have modelled the horizontal antennas over sea-water.
This article follows my paper on raised feedpoint vertical SSB antennas for fibreglass boats. In this article, I look at an alternative; a horizontal dipole antenna which I will call an offset doublet and a Mk2 version, with a vertical component at the rear. We will continue to use the ATU matching device (often supplied by Icom) which will remove all the hassle of mono-banding and tuning. Marine SSB relies on a number of frequencies so an ATU to dial out the mismatch is vital.
This article discusses in layman’s terms how an antenna transmits its energy and the various factors that might affect its performance. The target audience is Foundation students and marine sailors since I discuss the positive impact of the sea as a ground, particularly those sailors with fibreglass boats who have vertical antennas mounted up high off the waterline. This debate started in the Yahoo Group, NordhavnDreamers.
Dipoles and Vertical Antennas
Coax showing centre-conductor and ground / braid surrounding
The energy from your transmitter is sent via its coaxial cable and connects to the “feedpoint” of your antenna which will radiate electromagnetic energy. Most modern transceivers expect to “see” a 50 ohm load at the point where the coax connects to the transmitter. It’s this reason why coax cable suited for transmissions is more often than not quoted as 50 ohm cable.
Antennas are a little bit like piano strings. If all the piano strings were set at the same tension, the longer strings would play a lower note and the shorter strings will play a higher note.
I was very excited about taking delivery of my 2-piece 5300 Shakespeare Marine SSB HF Antenna for my narrowboat this week.
Close inspection showed that the antenna is a 2-piece 28’6″ (8.5 meter or thereabouts) white fibreglass hollow pole with a 2 foot aluminium heavy-duty sleeve at the base for mounting purposes. This main lower section (of around 17 feet or so) has three elements running the full length embedded inside the fibreglass at time of manufacture from the side feed to the top, in 120 degree arc segments. A heavy-duty male screw fitting at the top, electrically connects to the top section’s female thread.
Since the wind had died down this morning, it was an ideal opportunity to take down the 40 foot vertical and replace it with the 5300 as a test. I inserted one of my light duty aluminium poles up inside the aluminium housing approximately 12 inches in depth to where the fibreglass stopped inside the sleeve (from the other end) and I raised it to the vertical by walking up the antenna from the pointy end until it was raised vertical. I then lowered it using the same technique.
Lowering it at around 45 degrees, the fibreglass groaned, cracked at the sleeve point and fell to the ground.
Clearly, there’s a flaw just above the aluminium sleeve. A one-in-a-million manufacturing defect.
I’ve written to shakespeare and await their reply.
] On Monday morning, I was emailed by their UK sales department explaining that another 5300 would be shipped immediately. This is the first time one of these has ever broken in the history of the 5300 antenna production and clearly this is a freak. Good news Shakespeare, thanks.