Antenna choice

I use an outbacker antenna for HAM bands and I just ordered a new version which covers 75M through 10M + 6 and 2M from Jim Burns

For most communications I use the 20M (14.1 Mhz), and 40M (7.1 Mhz) bands, but sometimes the 10M (27 Mhz) band is good for long distance talks. The 2M band (144 Mhz) is good for local nets which are operating on every country in the world.

The antenna is a bit expensive at $400+ but I have been using one for 20 years and it has survived throughout my travels. I change bands by moving taps on the antenna which takes a few seconds only. The antenna has to be within reach - mine is on the aluminum solar panel mount. No antenna tuner is needed, so it works with any radio. And it uses no power, unlike a tuner which sucks power all the time.

To transmit on Marine frequencies I have to adjust the length of the “stinger” which means releasing a knurled nut, and then by checking the SWR on the HF VSWR meter I make sure I have selected the right length. This is a bit fiddly, but I’m never in a hurry.

The antenna is positioned to be clear of the backstay, to improve the polar plot of the transmitted RF.

The antenna is very quiet, in that it filters out most RF frequencies because of the high Q resonance (and so, very narrow bandwidth.) It is much quieter than the backstay which would pick up all the RF noise from the other boats in the marina and the RF noise from internal machinery. With the antenna mounted above my head and over the ground plane there is little sensitivity to generated RF noise from within the boat. It is safely out of the way when I transmit.  Personnel are protected by the ground plane overhead. The plastic covering prevents anyone getting “burnt” if they should touch it - unlike a backstay which is a hazard if touched during transmission. The outbacker has been in use in the Australian Outback for some 25 years. Of course, they are mounted on trucks, and cars.

This system makes a perfect radio installation for collecting weatherfax charts. Also noise levels are very low and it easy to hear weak chatter on the radio. For very weak signals I sometimes use an audio speech processor (iRig Amp) on my iPod, with headphones.

Note: If you compare the signal output for this type of installation with a shore based Yagi antenna on 20M, you will find that the Yagi signal is at least 6 S units stronger (ARRL antenna handbook,) because that Amateur will be using 1500 Watts and his antenna will be turned towards you and is 100 feet in the air. This means that he will hear your weak signal, where other boats will not. And you will be able to hear him easily. So shore based Amateurs can be very useful in an emergency.

Now here are bad recommendations from a Commercial Radio Company. It is an example of how not to install your tuner to the antenna. The wire on the right comes from the tuner. It is part of the antenna. In this design the lower insulator is 7 ft above the deck. On 12 MHz the wire is the major part of the antenna to radiate, the backstay above is ineffective, contributing perhaps 25% to the signal. The wire is “hot” with RF and a danger to personnel. The spacers which hold the wire off the lower part of the backstay may not isolate the wire from the grounded part of the backstay (if it connects to a metal deck, or if the backstay bulkhead plate below decks is connected to the sea via a bonding strap.) Instead it sucks RF signal to ground and reduces the strength of the EMF. In other words it will give out a weak signal. The writer of this erroneous article, warns that the voltage on the antenna can reach 5000 V, and that is why they recommend putting the insulator above head height. But he didn’t explain that the wire leading to the insulator is effectively the antenna (the bottom 30% of any vertical antenna produces most of the signal) and this can have the high voltage on it.



For engineers - large monitor neededAntenna_Electrical.html

iPhone users should return to menu. Here after are engineers notes on antenna design for boats and requires a 1920x1080 display