Counterpoise for SSB

duncan99210

duncan99210

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Winter in Falmouth, summer on board Rampage.
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OK. Back in the day, I did my Signals course. I've since forgotten much of what I was taught (it was about 35 years ago...) but I do recall that they taught us that you needed a good ground connections as a counterpoise to the antenna for HF radios. We're sort of contemplating moving beyond the Med in a year or two and have been giving some thought to the question of long range comms, which leads pretty quickly to SSB.

Now, Rampage has an iron keel and it occurs to me that actually, this might offer an effective counterpoise to a backstay antenna. Is it of any use or do I need to think in terms of copper plates fixed to the hull? Can anyone offer any explanations as to why it might or might not be effective? Also, if a copper plate is used, does it go inside or outside of the hull?

Look forward to hearing peoples thoughts on this one.
 
To answer your question we need to understand a little around the antenna theory and HF transmissions.

To simplify a complex subject, a radiating antenna requires a radiator and a ground to balance the signal. It works similarly to providing power to an electrical appliance, where you need to supply the positive and the negative to close the circuit and allow the current to get through the appliance. The appliance will present a "resistance" to the circuit by absorbing energy and converting it to do the "work" that the appliance do. In case of a light bulb the work is heat and light.

The most basic antenna type and also the simplest to understand is a dipole. A dipole is formed by two wires of equal length, one connected to the positive and one to the negative (ground). The positive radiates the signal and closes the circuit through the air with the second wire connected to the ground.

The actual length of these wires plays a very important role on how well an antenna is capable to convert a high current, high voltage alternate (modulated) signal into a radiated wave. Matching the length of these wires to the radio wave length is called "tuning". A tuned antenna will convert 100% of the power (voltage x current) into radio waves, thus presenting to the transmitter a pre-determined impedance. (The impedance is the "resistance" of a circuit at a given frequency. See the light bulb example above).

In your case you will have a fixed length of wire (your backstay) connected to the antenna feeder (the wire) positive, so how do you balance it to emulate a dipole? You do not have another wire of the same length therefore you use the ground. The sea water makes a perfect ground so you need to connect the antenna feeder ground to the sea water. The keel is a good conductor and therefore it can be used for this purpose.

However your antenna, as well as a dipole will be tuned only at ONE and one only frequency/wave length. For a dipole this wave length is easy to calculate, because it is twice the wire length. Your backstay + sea water instead it is harder to calculate because it depends on the length of the backstay segment, the salinity and the temperature of the water and thus it will change with time and conditions.

To take away most of these unknowns and increase the efficiency of such an unbalanced antenna one can fit a number of counterpoises (to the ground) of different lengths to match the HF frequencies most used.

I hope this helps to begin with I have been called for dinner ... Antenna Tuners will be in my next message.
 
Or, a slightly briefer response, yes an iron keel in reasonable contact with the sea (paint is no problem) is an excellent counterpoise.
 
Why is the paint 'no problem'?

If the keel had a perfect (unlikely!!!) coating of, say, epoxy - would this not affect the earthing arrangements?

Just curious.......:)
 
No, a layer of paint is pretty much transparent to RF frequencies, it's called capacitive linking. The painted keel would behave like a giant capacitor that would block dc but transmit high frequencies.
 
WS - thanks for that, I didn't know....

So a good earth for SSB on your average AWB can come straight off a keel bolt ?

Using copper strip and appropriate fittings of course.
 
stephenh said:
Why is the paint 'no problem'?

If the keel had a perfect (unlikely!!!) coating of, say, epoxy - would this not affect the earthing arrangements?

Just curious.......:)
Click to expand...

Because what you want to get through is alternate current, not direct. So the paint would block direct current, but not alternate radio frequency.
 
Pretty much, yes. You need good contact with the keel, multiple keel bolts would be better. With RF it is more about area than 'point' contact.
 
Antenna Tuners

Before getting into antenna tuners, we need to understand that modulated radio frequencies are alternate currents. In general connecting our keel to a battery ground would put it at risk of galvanic corrosion. The different metals of keel, copper wire and lead plates in a battery would form a low power battery, a direct current will flow through it and eventually cause galvanic corrosion of one of the elements, likely the keel.

However the ground that we require is an RF ground (alternate current) not a DC ground, therefore to protect our keel we will interrupt the ground circuit to the keel and bridge across using a number of capacitors like this:

http://www.sailmail.com/mg_image011.jpg

The image shows 0.15 uF ceramic capacitors. This is because capacitors will let RF/AC through (they charge and discharge with each cycle), but not DC (in which case they charge once and never discharge, thus blocking further current).

Antenna Tuners

Going back to our length of backstay, previously we explained that a fixed length of radiating element will tune only to one wave length, with such length being very difficult to calculate in case of a backstay using the sea water as RF ground.

What happens to the power from the transmitter if the antenna is not tuned? Practically the waves that cannot be radiated, bounce back through the feeder, combine with the waves going up the feeder creating a "standing wave" that returns to the transmitter, the transmitter overheats and eventually the final amplification stage fries. This can happen in a matter of a few seconds.

Given that it is not practical to re-calculate and change continuously our antenna length depending on the wave length used, or use only the ONE wave length for which our antenna is designed, we do use Antenna Tuners.

These, fitted on the antenna feeder, near as possible to the radiating element, are used to match the actual impedance of the antenna to the impedance expected from the transmitter, thus avoiding a "standing wave" returning to the transmitter.

Do antenna tuners allow an unmatched antenna to radiate all the transmitted power? No they do not. They just save the transmitter from a premature demise. We do still require to have a balanced antenna.

So, to answer you original question with your backstay you have to create something that it is as close as possible to an half wave length dipole, using sea water as ground will help the radiator (backstay) to put radio waves into the air, but the results and efficiency will be "random" and difficult to predict, and might not be satisfactory at the marine SSB frequencies that you require.

To increase the efficiency of your antenna you will have to fit also a number of counterpoises, the length of which is calculated depending on your backstay segment length and the wave length / frequencies that you will use. One counterpoise for each wave length.
 
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I do not have the technical expertise of some responders, but I can say that we had excellent performance with an external copper plate as a ground. One opinion was that it would have worked as well if fitted on the inside of the hull.
Our only major glitch was initial poor performance of voice transmission. This was put down to a marked voltage drop between radio and antenna tuner, which was cured by replacing the control cable (which had been supplied by the well known manufacturer) with a larger capacity cable. It was also curious that the optimum voltage given for our unit was 13.1v (probably rare for sustained periods on a sailing boat). One solution suggested by the manufacturer was to only use the radio when the engine was on!
 
I would disagree with heydude on the need for multiple counterpoises. It does need however a decent conection to ground (the sea) The modern approach with SSB transceivers at HF is to fit an automatic antenna tuner.
If we assume an infinite and perfect couterpoise also called a ground plane and in our case the sea then the antenna might be seen as a quarter wave antenna. (half a dipole). The wire must be exactly 1/4 wavelength long. Actually slightly shorter because of end effect of the wire. Now a wire can be made to electrically look longer than it really is by adding inductance (coil) in series with the wire. So we have a variable coil that can be tuned for best performance of the antenna at the chosen frequency. We can add a variable capacitor across the input end of the coil to get the impedance matching perfect. Older radios had a tuner box with a meter for getting best tune and variable inductors often switched and variable capacitor. Sometimes these elements are built into the radio box itself.
HF radio has different progation effects at different frequencies and time of the day and different distances. So for most reliable communication we use different bands of frequencies. 2to3 megahetrtz for shorter range 4 megahertz 6 megaherts 8 megahertz tending to be for longer range at night. From this you can see that the desired 1/4 wavelength can vary by a factor of 8 times or more.
An auto tuner has in effect motor driven adjustments or relay switched elements so that it can adjust perfectly to the frequency you choose and the length of the wire. It will also take into account the length of wire or strap that is in the earthing circuit. a manual tuner can do the same thing. However the tuner should be near the base of the antenna especially on a steel boat.
So the need to have the manual tuner near where you operate is in conflict with the need to put it near the antenna.
Op has not mentioned if he wants to use amateur radio bands or official ship to shore frequencies .
Here in Oz the coastal sea rescue organisations tend to monitor the 4125 6215 and 8291 megahertz channel while the official government ship communications frequencies use digital select calling where you call with id on one channel then change to a nearby frequency to actually talk. Seems to be more for commercial shipping. Amateur (ham) radio is much more relaxed but has far greater coverage provided by benevolent people. Concensus for cruising seems to be that you need ham radio. This requires that you get an operators licence witha lot of technical stuff to learn. (it is really meant or was to be an experimenters licence).
2 years ago I had a friend who entered the Fremantle to Bali race (rally). He was required to have an HF SSB to maintain contact with race control. He reckoned he knew what he was doing and fitted it himself using a transom mounted whip antenna. He showed me it working and all the LED indicators on the Benetou switch board lit up in unison with his voice. I did not realise to warn him that this was bad as it indicated power not going to the antenna. And yes he reported the radio was hopeless and he used VHF and relay to other boats. I am pretty sure he did not have a decent earth counterpoise or ground plane and relied on the auto tuner to do its thing. Wi5th disappointing results. Good luck olewill
 
blenkinsop said:
I do not have the technical expertise of some responders, but I can say that we had excellent performance with an external copper plate as a ground. One opinion was that it would have worked as well if fitted on the inside of the hull.
Click to expand...

Generally people use scintered bronze plates (Dynaplates) on the outside - they really only work properly on the outside because they are full of holes which increases the area of contact with water. Copper strips laid inside the hull are also common but fiddly to lay as access to long lengths of the bilge can be hard.

A small capacitor in the line to ground (brace for an essay on why that's the wrong word) is a good idea.
 
FWIW I have a lead keel that is connected to the ssb via the keel bolt, and it doesnt work that well. certainly not as well as the dedicated sintered bronze plate I had two boats ago. been pondering what to do myself.
 
Thanks for these responses. At this stage, this is only a mind clearing exercise and I won't be fitting an HF radio for at least a couple of years. Now that I know that I can probably use the keel as the counterpoise, it reduces the fitting costs somewhat and might make the idea a bit more viable. Now to consider whether do the Long Range certificate or go down the ham route or both...... Decisions decisions.....
 
wotayottie said:
FWIW I have a lead keel that is connected to the ssb via the keel bolt, and it doesnt work that well. certainly not as well as the dedicated sintered bronze plate I had two boats ago. been pondering what to do myself.
Click to expand...

If it is encapsulated lead 'bits' which is common I believe, I would not expect it to be ideal.
 
ostell said:
As my electrical engineering lecturer used to say: you find a condenser on the back end of a steam engine.
Click to expand...

What did your lecturer say about aerial and antenna?

whipper_snapper said:
If it is encapsulated lead 'bits' which is common I believe, I would not expect it to be ideal.
Click to expand...

no its a solid lump of lead albeit coated in epoxy and then coppercoat.
 
As said attaching the ground to a keel bolt is good. The only place you need a copper strap is from the Automatic Tuner Unit to the aerial. Clearly you cant put that through the coachroof to the aerial so fold the end nearest the aerial to a triangle shape, attach wire to it, pass through the coachroof nearest the stay with the aerial and clamp it to the aerial wire above the insulator. Cover it with self almagamating tape to waterproof it. Bear in mind to not let anybody touch the aerial when transmitting, you can light a cigarette on it, as I used to do to demonstrate the the power in it.

Do the LRC not the Ham exam, Hams use different LSB frequencies but Marine SSBs have both LSB and USB bands so you get the Ham frequencies too.
 
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