HF in marinas

Kelly's Eye said..
"Because of the ATU there are no such things as bad a aerials only bad grounds."

I think there are bad antennae despite the use of an ATU. Take this to a riduculous extreme, and use a 1m long wire and ATU. At HF that's a bad antenna.

But backstays are typically 8- 12metres. And an ATU can make sense of that for most HF frequencies in common use.

But there certainly are bad "grounds". I've found that this is quite critical, and have done many experiments. My current configuration is 5 counterpoise wires along the hull, one centreline and 2 either side, all from the ATU (in stern) to the bow, and therefore approx same length as backstay portion between insulators (about 10m). A high surface area direct connection to the sea is no improvement on this.


Someone mentioned hoisting a wire up. I've tried that to good effect once, when running two transmitters (different bands), one using the existing permanent setup on the backstay, the other a wire from bow to masthead on a halyard, parallel to the forestay foil. Other than occasional interference between the two rigs, this worked OK, both getting a reasonable signal out.

Someone else mentioned that the backstay isn't a vertical. It's true that it is not perpendicular to the sea surface, but the term "vertical", is being used in this context to mean an antenna producing a vertically polarised signal. That its isn't exactly vertical doesn't matter too much. The alternative is usually a horizontally polarised signal such as that produced by a horizontal dipole, or a yagi. There's nothing to suspend these types on a yacht normally.
But its true that on or near the sea, a vertical outperforms a moderate yagi on HF.
 
Backstay antenna performance

The attached plots are of the backstay on my boat, allowing for the wire diameter and material etc. They were all obtained using the program 4NEC2.

Although Moxon is of course right in principle that the antenna isn't that good at all frequencies, its behavior is not disastrous on any of 3.5, 7, 14 or 21MHz amateur bands as can be seen from the impedance plot, and is within the capability of an ATU.

The polar plots are gain in a fore-and-aft vertical section. The gain in the low-ish angle lobe is pretty much the same at around 0dBi for all 4 frequencies although the pattern changes quite a bit at higher angles. But it's only the low angles which count, which is why one can in practice use a backstay (over a ground-plane) for quite a range of frequencies even 'tho it's far from a 1/4 wavelength long for most of them.

backstay1_geometry.jpg

Configuration

backstay1_impedance.jpg

Impedance

backstay1_3M5.jpg

radiation pattern on 3.5MHz band

backstay1_7M.jpg

radiation pattern on 7MHz band

backstay1_14M.jpg

radiation pattern on 14MHz band

backstay1_21M.jpg

radiation pattern on 21MHz band
 
Last edited:
Conachair said:
So how many have used http://wsprnet.org/drupal/ switching between different antennas to check which work best?

Me neither:o

Might get it up and running again tonight though.
Click to expand...

Timestamp Call MHz SNR Drift Grid Pwr Reporter RGrid km az

2012-10-30 23:30 M0PDD 3.594110 -11 0 IO91oo 5 OE1MSB JN88df 1276 100

Straight away picked up 1276Km away in Austria transmitting on 80m at 5w surrounded by buildings in South Dock, but not many masts up this end.

Cheery stuff :cool:

Another one just recieved me in from Norway, 2000+Km. If I have a none lazy moment I´ll put up the inverted V again and try half hour on each, see what happens.

http://wsprnet.org/drupal/wsprnet/spots
 
With a 12,000Km TX this morning 40m/5w and furthest RX seems to be about under 750Km I seem to have the opposite problem from the OP. :confused:
Still, noisy with charger & fridge going.

Slight thread drift but a new mode, JT65, looks interesting - http://wsprnet.org/drupal/node/3723 , submodes down to 0.4 Hz total bandwidth.
 
>I'm not recommending anything to you because you'd only argue about it!
In fact I don't think I'm recommending anything to anyone - just trying to share some very basic aerial theory. Basic because it's all I've got :-)

Theory is one thing practical experience is a whole different ball game. For example mentioning different wire lengths for different frequencies is intresting and correct but ignores an ATU. It implies an arial is needed for each frequency band of which there are five marine bands in common use and two more for very long distances.

>Take this to a riduculous extreme, and use a 1m long wire and ATU. At HF that's a bad antenna.

I don't see the point of that, of course it's a bad antenna but who would be stupid enough to do that.
 
Conachair said:
.

Slight thread drift but a new mode, JT65, looks interesting - http://wsprnet.org/drupal/node/3723 , submodes down to 0.4 Hz total bandwidth.
Click to expand...

It is a good mode for DX chasing but not for chatting as prescribed massages are sent in most cases and I believe that user designated messages are restricted to 13 characters.

It decodes signals which are inaudible and it is interesting watching it note stations all over the globe. I used MSK which tested the grey matter a wee bi=t getting it set up and working. The puter clock needs to be accurate to a couple of seconds and all transmissions start on the start of an odd or even minute. Very slow and pedantic!
 
>>Take this to a riduculous extreme, and use a 1m long wire and ATU. At HF that's a bad antenna.

>I don't see the point of that, of course it's a bad antenna but who would be stupid enough to do that.



Of course there's little point in a 1m long antenna for HF, but where do you draw the line? 5m? 8m? 3m? 10m?

There are bad antennas. Perhaps better described as unsuitable antennas!?

An ATU of sufficient range could match the impedance of just about anything, even the 1m whip. But the radiation efficiency would be low.

There's no accurate dividing line, but I guess you have agreed that a 1m antenna would be "bad".
 
>Take this to a riduculous extreme, and use a 1m long wire and ATU. At HF that's a bad antenna.

I don't see the point of that, of course it's a bad antenna but who would be stupid enough to do that.

Dunno, maybe somebody who had idea or interest in how it worked? LOL!
 
JDC

Thanks for the plots. I never mastered 4NEC2 and am struggling to understand them. If they are polar plots for a vaguely vertical aerial wouldn't the radiation pattern be more symetrical - i.e. at least some radiation in all directions?

I suspect the problem is that I am reading the graphs wrongly but can you help me see how?

Thanks.
 
Elevation not Azimuth

The polar plots are for a vertical slice aligned fore and aft. So the half in which there's no radiation is down into the sea! Ie they are plots of radiated power versus elevation.

You're quite right that the near vertical back stay gives a near symmetrical radiation in azimuth, so it wasn't that interesting to plot (and so I didn't:-)
 
"All the SSB rigs I've seen on boats (150+) are on a backstay which is not vertical. With a good ground they work fine. As I mentioned ours is Z shaped the bottom being the steel hull.

Well it sure as Hell isn't horizontal, unless you are in a very difficult position.

And with that I'll call it quits.

I can't compete with pedantry of that degree
 
Last edited:
You must log in or register to reply here.

Share this page

Top