Wire length for HF aerial

[whipper_snapper]

I am about to cut my 12m long cap shroud to insert an insulator so that I can use it as an HF antenna. Conventional wisdom says to put the insulator a foot or two below the mast, presumably to keep the active bit away from the grounded mast.

But I could choose to put it anywhere. Are there any particular final lengths that you would go for or avoid? I mostly use ham frequencies on 20m and 40m for transmit, but receive all over the place for weather etc. I have an SGC tuner (I guess that the run from tuner to shroud base would actually add that 2 feet back on to leave me with 12m).


Thanks

 

[tri39]

Most people use the backstay. A cap shroud may leak current into the spreader, unless you have a cat?
I believe 7mtrs/23 ft is the length of off the peg whips for HF, gives a good match for 20 and 40 mtrs.
If you're just using a single insulator at the top, make sure there's no metal contact at the bottom, just GRP hull.
Most Hams use two insulators as I have, and run a good wire (such as spark plug lead) from ATU to above lower insulator, keeping it away from metal such as pushpit or backstay below the lower insulator.
The Tuner wil do the rest. But you need a healthy battery and may have to run the engine to transmit with enough power.
I assume you have a good earth sorted out!?

 

[whipper_snapper]

Thanks. I have a cat so the shroud is pretty we'll clear of the spreaders, and I have no backstay. The bottom of the shroud is a couple of feet of rope lashing which should give excellent insulation. Grounding is by two linked sintered plates underwater.

I previously used a whip antenna, but that was ripped off in a close encounter with a mangrove forest and I think I want to switch to using the rig for performance and simplicity.

 

[KellysEye]

The length should be around 35 feet. I can't see how you could fit the insulators on a cap shroud, because it is held by a spreader, almost every boat I've seen the antenna has been on the backstay. The only others I've seen are two cats with SSB and they both had the big white antennal on the stern.

 

[Roberto]

theoretically, it's better to avoid half wavelengths or multiples; in practice, with a decent atu after having avoided multiples of half wavelengths, transmissions on those frequencies are often ok too

anyway, about 12m will keep you free on 7, 10 and 14MHz, with a possible rough spot on the marine 12MHz band which has about 11.60m half lambda


View attachment 36271

 

[Ariadne]

When I was going to replace my back-stay insulators in Las Palmas; it was cheaper to buy a new 7m whip aerial and replace the back-stay with a single piece wire than it was to buy two new stay-loc insulators.

Have you done the sums?

 

[whipper_snapper]

theoretically, it's better to avoid half wavelengths or multiples; in practice, with a decent atu after having avoided multiples of half wavelengths, transmissions on those frequencies are often ok too

anyway, about 12m will keep you free on 7, 10 and 14MHz, with a possible rough spot on the marine 12MHz band which has about 11.60m half lambda

View attachment 36271

What is the table showing?



For the other questions, yes I have done the sums. One 7mm Norseman insulator is £150, that is all I need to buy for the system. I used a whip before with a previous rig on the same boat. To replace the whip would be around £350. The whole installation is cheaper, neater and simpler using a shroud and it should give better performance. The rig is slightly unusual in that the cap shroud does not run over the spreaders and there is no backstay.

 

[geem]

I just sold a 26ft commercial whip antenna for £100. This had exceptional transmit and receive. Why do you think cap shroud will be better?
New boat has insulated backstay but not hooked ssb up to it yet to try

 

[syfuga]

I would have thought that the rope would not provide good insulation, when doused with salt and a little atmospheric dampness?

 

[tri39]

Roberto's table is frequencies in left column followed by half and full wavelengths.
Thus 14.30mhz has half wavelength of 9.97mtrs which is 33ft, and a lot of hams use this length since it's also the quarter wavelength at 7mhz.
Altho Roberto who's experienced seems to be saying 12mtrs length is better for ham frx but worse for MarineSSB on 12mhz.
Since Herb on 12359khz is retired this may not matter!
Mind you I know Roberto has a sat phone now!! (not affected by propagation probs, but def affected by cost!).