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Old 01-03-2013, 06:11   #1
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Random Wire Antennae

I was unable to buy an insulator for my jumbo diameter backstay -- it was all special order and several weeks leadtime and incredibly expensive.

I wasn't all that keen on chopping the backstay anyway. I am installing a spare halyard at the aft end of the mast truck which will be perfect for hauling up a random wire antenna from the pushpit, so I think that's what I'm going to do.

I have a GAM antenna which came with my radio, but I think it is suboptimal with all that extra rubber on it (windage), plus it is quite short compared to the space between my pushpit and the masthead.

I now have a vastly better idea about how antennae work after cramming for the amateur radio exams, but still lack practical knowledge. So maybe you radio experts can give me some advice. Here are my questions:

1. Is stainless steel lifeline (plastic coated?) the best material for these? Does it make any difference? What about the diameter?

2. I understand that the secret to a good random wire antenna is avoiding lengths which will amount to a half wave of any of the bands I will be using, or a multiple of a half wave. Here is a good resource I found: Randon Wire Antennas - Best Lengths To Use For Random Wire

Although the marine bands are not represented there; I will have to make some additional calculations. I have up to about 80 feet to deal with so a good deal of flexibility in choosing a length. Actually at 80 feet or so, such an antenna will be a long wire antenna on most bands. I'm hoping that it will work decently.

Do I correctly understand that the feedline between the tuner and the wire itself is also counted as part of the total length? And should it not be of the same material as the wire in order to avoid any possible difference in impedance? Or how to deal with that?

Thanks in advance for your advice!
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Old 01-03-2013, 11:13   #2
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Re: Random Wire Antennae

Dockhead -

You have it right. Typically you would avoid 1/2 wavelength antennas (and their odd multiples like 3/2 wavelength) because they can be hard for the ATU to handle. Your AT-140 might do fine with it anyway, it is a very wide range tuner. You also count the length of wire from the ATU to your "antenna" as part of antenna length. Use GTO-15 which has a 15,000 volt breakdown resistance, so if it happens to touch something grounded on the way to the antenna, you won't arc while transmitting. No special need to match the material of the antenna.

You don't reallywant coated SS. Just use uncoated SS lifeline, it will last longer. If you worry about someone grabbing it while transmitting, get a 5 foot length of PVC pipe to pass it through and insulate the lower end. Diameter is not critical either. Larger diameter gives greater bandwidth, but that is not really significant here. Choose a size you can easily work with.

Chip
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Old 01-03-2013, 11:34   #3
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Re: Random Wire Antennae

slight derail - I have some electronics/radio experience but not alot of marine SSB specifics. I see the ATUs - are they common because they simplify antenna-matching for the non-radio guru, or is there another reason not to use a manual match network (with marked presets, maybe) and a VSWR meter in the setup?
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Old 01-03-2013, 11:55   #4
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Re: Random Wire Antennae

Quote:
Originally Posted by Lake-Effect View Post
slight derail - I have some electronics/radio experience but not alot of marine SSB specifics. I see the ATUs - are they common because they simplify antenna-matching for the non-radio guru, or is there another reason not to use a manual match network (with marked presets, maybe) and a VSWR meter in the setup?
If you don't mind fiddling with a manual match network every time you change bands, go for it. I think that would be a royal PITA, myself, especially at sea. Remember you may be flipping through the bands quite rapidly, trying to find propagation.
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Old 01-03-2013, 11:56   #5
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Re: Random Wire Antennae

Quote:
Originally Posted by SoonerSailor View Post
Dockhead -

You have it right. Typically you would avoid 1/2 wavelength antennas (and their odd multiples like 3/2 wavelength) because they can be hard for the ATU to handle. Your AT-140 might do fine with it anyway, it is a very wide range tuner. You also count the length of wire from the ATU to your "antenna" as part of antenna length. Use GTO-15 which has a 15,000 volt breakdown resistance, so if it happens to touch something grounded on the way to the antenna, you won't arc while transmitting. No special need to match the material of the antenna.

You don't reallywant coated SS. Just use uncoated SS lifeline, it will last longer. If you worry about someone grabbing it while transmitting, get a 5 foot length of PVC pipe to pass it through and insulate the lower end. Diameter is not critical either. Larger diameter gives greater bandwidth, but that is not really significant here. Choose a size you can easily work with.

Chip
Great; thanks a lot for that useful information.
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Old 01-03-2013, 12:01   #6
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Re: Random Wire Antennae

Quote:
Originally Posted by Dockhead View Post
If you don't mind fiddling with a manual match network every time you change bands, go for it. I think that would be a royal PITA, myself, especially at sea. Remember you may be flipping through the bands quite rapidly, trying to find propagation.
...makes sense. Thanks.

I'm still a few years (or maybe a reincarnation) from a trans-oceanic voyage, so it's just a matter of curiosity for right now.
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Old 01-03-2013, 13:17   #7
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Re: Random Wire Antennae

Dockhead,

Re: random length antennas, I've been using them on my own boat and house(s), testing, writing about them, and installing them on friends' and clients' boats for decades.

Just Google "alternate backstay antenna".

I use insulated 3/16" s/s lifeline. They last forever, and are very strong. You can set them up hard so they don't flop around. I favor the insulated variety because I believe they're a bit quieter in precipitation, and don't detune as easily in the changing marine environment.

Just about any length over 23' will work. Best performance on all ham and marine bands tends to be around 40-45' overall (including the GTO-15 feedline from the tuner). Longer lengths favor the lower bands (8mHz and below), while shorter lengths tend to favor the higher bands (above 8mHz or so).

Don't sweat the length too much. Theorists tend to attribute too much importance to avoiding certain lengths, but practically to avoid the troublesome lengths you would need to take into account the velocity factor of the antenna and feedline (not just the actual physical length). Today's auto tuners tend to handle all frequencies very well. If you should happen upon one which won't tune properly, just add some feedline or cut some off.

Here's one way to attach the GTO-15 feedline to the base of the antenna.
Click image for larger version

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The loops are formed with Nicopress sleeves. Just put a length of 3/8 Dacron or nylon line at each end for insulators, and hoist away.

These antennas will survive a major hurricane (mine went thru 5 major hurricanes in the BVI during the 11 years I was there, with over 100 knots in the marina!).

They also will perform every bit as well as a traditional backstay antenna.

Bill
WA6CCA
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Old 01-03-2013, 16:04   #8
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Re: Random Wire Antennae

Your name would be Lucky if you succeeded at picking a random wire length antenna that worked on all ham and marine frequencies. Its very hard picking lengths based on simple calculations alone.

Once you install the antenna in its final position all the interactions and other installation impacts will make a liar out of you. Its a very hard thing to do or calculate practically without test equipment.

The really only practical way is to get yourself an impedance antenna analyzer and sweep the frequencies you will using searching out the extreme impedance and reactance points.

I have measured backstay antennas and very few of them succeed at avoiding the half wave feedpoint syndrome. Some people do get lucky and hit the jackpot target!

The feed in length is part of the calculation and many installers ignore the feed in length. You can use this feed in length which is part of the antenna to try and mitigate the high impedance problem. You will need an impedance analyzer to really see whats going on. The alternative is to just to try and tune the antenna with your antenna tuner. When it fails to get a match you can either lengthen or shorten the
feedline length for best results. On my ham installation I use this trick with a massive loop antenna to try and bring them impedance within the range of the antenna tuner.

You can see how this is done on this link. You could do something similar if
you are a mad ham on a yacht!
http://www.w5dxp.com/notuner.htm
There are simpler ways of doing it by just using a roller inductor or a capacitor to increase or decrease the length of your antenna electrically. You then using your
automatic ATU to tune after you have brought the antenna into the ballpark optimum length.

ALL VERY MESSY AND NOT very convenient. There are many ways to skin the cat when it comes to mysticism of radio antennas. Its just not practical nor convenient
for most people.

Quote:
Originally Posted by Dockhead View Post
I was unable to buy an insulator for my jumbo diameter
2. I understand that the secret to a good random wire antenna is avoiding lengths which will amount to a half wave of any of the bands I will be using, or a multiple of a half wave. Here is a good resource I found: Randon Wire Antennas - Best Lengths To Use For Random Wire

Although the marine bands are not represented there; I will have to make some additional calculations. I have up to about 80 feet to deal with so a good deal of flexibility in choosing a length. Actually at 80 feet or so, such an antenna will be a long wire antenna on most bands. I'm hoping that it will work decently.

Do I correctly understand that the feedline between the tuner and the wire itself is also counted as part of the total length? And should it not be of the same material as the wire in order to avoid any possible difference in impedance? Or how to deal with that?

Thanks in advance for your advice!
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Old 02-03-2013, 07:13   #9
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Re: Random Wire Antennae

Quote:
Originally Posted by plebian99 View Post
Your name would be Lucky if you succeeded at picking a random wire length antenna that worked on all ham and marine frequencies. Its very hard picking lengths based on simple calculations alone.

Once you install the antenna in its final position all the interactions and other installation impacts will make a liar out of you. Its a very hard thing to do or calculate practically without test equipment.

The really only practical way is to get yourself an impedance antenna analyzer and sweep the frequencies you will using searching out the extreme impedance and reactance points.

I have measured backstay antennas and very few of them succeed at avoiding the half wave feedpoint syndrome. Some people do get lucky and hit the jackpot target!

The feed in length is part of the calculation and many installers ignore the feed in length. You can use this feed in length which is part of the antenna to try and mitigate the high impedance problem. You will need an impedance analyzer to really see whats going on. The alternative is to just to try and tune the antenna with your antenna tuner. When it fails to get a match you can either lengthen or shorten the
feedline length for best results. On my ham installation I use this trick with a massive loop antenna to try and bring them impedance within the range of the antenna tuner.

You can see how this is done on this link. You could do something similar if
you are a mad ham on a yacht!
http://www.w5dxp.com/notuner.htm
There are simpler ways of doing it by just using a roller inductor or a capacitor to increase or decrease the length of your antenna electrically. You then using your
automatic ATU to tune after you have brought the antenna into the ballpark optimum length.

ALL VERY MESSY AND NOT very convenient. There are many ways to skin the cat when it comes to mysticism of radio antennas. Its just not practical nor convenient
for most people.
That's another argument against backstay antennae -- other than changing the feedline length, you can't change the length of them. For a wire antenna hauled up on a halyard, it is otherwise. A snap to chop some wire off. It's even realistic to carry different ones for different sets of bands, if you can't find a length that works well everywhere.

SWR meters are cheap so I guess I'll buy one. I don't think it will be a big deal to experiment until something works well.

Thanks everyone for all the good advice.
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Old 02-03-2013, 07:25   #10
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Re: Random Wire Antennae

Two excellent reference sources for electronics parts: Newark.com and Frys.com . If you don't find exactly what you're looking for, you can cannibalize parts to suit your project. Mauritz
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Old 02-03-2013, 09:29   #11
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Re: Random Wire Antennae

Is your hull fiberglass? If so, you might try skipping the insulators and feeding the backstay at the chainplate. Of course since there will be no upper insulator your antenna will include the mast and the rest of the stays / shrouds, these may well be tied into your ground system, but that's not necessarily a problem, and it's likely that your tuner will be able to tune anyway.

Anyway, it's worth a try.
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Old 02-03-2013, 09:31   #12
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Re: Random Wire Antennae

Dockhead,
+1 to what Bill and Cecil (Plebian) said....

And, in my experiences the AT-140 (and the earlier AT-130 / AT-120) as well as the SG-230, etc. have no problem matching 1/2 wave length antennas....so I have no real concern....
(perhaps the thickness of the backstays vs. a thin wire, providing a lower imped than would normally be seen with end-fed 1/2-waves, allows for such "lucky" results...)


Fair winds..

John
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Old 02-03-2013, 09:33   #13
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Re: Random Wire Antennae

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Originally Posted by Teknav View Post
Two excellent reference sources for electronics parts: Newark.com and Frys.com . If you don't find exactly what you're looking for, you can cannibalize parts to suit your project. Mauritz
Two more good sources: Mouser Electronics and Digi-Key. For pre-made cables and R.F. connectors I sometimes use Pasternak.
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Old 02-03-2013, 14:29   #14
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Re: Random Wire Antennae

Quote:
Originally Posted by btrayfors View Post
Dockhead,


Don't sweat the length too much. Theorists tend to attribute too much importance to avoiding certain lengths, but practically to avoid the troublesome lengths you would need to take into account the velocity factor of the antenna and feedline (not just the actual physical length). Today's auto tuners tend to handle all frequencies very well. If you should happen upon one which won't tune properly, just add some feedline or cut some off.
Right -- I learned that studying for my ham exams Electrical length versus physical length.

Lifeline is cheap enough that I think I'll have a couple of these made up with loops on both ends. Thanks for the great advice.
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