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Old 01-05-2014, 16:17   #1
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Long wire HF Antenna Length

There have been many discussions about backstay antennas but I would like to know how to calculate the length of a "long wire" HF antenna for 3-30mHz bands.

So if there is anyone who point me in the right direction for calculating the length of a "long wire" HF antenna that would be appreciated.

Thank you.
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Old 01-05-2014, 16:30   #2
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Re: Long wire HF antenna length

Do you mean the kind a old airplane might trail or let out?
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Old 01-05-2014, 16:38   #3
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Re: Long wire HF antenna length

Here is a very basic quick start; I'm sure the radio bods on board will soon elaborate.

The length of the long wire is directly proportional to the wavelength of the frequency used.

So to calculate the wavelength, divide 300,000,000 by the frequency used and the result is in metres. The actual formula is "speed of light = frequency times wavelength"

Hence the wavelength of 3 MHz is 100 metres and the wavelength of 30 MHz is 10 metres.

Right away you can see a problem is using a single length for such a range of frequencies so we use a antenna matching unit to "electrically alter the length" of the wire at the desired frequency by inserting inductors (coils) and capacitors either in series or parallel. While there is mathematical ways of working what values are required, most people leave it up to the "antenna box" to sort that out .

All you want now is to know what proportional of the wavelength is best to use. Here you will get into a huge range of ideas and types of antennas but I will leave that to the radio boffins to explain better (and faster) than me.

In the meantime, I suggest a quarter wavelength is a good starting point
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Old 01-05-2014, 16:42   #4
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Re: Long wire HF antenna length

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Originally Posted by Guy View Post
Do you mean the kind a old airplane might trail or let out?
Or how long to make the antenna section of your backstay
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Old 01-05-2014, 17:19   #5
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Re: Long wire HF antenna length

Thanks wotname

A bit more research would indicate 16mHz requires an antenna length of 18.75m (1/4 wavelength).

I assume that length also would include the lead section to the antenna tuner and ground wire lengths.

If that is the case then 18.75m is workable on a boat with a 15m mast.
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Old 01-05-2014, 17:28   #6
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Re: Long wire HF antenna length

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Originally Posted by 40 South View Post
Thanks wotname

A bit more research would indicate 16mHz requires an antenna length of 18.75m (1/4 wavelength).

I assume that length also would include the lead section to the antenna tuner and ground wire lengths.

If that is the case then 18.75m is workable on a boat with a 15m mast.
Just the lead section from the antenna tuner to the backstay.

The ground wire is electrically on "the other half" on the active (long wire) element so is not part of the "quarter wavelength".
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Old 01-05-2014, 17:41   #7
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Re: Long wire HF antenna length

Over on hamuniverse.com a couple of hams calculated all of the optimum lengths for a long wire antenna based on multiples of half and quarter wavelengths of all of the bands between 160m and 10m.

These are the optimum lengths in feet for anything under 500 ft.

Quote:
Here are the final numbers (in my opinion) in green below that would be good for a long-wire antenna: (You may want to make a note of them)

REVISED: 29 35.5 41 58 71 84 107 119 148 203 347 407 423
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Old 01-05-2014, 17:49   #8
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Re: Long wire HF antenna length

Quote:
Originally Posted by socaldmax View Post
Over on hamuniverse.com a couple of hams calculated all of the optimum lengths for a long wire antenna based on multiples of half and quarter wavelengths of all of the bands between 160m and 10m.

These are the optimum lengths in feet for anything under 500 ft.
that should be stickied or otherwise saved...



Here are the final numbers (in my opinion) in green below that would be good for a long-wire antenna: (You may want to make a note of them)

REVISED: 29 35.5 41 58 71 84 107 119 148 203 347 407 423
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Old 01-05-2014, 18:13   #9
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Re: Long wire HF antenna length

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Originally Posted by Wotname View Post
Just the lead section from the antenna tuner to the backstay.

The ground wire is electrically on "the other half" on the active (long wire) element so is not part of the "quarter wavelength".
Even more workable!

Add a KISS counterpoise and it should be good to go.

Could a "long wire" be run to the masthead, through an insulator and returned towards the deck to gain greater length, or would the returned section create interference? Sort of like an end fed inverted "V" running from each of the stern quarters towards the masthead and back.
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Old 01-05-2014, 18:29   #10
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Re: Long wire HF antenna length

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Originally Posted by 40 South View Post
Even more workable!

Add a KISS counterpoise and it should be good to go.

Could a "long wire" be run to the masthead, through an insulator and returned towards the deck to gain greater length, or would the returned section create interference?
Yes(ish)
The angle between the two sections becomes important here. Ideally the angle should be 180 degrees (i.e. straight ) which defeats the purpose in the first place. Somewhere around 90 degrees works (sort of). Usually this is impractical on your average sailing boat so it isn't done. There are, however many antenna designs that do this sort of thing. Inverted "V"s are not uncommon.

You are also wandering into areas like prorogation angles and other more esoteric matters of RF radiation and this can be a minefield to the uninitiated so unless you want the full monty, stick with getting your "simple" long wire working well and quietly browse all the loooong HF / SSB / Ham threads for some "light" reading.
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Old 01-05-2014, 18:31   #11
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Re: Long wire HF antenna length

Opps... missed your edit, I see you are already onto the inverted V... tread carefully :
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Old 01-05-2014, 18:31   #12
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Re: Long wire HF antenna length

I run one from the cockpit to the wingtip to the tail on aircraft, makes a V and works great
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Old 01-05-2014, 18:46   #13
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Re: Long wire HF antenna length

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Originally Posted by Wotname View Post
You are also wandering into areas like prorogation angles and other more esoteric matters of RF radiation and this can be a minefield to the uninitiated so unless you want the full monty, stick with getting your "simple" long wire working well and quietly browse all the loooong HF / SSB / Ham threads for some "light" reading.

Opps... missed your edit, I see you are already onto the inverted V... tread carefully :
Thanks again wotname.

I though as much.

Straight long wire, 1/4 wavelength for the mid frequency and let the ATU do the rest!
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Old 01-05-2014, 18:53   #14
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Re: Long wire HF antenna length

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Originally Posted by a64pilot View Post
I run one from the cockpit to the wingtip to the tail on aircraft, makes a V and works great
And you aviation load box is designed (internally) somewhat differently to accommodate the different nuances of the long aircraft antenna .

Some just use a short length of wire (say 6' or 8') and the far end is grounded; often located inside the vertical stabiliser

As is often the case, especially RF, the devil is in the detail.
Safe landings and tailwinds...

Please excuse the thread drift 40S.
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Old 01-05-2014, 19:02   #15
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Re: Long wire HF antenna length

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Thanks again wotname.

I though as much.

Straight long wire, 1/4 wavelength for the mid frequency and let the ATU do the rest!
More mundane detail, generally choose the mid frequency of the range of frequencies you expect to use.

I.E. you wont be using frequencies say above 18 MHz and unlikely to use any below 4MHz - at least not in Aussie waters. More likely to be using 4 & 8 and 12 & 16 MHz.

Disclaimer, I am currently out of touch with usual working AU frequencies and hopefully more up to speed folk will post accordingly.
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