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Old 16-09-2015, 15:07   #16
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Re: Length of HF longwire antenna

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Originally Posted by colemj View Post
I'm not sure I agree with being so concerned about antenna redundancy. If the OP's random wire antenna fails for any reason (it is only a wire, so I don't know what a failure mode would be), he can just run up another length of any old wire..........
Failure of a long wire antenna would typically involve the mast falling off.... then running up another long wire becomes problematic. However a long wire laid out along the deck or draped over a spinaker pole on its way frd will work..... both Jim Cate and myself have been there - done that.

I now have backstay and a whip.
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Old 16-09-2015, 16:25   #17
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Re: Length of HF longwire antenna

As far as frequencies you'll be using: 20 meters/14MHz for long distance daytime; 40 meters/7 MHz for night and shorter distance daytime. You may have some lower freq. comms. FWIW ham frequencies and marine SSB use the same frequency bands just different parts of them. IIRC, the 1/4 Wave length for 20 Meters was longer than my backseat and lower Frequencies even longer. The antenna tuners whole reason for existing is to make improper length antennas work.
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Old 16-09-2015, 16:59   #18
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Re: Length of HF longwire antenna

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Originally Posted by El Pinguino View Post
Failure of a long wire antenna would typically involve the mast falling off.... then running up another long wire becomes problematic. However a long wire laid out along the deck or draped over a spinaker pole on its way frd will work..... both Jim Cate and myself have been there - done that.

I now have backstay and a whip.
What if your mast falls down and you get struck by lightning through your whip?

The above is meant to highlight the absurdity of thinking one has "redundancy" by focusing on just one aspect. A lightning strike alone can take out all of it in your case.

In the PO's case, he is just asking a simple question and getting some off-focused and naval-gazing answers. Besides, he wants to use an end-fed wire, which you even point out can be redundantly re-rigged almost instantly with any old wire he has lying around. That is, if his wire is even raised when the mast comes down...

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Old 16-09-2015, 18:04   #19
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Re: Length of HF longwire antenna

One comment really strikes a chord with me: it really does boil down to a certain amount of "black science ". I reckon you could know Mr. Maxwell's equations inside out with very little effect on your actual ability to design and install a well-functioning HF system, let alone maintain and operate it well.

JK
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Old 16-09-2015, 18:46   #20
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Re: Length of HF longwire antenna

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One comment really strikes a chord with me: it really does boil down to a certain amount of "black science ". I reckon you could know Mr. Maxwell's equations inside out with very little effect on your actual ability to design and install a well-functioning HF system, let alone maintain and operate it well.

JK
I won't agree with that. HF radio obeys all the laws of physics (including Maxwell's equations). But there is a lot of "voodoo science" proffered for sale out there. I won't mention them here.

To the OP, go back and read Btrayfor's post. He gave you the answer to your question. It is not a black art. The tuner really will help tuning the antenna.
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Old 16-09-2015, 20:15   #21
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Re: Length of HF longwire antenna

Orchidius,
Please read Bill's (and W3GAC's) postings....as the definitive answers are right there!!
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Originally Posted by btrayfors View Post
Longer is better for the lower bands/frequencies, i.e., below 10mHz. Shorter is better for the higher bands/frequencies, i.e., above 10mHz.

Several things to keep in mind about length:

1. overall length is calculated from the terminal on the tuner (i.e., includes the GTO-15 feed line);

2. electrical length is different than physical length; you must take into account the velocity factor of the antenna wire itself if you want to accurately calculate electrical length; and

3. length generally isn't critical; don't worry too much about it.

A good compromise length for all frequencies has been found to be 40-45' overall.

Bill
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JK, I'm not interested in starting a discussion about the esoteric issues, but I didn't see anyone comment on any of this being a "black science"???
Regardless, as Dan writes the physics of this is well known (and has been for many, many decades....some of the basics of HF comms from the 1920's have been proven on paper, in the labs, and in the real-world, for almost 100 years now..)
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Originally Posted by Sojurnr View Post
One comment really strikes a chord with me: it really does boil down to a certain amount of "black science ". I reckon you could know Mr. Maxwell's equations inside out with very little effect on your actual ability to design and install a well-functioning HF system, let alone maintain and operate it well.

JK

I hope this helps...

Fair winds...

John
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Old 16-09-2015, 20:42   #22
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Re: Length of HF longwire antenna

Bill's comments are right on as are John's... as they usually are. The science of RF communication is well understood in amateur, emergency communication, and military circles. It's only in the boating community where we seem to ignore 75 years of expertise. It isn't black magic, just another skill to learn (like sail trim, engine mechanics, etc).

Oh, and I do have multiple transceivers (3), tuners (3), etc. But I'm as much of a radio nut as a cruiser. One comment... if you have a KETCH you have the option of a GREAT antenna, Mizzen Cap Shroud (insulated) jumped to the Triatic Stay (insulated). Gives you a near 'inverted L', a great antenna with multiple lobes that can work short and long skip well. On my boat it's a radiator of over 80' and tunes easily from 10-160. I feed mine through a 4-1 Un-Un... Contact me if you're interested in learning more (or listen in on the maritime nets to hear what a ketch can sound like) <grin>
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Old 16-09-2015, 20:49   #23
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Re: Length of HF longwire antenna

Scott might just be a bigger radio nut than I am???
Is that possible?
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Oh, and I do have multiple transceivers (3), tuners (3), etc. But I'm as much of a radio nut as a cruiser. One comment... if you have a KETCH you have the option of a GREAT antenna, Mizzen Cap Shroud (insulated) jumped to the Triatic Stay (insulated). Gives you a near 'inverted L', a great antenna with multiple lobes that can work short and long skip well. On my boat it's a radiator of over 80' and tunes easily from 10-160. I feed mine through a 4-1 Un-Un... Contact me if you're interested in learning more (or listen in on the maritime nets to hear what a ketch can sound like) <grin>
I've only got two M-802 transceivers, and two tuners....and while I've got two backstays only one is insulated and fed as an antenna, but I do have an insulated lower shroud as well...and have two dipoles as well...as well as a whip/wire antenna stowed below for a back-up...
I've only got a 47' sloop....so Scott has more "realestate" to work with than I do, in two planes!


Fair winds...

John
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Old 17-09-2015, 04:57   #24
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Re: Length of HF longwire antenna

If my backstay has a 12' Waltz Radar Mount (1.25" alu pipe) alongside the bottom with a radar gimbal hanging towards the aft, where should the bottom SSB antenna insulator be located? Is length more important? This backstay antenna my be less than 35' in any case. Would there be operational conflicts between radar and Ham? Thanks.
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Old 17-09-2015, 05:45   #25
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Re: Length of HF longwire antenna

Assuming the connection of the mount to the backstay is insulated (it's supposed to be) I think the Waltz mounts have SSB antenna coexistence in mind. You should be able to put the backstay insulator wherever you like. The exact position of the lower insulator is not that important. The upper insulator is the one that determines the length of the antenna. The antenna begins at the tuner and ends at the upper insulator. So that length (tuner to upper insulator) is what matters for SSB purposes.

Interference from radar into SSB will not be a significant problem I think. The main concern is whether the radar unit or control cables create RF interference that will be picked up by the SSB receiver. This is possible and can probably be mitigated.

However, I suspect SSB interference into the radar system could be a problem. The radar control and data cables will probably capture RF energy when the SSB is transmitting. It may take some trial and error to find the right place along the radar cabling to install clamp-on or torroidal ferrite cores to block HF interference getting inside the boat. I would guess this to be a solvable problem.
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Old 17-09-2015, 07:10   #26
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Re: Length of HF longwire antenna

rgleason,
Transmitterdan, has given you some very good info....read it, and use it!

I'd like to highlight his last paragraph, as this can be the real issue...

And, here my usual admonishment of not needing to "stand-off" the GTO-15 wire from the stay, can be ignored!
Meaning that separating the "SSB antenna" (GTO-15 wire) from the radar cabling (and the radome electronics), by an extra few inches can make a significant improvement in RFI troubles...this is not to say that using stand-offs is a necessity, but it is a good idea!



Fair winds...

John
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Old 18-09-2015, 06:01   #27
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Re: Length of HF longwire antenna

Yes, Thankyou TDan and ka4wja. This thread is bookmarked! I am familiar with need for standoff, but did not know antenna started at tuner. Yes Waltz attachment has insulation. I have full adv ham lic but need to get started.
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Old 18-09-2015, 09:01   #28
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Re: Length of HF longwire antenna

You'll notice no one is answering your question, which is typical for this forum. Gordon West has a good article which explains antenna length. It is not a full wavelength. It has to be off-set a little. I believe I made mine around 33 ft. It's not that critical with a tuner. Try goofling the Gordon west article on the subject.
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Old 18-09-2015, 09:41   #29
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Re: Length of HF longwire antenna

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You'll notice no one is answering your question, which is typical for this forum.
The question was rather vague, as in "how long is a longwire antenna?"

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...antenna length... is not a full wavelength. It has to be off-set a little.
Now that's a really specific, non-vague answer.

To make a recommendation for antenna length, it will be necessary to know the intended operating frequency.
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Old 18-09-2015, 11:29   #30
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Re: Length of HF longwire antenna

"I understand the whole wavelength theory, but doesn't that mean you're "tuning" the rig for a specific frequency, making it less efficient on others?"
That's the way it works. Regardless of how "specific" an antenna is, it is optimized for some frequency and that makes it less optimal for certain others. It will work best for your target, and for certain other frequencies that are multiples of that target, but still it is optimized for those frequencies.
The antenna tuner is also not the magic box they are sometimes thought of. It isn't really tuning the antenna, isn't really making the antenna better of worse. You want to make the antenna as good as you can, for the frequency(s) you need the most. Then the tuner simply makes the radio happier [sic] by providing the right impedance match to that antenna. It does NOT make the antenna any more efficient, there will still be power losses involved when the antenna has to be "matched" by the magic box. So it is still worthwhile targeting the antenna length as if you had no tuner at all, and then the system will still be optimized even with the tuner.


Also remember that despite any theories, the antenna will be interacting with the hull, mast, and rigging, so you might want to start with some cheap wire that is too long, and trim it down, to see just what the best "real world" length will be. Then replace with the good stuff.
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