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Old 16-09-2015, 04:54   #1
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Length of HF longwire antenna

Hi there,

With the refit now mostly behind us and the HF radio, ATU and through-deck connection made, it's time to install the longwire antenna and wrap up the whole HF install.

Obviously the length of the longwire is absolutely crucial to its preformance, and even though the ATU can play with it a little, picking the right length for the frequencies you hope to use most helps tremendously in preformance. I'm very new to the whole HF game myself, and haven't been able to thoroughly look into what frequencies I hope to use most on my upcoming big trip. We'll be coming from Europe, going to the Caribbean and back again to complete the loop. I installed the HF radio for weatherfax, email, cruising nets and occasional contact with the home-front (that last one is the least of my worries).

With this in mind, what do you guys suggest I use as antenna length? What do you have and why?

Note: I'll be using a regular longwire antenna, not an insulated backstay. From the longwire it'll go through an insulated deck-fitting which leads to the ATU which I installed below-deck. The ATU is grounded to the steel hull of the boat.
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Old 16-09-2015, 05:38   #2
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Re: Length of HF longwire antenna

Generally an antenna must be at least one wavelength long to be called a longwire antenna. You can compute the length of a 1-wavelength-long longwire antenna from

Wavelength (in meters) = 300 / Frequency in MHz

It is generally easier to feed a longwire antenna if its length is not an integer multiple of a half-wavelength, as this will avoid having a very high-impedance feed point. More advantageous lengths are those that are odd multiples of quarter-wavelengths, as that will produce a low-impedance feed point, which can more easily be matched to the typical 50-Ohm transmitter output impedance.

You can also make a longwire antenna have a more broadband feed point impedance if your terminate the far end in a resistor. Usually a 600-Ohm resistor is used. The resistor should be non-inductive. Terminating a longwire tends to make the antenna unidirectional, with its main lobe toward the terminating resistor.

On a steel hulled boat with a tall mast, you could have a nice antenna system if your constructed a longwire antenna which was fed at the transom, ran to the masthead, and then continued to the bow, where it was terminated in a 600-0hm resistor. This would be an interesting antenna to try. It could be fed with a 4:1 broadband unbalanced-to-unbalanced transformer.

It is probably more common for boaters to use an antenna that is really better described as an end-fed random length tilted vertical. The same considerations about length apply. Avoiding integer multiples of half-wavelengths will keep the feed point impedance from becoming too high to effectively match to 50-Ohms with a tuner.
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Old 16-09-2015, 05:45   #3
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Re: Length of HF longwire antenna

Its not so much how long should it be but how long it shouldn't be.

From the Icom 140 manual....
'Undesirable antenna element lengths ....... AVOID multiples of 1⁄2λ (half wavelength) for antenna element lengths, since tuning becomes difficult.'
Sounds like you want only marine HF and not ham which makes things easier...what radio and tuner do you have?


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Old 16-09-2015, 05:51   #4
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Re: Length of HF longwire antenna

and also... long wire is good.... you can trim and adjust at will if you find that its not working.... Fitting something like a 'diamond' SWR meter is also good and if you can beg steal or borrow an antenna analyzer such as a Rig Expert AA-54 so much the better.

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Old 16-09-2015, 05:53   #5
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Re: Length of HF longwire antenna

Have an IC-M700 with an ICOM AT-140E atu.

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?

And yes, only interested in marine HF (apart from the homefront comms and the email I guess).
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Old 16-09-2015, 06:07   #6
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Re: Length of HF longwire antenna

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.

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Old 16-09-2015, 07:01   #7
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Re: Length of HF longwire antenna

One thing that may help you going forward is to point out the implications in the responses you have seen so far - the length of the longwire is NOT absolutely crucial to its performance.

Bill pointed this out more clearly, but I thought it was worth repeating.

If you were operating a dipole antenna, then the length is crucial, but not for a random length end-fed antenna (except for trying to avoid 1/2 wavelengths of desired bands).

More critical to performance are the power supply and the counterpoise. You sound like you have the counterpoise well in hand. I would suggest moving the ATU either to the very top of your underdeck or to above deck.

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Old 16-09-2015, 07:50   #8
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Re: Length of HF longwire antenna

Distance between the ATU and the deck is maybe 40cm, check!

So length-wise, anything between 40-45ft...?
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Old 16-09-2015, 08:33   #9
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Re: Length of HF longwire antenna

Yes. That should work fine.
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Old 16-09-2015, 08:46   #10
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Re: Length of HF longwire antenna

Quote:
Originally Posted by Orchidius View Post
Hi there,

With the refit now mostly behind us and the HF radio, ATU and through-deck connection made, it's time to install the longwire antenna and wrap up the whole HF install.

Obviously the length of the longwire is absolutely crucial to its preformance, and even though the ATU can play with it a little, picking the right length for the frequencies you hope to use most helps tremendously in preformance. I'm very new to the whole HF game myself, and haven't been able to thoroughly look into what frequencies I hope to use most on my upcoming big trip. We'll be coming from Europe, going to the Caribbean and back again to complete the loop. I installed the HF radio for weatherfax, email, cruising nets and occasional contact with the home-front (that last one is the least of my worries).


With this in mind, what do you guys suggest I use as antenna length? What do you have and why?

Note: I'll be using a regular longwire antenna, not an insulated backstay. From the longwire it'll go through an insulated deck-fitting which leads to the ATU which I installed below-deck. The ATU is grounded to the steel hull of the boat.
It has been a long time but as I recall, length on a long wire is not overly critical. Longer is better.
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Old 16-09-2015, 08:52   #11
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Re: Length of HF longwire antenna

Just to jump in with another question:
Is it practical/desirable to install two individually selectable antennas, one for shorter wave length tuning and another for longer?

JK
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Old 16-09-2015, 09:02   #12
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Re: Length of HF longwire antenna

It depends.

On my boat, I have three types of antennas:

1. the traditional insulated backstay for use on all frequencies;

2. tuned vertical dipole antennas for use on the amateur 20m and 30m bands; and

3. a stern-rail mounted mobile vertical antenna with changeable resonators for the ham 75m, 40m, 20m and 15m bands. This one is for emergency use; it's wired up to the base, but the mast and resonators are stored below, ready for fast deployment if needed.

The backstay antenna is pretty good on all bands.

The vertical dipoles cannot be beat for DX (long distance) contacts -- their performance is truly astonishing. But, of course, they are single-band only.

The relatively short Hustler mobile antenna is OK on the chosen bands, better on the higher bands (14mHz and above).

If you're talking about two separate backstay antennas, like are found on some boats, yes -- that would be possible. Only you could judge whether it would be worth the extra work and expense. In most cases, it would not.

Bill
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Old 16-09-2015, 10:19   #13
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Re: Length of HF longwire antenna

I agree with BTRAYFORS. As an engineer and serious cruiser, I like redundancy. My 46' ketch has permanent insulated backstay antenna about 55' long, port and starboard rail mounted short (hamsticks/ for wx fax 17, 20, 15, 10m) and a ready made temporary dipole ((cut /coax to pull up to top of mast).

They can all go to a coax switch to allow quick comparison which I do from time to time. Generally backstay antenna is better than hamsticks and maybe not quite as good as dipole... but so close to dipole performance as not worth extra trouble/ risk of keeping dipole up while underway.

While there have been many discussions here on pro/ con insulated backstay for serious blue water sailors, my opinion is other antenna options have far higher risk of causing catastrophic issues than a backstay insulator/ swag failing. Makeshift wire antennas failing and fouling prop/ rigging... usually at worst possible time/ conditions are real and serious risk.

If you try/ compare various antenna configurations yourself don't just use your receiver for the test... get a A/B signal report from stations on the other end as well. Record freq, time of day, date, distance/ direction to station. All of these factors affect which antenna configuration might be better in a particular contact. Over time/ reading propagation books/ discussions with hams,,you will learn the basic rules of thumb and your specific antenna(s) performances.

FYI, prepared to chase down RF interference from various things on your boat (LED lights, autopilot, Chartplotter microprocessor, inverter, any wall-wart chargers, battery chargers...seem to be the worse potential offenders) into your sensitive HF receiver. I've managed to reduce most, but on weak HF contacts I still have to turn of the autopilot, LED cabin lights when using some frequencies.

Also, be prepared to spend some time taming down your own transmit RF from getting into your other sensitive shipboard systems (auto-pilot, 3-phase alternator charge circuit, ... That's a whole black science of its own. Every boat environment/ shipboard equipment list and effective procedures to keep all systems on their own reservation and happy is different and usually takes time, trying various techniques, and some luck.


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Old 16-09-2015, 12:15   #14
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Re: Length of HF longwire antenna

While this may not be the most technically correct or exact, I did not use an insulated backstay or long wire on my Gulfstar 50 many years ago. I just installed a ham mobile antenna with different coils for different bands and was able to get outstanding coverage. One thing to consider is that you should not just consider screwing the antenna to a " bumper style mount". There is too much movement and a friend had his bumper style antenna fall off about 2/3 of the way to Tahiti. I fixed that by insulated mountings on an offset and below the top rail, then did another offset to hold the antenna about 2 feet higher.
While this is not as technically sophisticated, the mobile antenna with a good antenna tuner gave me significant reach during the 1 1/2 years I used that system.
I then swallowed the anchor and became land based for several years.
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Old 16-09-2015, 12:45   #15
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Re: Length of HF longwire antenna

Quote:
Originally Posted by W3GAC View Post
I agree with BTRAYFORS. As an engineer and serious cruiser, I like redundancy.
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.

For those talking redundancy, how many have redundant couplers, transceivers, battery/power connections and counterpoises? These are the parts more likely to fail over a piece of wire used for an antenna. I don't think I have ever come across anyone with HF problems (seemingly about half of all cruisers out there) because their antenna is broken.

A redundant antenna without the other stuff is just playing with one's self, unless those antennas are specific use ones like Bill mentions. But then they really aren't "redundant".

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