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Old 15-01-2020, 07:10   #1
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Backstay antenna length decision

My rigger is coming tomorrow to put in the new backstay! Opinions needed now, thanks in advance.


I am a newbee to marine and ham SSB, but have a ham General License, Marine FCC License, and I am a used-to-be electrical engineer. I am preparing my vessel for offshore and need voice and data SSB. US East Coast, Atlantic, and Caribbean destinations. I am replacing my backstay tomorrow, and have the option to make the insulated part up to ~56 ft (17 m), or arbitrarily shorter. This includes to GTO wire length from the tuner in the aft lazerette.


The quarter wave resonant freq should be 4.4 MHz w 56 ft, slightly above the 80 meter ham and 4 MHz Marine bands. My tuner docs warn not to transmit on the half wavelength frequency (8.8 Mz) to avoid output stage damage to the rig - makes sense.


At 56 ft, I should be able to tune and Tx on Marine SSB 4, 12, and 22 MHz, and Ham 80, 30, 20, 15, and maybe 10 meters.


I will not be able to Tx on Marine SSB bands 8, 16, 18, or 25 Mhz or Ham 40, 17, or 12 meter. I hope I can Rx these to some degree.


Iffy for Tx are are Marine band 6 MHz and 60 and 10 meter Ham.


WHERE THE TRAFFIC IS: I note that the Marine 8 MHz has the most nets, followed by Marine 6 MHz, and Ham 40 meters. The most Winlink gateways are on 40 meters. I will not be able to Tx to any of these.


DECISON: I am screwing up with such a long antenna? I could shorten it to, for example, 34 ft, centered in the 40 meter Ham band, and be able to Tx on Marine 6 MHz and 8 MHz.


I would lose Tx-ing on the 80, 20, and 10 meter Ham bands, and lose Marine bands 4, 6, 12, and 25 MHz. These have a lot of nets and Winlink gateways also.


Voice of Experience would be very welcome. Thx in advance.
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Old 15-01-2020, 07:20   #2
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Re: Backstay antenna length decision

What bands do you get with 27 feet?
Are any of them optimal?
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Old 15-01-2020, 07:24   #3
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Re: Backstay antenna length decision

With a good automatic antenna tuner you should be able to TX at all frequencies. My smart tuner (sgc works on every radio) does for sure.

That said, I would go down 2 meters from the top so 15m length. The reason is that I know it works well on both the SGC and Icon tuners I have. If you have a different one then you may have different requirements.
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Old 15-01-2020, 08:09   #4
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Re: Backstay antenna length decision

Both Icom and SCS auto tuners state 23ft minimum lengths...
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Old 15-01-2020, 08:40   #5
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Re: Backstay antenna length decision

Quote:
Originally Posted by Rucksta View Post
What bands do you get with 27 feet?
Are any of them optimal?

Remember ... I am a newbee, so ... grain of salt...
This is a useful web calculator. Use 1/4 wavelength for a backstay.
Wavelength Frequency Calculator
Don't forget to include your feed wire from the tuner to the backstay.
27 ft resonates at 9 MHz. With a tuner you should be able to Tx Marine 8 MHz, and Marine 12 MHz, and also Marine 22 Mhz (the 5/8 Wave). Ham 30 meters will work. Ham 20 Meters less likely.

The bad one is the 1/2 Wave. For you that is 18 MHz. Probably 16 MHz will not be good. When you transmit near the 1/2 wavelength, your tuner is trying to compensate for a near zero impedance at the antenna feed. My Icom AH-4 tuner data sheet says it cannot do it. Even if it does compensate enough to keep the radio output stage from blowing, the antenna will not radiate much power at 1/2 wavelength.
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Old 15-01-2020, 08:49   #6
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Re: Backstay antenna length decision

I guess the short version of my question is...


Which frequencies are the most valuable for offshore and long range cruising on the East Coast and Caribbean?


Looks like 6, 7 (40 meter Ham), and 8 MHz are all pretty useful.


My 1/2 wave dead zone will be 8.6 MHz unless I shorten my antenna.
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Old 15-01-2020, 09:13   #7
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Re: Backstay antenna length decision

The minimum 23’ length is all there is. The wavelength calculator is pretty much useless for antenna design because it calculates length for a wave in vacuum or air, not in copper, stainless steel etc. let alone a combination of those like you get with a wire between tuner and backstay.

Also, where you read “tuner” they mean something different than the smart autotuners we use on boats. Please describe your radio rig as well as details about your mast height, backstay and in particular how the stay attaches to the boat and to which materials (fiberglass, steel, aluminum).

If you have a HAM manual tuner then you can simply test by hoisting a wire. Some boats even tie-wrap a wire against a stay (completely wrong in theory) and are happy with results.
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Old 15-01-2020, 10:05   #8
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Re: Backstay antenna length decision

I used a centre fed dipole attached to the back stay. It was great.
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Old 15-01-2020, 10:26   #9
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Re: Backstay antenna length decision

Quote:
Originally Posted by derfy View Post
Remember ... I am a newbee, so ... grain of salt...
This is a useful web calculator. Use 1/4 wavelength for a backstay.
Wavelength Frequency Calculator
I think the disconnect here is that you seem to be focused on making a resonant antenna out of your backstay. That is really impractical for a marine HF antenna. Your ATU will be able to tune your backstay antenna for the wide range of both marine and ham frequencies.

Generally speaking...the longer the better. Avg. seems to be in the 40-45 foot range.
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Old 15-01-2020, 10:48   #10
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Re: Backstay antenna length decision

I think you have the right idea. I cut my backstay to be resonant at about the lower end of the 20M band, The idea being if you lose your Automatic tuner you will always be able to get out on the most common long range freq used. The MM Net on 20 M.
I also found the most successful Freq's for long range Winlink connections was in the 10 Mhz range.
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Old 15-01-2020, 10:50   #11
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Re: Backstay antenna length decision

Is it good to have the lower insulator fairly high so no one grabs or touches it while you are transmitting?
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Old 15-01-2020, 10:53   #12
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Re: Backstay antenna length decision

Researched this subject last year and 27' was the general consensus. I will say the most important (found as a result of research) part and hardest part is the ground plane. As in Keep it simple as in the KISS device seems to work pretty well.
I don't use the SSB as much as thought. Many are going to phones with data.
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Old 15-01-2020, 10:54   #13
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Re: Backstay antenna length decision

Use multi-band loaded vertical(s). Using stay is asking for trouble.
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Old 15-01-2020, 11:13   #14
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Re: Backstay antenna length decision

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Originally Posted by s/v Jedi View Post
The minimum 23’ length is all there is. The wavelength calculator is pretty much useless for antenna design because it calculates length for a wave in vacuum or air, not in copper, stainless steel etc. let alone a combination of those like you get with a wire between tuner and backstay.

Also, where you read “tuner” they mean something different than the smart autotuners we use on boats. Please describe your radio rig as well as details about your mast height, backstay and in particular how the stay attaches to the boat and to which materials (fiberglass, steel, aluminum).

If you have a HAM manual tuner then you can simply test by hoisting a wire. Some boats even tie-wrap a wire against a stay (completely wrong in theory) and are happy with results.

Thanks Jedi,

I like the idea to trim the overall length to 15 m. Q: Can you Tx to the 80 Meter Ham or Marine 4 MHz bands using this? The 1/4 wave freq is 5 Mhz - it should be tuneable down to 80 Meters.



This also would move the 1/2 wave "dead zone" up to 10 MHz - further away from the 6-8 MHz Bands, but still below the useful 20 meter ham band (14 MHz).


My configuration...

Icom 718 mounted midship at nav station with 9-14vdc>13.8vdc regulated power supply. Chassis grounded to 12 V ground at the keel w 10 AWG stranded - 6 ft. Ferrite Core Filters on Power, tuner cable, and coax feeds.

Icom AH-4 tuner in aft lazarette just below deck at backstay chain plate

KISS ground counterpoise assembly at AH-4 in stern lazarette.
GTO-15 feed to cable above the lower backstay insulator (above my hydraulic backstay tensioner), about 6 ft, using standoffs from un-insulated backstay section.


Thx
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Old 15-01-2020, 11:17   #15
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Re: Backstay antenna length decision

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Originally Posted by sailingchiro View Post
Researched this subject last year and 27' was the general consensus. I will say the most important (found as a result of research) part and hardest part is the ground plane. As in Keep it simple as in the KISS device seems to work pretty well.
I don't use the SSB as much as thought. Many are going to phones with data.

Good to know. I have a KISS Ground I plan to try first. Previous owner used one on my boat with good success.
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