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Old 03-05-2013, 10:28   #61
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First the tuner question, a tuner for a back stay is called a random wire tuner and it has two wing nut connectors one for the backstay and one for ground. The other type tuner is one with a coaxial output and is normally used for taking a already resonate antenna and keep the SWR low over the entire band it's cut for.

A example would be a 4mhz dipole for 75 meters but you want to operate down in the 8 meter 3.5 MHz CW part of the band.

But back to the backstay tuner, where the wire hooks to the output wingnut on the tuner is where the antenna starts and will radiate and no amount of coax or balun will stop that. By having that wire from the output of the tuner running through the boat will cause RFI issues because its radiating in your boat.

As said several times in this thread using coax cable from a random wire tuner to the backstay adds excessive amounts of capacitance to the antenna and makes the tuner work much harder at the cost of efficiency and heat in the tuner.



The only practical thing to do is mount the tuner right under the deck where backstay is.
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Old 03-05-2013, 10:30   #62
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Re: Technical clarification: Special case of LONG feed line

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Originally Posted by Marqus View Post
I would like a clarification of a purely technical point regarding Tuner to Antenna connection when the feed line is unusually long.

I am considering placing my Antenna Tuner far away from the Antenna base (physical installation constraints). So, I found a 2008 post from forum member "Fairbank56" where he addressed precisely this situation:

Quote: "Most manual tuners either have a balanced output or coaxial output or both. On a boat, placing the tuner near the radio, we would use coaxial cable to feed the antenna, connecting the center conductor to the backstay lead-in wire and the shield to the ground plane. In this case, the coax does NOT radiate the signal. The signal travels between the center conductor and the inside surface of the shield til it gets to the feed point. Some of the signal can then travel back down the outside of the shield which can be radiated. To prevent this we would use a choke balun in the form of ferrite beads or several turns of the coax onto a cylindrycal form whose diameter is several inches." End quote.

Earlier in that thread he also cautions about high voltage nodes and possible damage to the coax. I am not sure how that relates to the above quotation.

Where do we locate the choke balun - at which end? I assume you must use ultra heavy duty coax?

And any expert opinions - will the proposal work; i.e. will signal loss be minimized and antenna radiation maximized by this type of feed line?
Note that he is discussing manual tuners with a coaxial antenna connection. I have a couple of these for my ham rig at home. Usually these are used when the antenna is an approximate match to the coax at the frequency you are using. The tuner is used to fine-tune the impedance match. If the antenna is wildly mis-tuned (using it on the wrong band, for instance), the tuner may still be able to find a match, but the tuner and coax losses can be quite high. In this case you would want to use low-loss coax.

The balun / choke is best located at the antenna in this configuration.

But none of this really applies if your antenna is an end-fed backstay. This is generally not a good impedance match for coaxial cable, the marine auto-tuners we usually use are not designed to drive a coaxial cable, and your R.F. ground point is usually nearer the tuner than the backstay.

You might be able to make it work though. Use low-loss coax, and ground both ends (ground at the tuner, and close to the backstay), The balun/choke could go at the antenna end of the coax.

But I really don't recommend that you try this. It's probably not going to work well. Forget that I even mentioned the possibility. Instead, do whatever you can to put the tuner close to the antenna and ground.
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Old 03-05-2013, 13:23   #63
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Re: My SSB Install Was a Breeze

Marqus,

The trick is to transform the impedence of the backstay or other random length antenna/ground system into something closer to the 50 ohms the radio likes.

For this purpose, one way which works very well is to use an UN-UN (unbalanced-to-unbalanced) device. It's like a balun (balanced-to- unbalanced), but can be designed specifically to handle random length end-fed antennas.

The backstay/ground system is an unbalanced system. Coax, by definition, is unbalanced, as the shield and the center conductor are constructed of different materials and sizes, etc. Thus, the UN-UN.

Balun Designs makes several great baluns and un-uns for around $70. These will handle very large wattages just fine.
http://www.balundesigns.com/servlet/StoreFront

The idea is to connect the random-length antenna (backstay, etc.) to one pole of the UN-UN; connect the RF ground system to the other pole of the UN-UN; then run coax of any length back to the tuner & radio.

In practice, this works very well, usually even making it possible to use the internal tuners in many transceivers work just fine. You can hear such a system on the air practically every day on the WaterWay Net: WA0LSS, Scott, has one on his 56' ketch.

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Old 03-05-2013, 13:33   #64
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Re: My SSB Install Was a Breeze

You will get an expert opinion from btrayfors. From me all I can offer is try and figure out a way to place the auto tuner/coulper close the the antenna feed point (be it an insulated backstay or vertical) as possible.
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Old 03-05-2013, 13:38   #65
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Re: My SSB Install Was a Breeze

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You will get an expert opinion from btrayfors. From me all I can offer is try and figure out a way to place the auto tuner/coulper close the the antenna feed point (be it an insulated backstay or vertical) as possible.
Ha! Bob, Don't know about the "expert opinion", but you're right: the preferred way is to locate the tuner as close as you possibly can to the feedpoint of the backstay or other end-fed antenna antenna.

However, in some cases this just isn't practical and the UN-UN solution WILL work :-)

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Old 03-05-2013, 13:40   #66
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Re: My SSB Install Was a Breeze

This plan can work reasonably well, albeit with some losses you wouldn't have with the tuner at the base of the backstay. When my AT-140 was not functioning properly, I used a manual tuner at the radio end, which connected to relatively lossy RG-8X coax to the backstay, where the shield went to counterpoise/RF ground, and the center conductor to the backstay. This worked well while on a near offshore passage for keeping in touch using the Pactor modem. Its deficiencies might have become more apparent on a voyage farther afield.

Having said all that, there are steps you can take to make this work better. Using a low-loss coax like RG-213 as Paul suggests would be one step. Installing a 4:1 balun (unun) at the antenna feedpoint would also help reduce SWR losses on most bands. The unun can be both a current choke to prevent radiation of the feedline, but also a 4:1 impedance transformer to reduce SWR losses along the coax between tuner and unun. This has been discussed on this forum before. You would still have some SWR losses, but I think they would be manageable. If you were to go this route, I would suggest contacting balundesigns.com to tell them what you want, and they could suggest which of their baluns would be most suitable. It should cost well under $100.

I'm sure others will be able to elaborate/correct the above, I have only very limited experience with such techniques and that has been with random wires on land.

Chip

Edit: I see while I stepped away from creating this post, more experienced hands have provided the info!
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Old 03-05-2013, 13:50   #67
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Re: My SSB Install Was a Breeze

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The trick is to transform the impedence of the backstay or other random length antenna/ground system into something closer to the 50 ohms the radio likes. [...]
But for this to work well, you need a tuner that can work into the lower impedance of the coax / transformed antenna. Does the AT-120 (or similar tuners) work well in this case? I suspect it might, since at some frequencies the backstay is close to being a quarter-wave long, which has an impedance of around 20 Ohms (with ideal ground).

I know my ham tuners do this well, but I'm not sure about the tuners designed for end-fed random wire backstays.
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Old 03-05-2013, 13:52   #68
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Re: My SSB Install Was a Breeze

Chip,

No problem...you essentially had it right :-)

One note, however: the loss difference between RG-8X and RG-213 at HF frequencies is negligible. For example, RG-8X at 10mHz only has a loss of 1.1db per 100 feet, and we're only talking about typically 10-30' in marine applications here. The difference at VHF frequencies, though, is significant.

But, there's another reason for using RG-213 instead of the much thinner RG-8X, and that is physical strength. I much prefer to use it or, even better, double-shielded RG-214 for HF applications.

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Old 03-05-2013, 13:56   #69
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Re: My SSB Install Was a Breeze

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Originally Posted by Paul Elliott View Post
But for this to work well, you need a tuner that can work into the lower impedance of the coax / transformed antenna. Does the AT-120 (or similar tuners) work well in this case? I suspect it might, since at some frequencies the backstay is close to being a quarter-wave long, which has an impedance of around 20 Ohms (with ideal ground).

I know my ham tuners do this well, but I'm not sure about the tuners designed for end-fed random wire backstays.
Paul,

Modern marine tuners can handle just about anything....from bedsprings to a wet noodle!

BUT, IMHO it's a sacrelige to use them in this way. Better to find a way to place them near the base of the antenna :-)

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Old 03-05-2013, 19:58   #70
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Extra long feed line

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Marqus,

The trick is to transform the impedence of the backstay or other random length antenna/ground system into something closer to the 50 ohms the radio likes.

For this purpose, one way which works very well is to use an UN-UN (unbalanced-to-unbalanced) device. It's like a balun (balanced-to- unbalanced), but can be designed specifically to handle random length end-fed antennas.

<.....>

Bill
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This sounds like a good plan for my non-ideal situation. Thank you Bill for your detailed description, and for the good info from other responders.
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