UNinsulated Backstay for HF Radio

[Dockhead]

I have a "parallel backstay" antenna for my HF radio, but I have be musing about the idea of using the real backstay. I don't really like the parallel backstay -- it's in the way, and more windage.

The mast is grounded to the keel, but that's a LONG round trip -- the mast is 75 feet tall.

Would it be completely stupid to try it?

[transmitterdan]

Probably, but nothing "bad" will happen if you try it. I assume the backstay itself is not grounded and just the mast is connected to the keel.

There is a long wave antenna in Ireland built this way and it worked pretty well. The difficulty will be that such an antenna will have frequencies where it works and others where it simply will not. The easiest way to find out is to try it.

[Jim Cate]

DH, this is essentially what we've been doing for years. On our previous boat, we fed the tuner into the backstay chain plate. The mast and all the other shrouds and stays were grounded to the lead keel. It seemed to work about as well as the regular insulated backstays on other boats in the same anchorages. There is more RF floating about the boat if that worries you. At one point we put several ferrites on the backstay just below the masthead (at the suggestion of a fellow ham). It changed the way it tuned up, but did not seem to make much difference in radiated power. This, by the way, all with a manual tuner.

On this boat we feed the shroud chainplate with similar results. Both rigs would tune on any frequency that we ever tried (ham bands between 80 and 10 meters, 4,6,8 and 12 MHz marine bands). NEither one would tune well on the 2 MHz band, and for a while I worried about lack of a useful 2.182 distress frequency. Over the years I stopped worrying!

I don't know if the typical auto tuner would do as well, for I have not tried one.

BTW, we do have an insulated backstay on this boat, so it is excluded from the rest of the antenna array... kinda the opposite of the normal situation. At the suggestion of one of the antenna expert hams on CF, I ran some 50 ohm coax up the mast and tried feeding the backstay as a top fed sloper. It makes a very good and quiet rx antenna on 40 meters, but does not seem to get out well over the short to medium path length we encounter on our daily maritime mobile net. On 20, it seems to work well on both rx and tx, but that is over longer paths. I've not spent much time fooling with it for the "load up the whole works" antenna does so well.

Cheers,

Jim vk4/gft/n9gft

[Viking Sailor]

DH,

Assuming you have a typical HF setup consisting of a SSB transceiver and an automatic antenna tuner - the use of a grounded, uninsulated, and shunt or slant fed backstay is a very feasible antenna configuration. These types of antennas are not new and have been used by broadcast and shore stations for many decades.

http://www.hatdaw.com/papers/Dawson_Slant_Wire_Shunt_Fed_IEEE_BTS_2010.pdf

Radio Antenna Engineering - Shunt-fed Radiators

The backstay ground should be as direct as possible and something less then 8 feet of 2 inch stainless steel foil tape to a seawater fitting. The RF ground for the tuner needs to connect to the same seawater fitting using the same SS foil tape. If the distance to the seawater fitting exceeds 8 feet use parallel strips of foil tape.

https://www.grainger.com/product/GRA...oil-Tape-4CLH2

The mast and all stays, and thru-hulls need to be bonded and taken to a seawater ground.

The turner RF output needs to be connected to the backstay with GTO-15 high voltage wire at 8 feet above the chainplate. The lower 8 feet of backstay can be insulated using Davis cable covers.

Cable Covers by Davis

A never mentioned fact about an antenna on a sailboat with metal masts and rigging is the effect all that metal has on the antenna's radiation pattern. These effects are known as near field effects. They are do to the phase interaction between the primary signal and the induced secondary radiation from the metal masts and rigging. (Think of the multi-element Yagi type TV antenna.) These near field effects are present with or without the installation of backstay insulators. The backstay insulators are used to avoid feeding the antenna at a high impedance (half wave) frequency. The off center shunt or slant fed antenna eliminates the possibility of it being fed at a high impedance point for any HF frequency.

Years ago I used MININEC software to model a number of different backstay antenna configurations to determine how well they compared. The uninsulated, shunt or slant fed, grounded backstay antenna performance was quite good. The model showed that the near field effects of mast and rigging favored an antenna radiation pattern in the stern direction.

I have used this antenna configuration while cruising with great success.

[RaymondR]

Inverted V using the mast as the feed line?

[Russ]

Get this easy peasy and it works. Russ
gamelectronicsinc.com

[Dockhead]

Originally Posted by Russ

Get this easy peasy and it works. Russ
gamelectronicsinc.com

I just sold one, which came with my radio. Didn't work very well for me. Was pleased at how fast I sold it, however, and how much it brought.

[transmitterdan]

DH if the GAM did not work then feeding the same non-insulated backstay probably won't work either.

As a test you could try pulling up a piece of wire using a halyard to see if a "real" antenna will work. The "optimum" length is about 40+ feet including the wire from the tuner plus the antenna wire itself. A piece of lifeline wire will work.

Also, tell us about how the tuner is grounded (width of strap, length and to what is it connected).

[Dockhead]

Originally Posted by transmitterdan

DH if the GAM did not work then feeding the same non-insulated backstay probably won't work either.

As a test you could try pulling up a piece of wire using a halyard to see if a "real" antenna will work. The "optimum" length is about 40+ feet including the wire from the tuner plus the antenna wire itself. A piece of lifeline wire will work.

Also, tell us about how the tuner is grounded (width of strap, length and to what is it connected).

Different boat!

I have a parallel backstay and it works fine.

HOWEVER, I am concerned that the GAM antenna may be the same as this -- since it will inductively couple to the uninsulated backstay (right?).


I am just trying to find away to avoid rigging it this year, as it is in the way and adds windage.

I think I'll give this a try. It's not that much trouble.

Grounding is via copper strap to through hull.

[denverd0n]

Originally Posted by Viking Sailor

Years ago I used MININEC software to model a number of different backstay antenna configurations...

Same here, though I used EZNEC. The difference between an isolated backstay, and not bothering with isolation (meaning, in essence, that the entire stangind rigging becomes the antenna), was pretty minimal.

Certainly no harm in giving it a try, and since it is easy and practically free, why not?

Good luck.

[belizesailor]

Originally Posted by Jim Cate

....I've not spent much time fooling with it for the "load up the whole works" antenna does so well.

Cheers,

Jim vk4/gft/n9gft

I initially installed my SSB as such as an initial test years ago with the intent of later going to an insulated stay or similar. Worked so well, Ive never changed it. Feed to side stay on a cat which effectively "loads up the whole works". Being a cat, no path to ground from rigging. Tuner grounded to dyna-plate.

No mucking with my rigging and works well.

[Dockhead]

OK, thanks to all for the interesting data.

I'll give it a try sometime this month before heading out.

I don't much like the idea of a loop between antenna and ground, but it seems to be inevitable, even if I'm using an unbonded through hull, since I have to ground the tuner. My mast and keel are bonded together, and are bonded to the negative side of both AC and DC systems.

If it doesn't work, I can go back to the parallel backstay.


The idea of using a shroud is especially interesting, as it would mean I could put the tuner quite close to the radio, but I don't like the idea of the short loop through the first spreader. Also it would be hard to get the copper strip to a through hull from there.


I don't think I'm going to bother with insulation, as the voltage is quite low at the bottom part of such an antenna, isn't it? The voltage only goes up higher up? I wonder if the base of the mast, then, will be very hot during transmission. Hmmm.

[transmitterdan]

If the mast is bonded to the keel the voltage at the step will be low. If the base of the mast is insulated then it will be significantly higher voltage. For open ended antennas less than 1/4 wavelength the maximum voltage is at the end away from the tuner.

[Dockhead]

Originally Posted by transmitterdan

If the mast is bonded to the keel the voltage at the step will be low. If the base of the mast is insulated then it will be significantly higher voltage. For open ended antennas less than 1/4 wavelength the maximum voltage is at the end away from the tuner.

That's what I thought -- so since the base of the mast is bonded to the keel, which is bonded to ground, I guess I shouldn't have any "hot" rigging problems.

[belizesailor]

Originally Posted by Dockhead

...

The idea of using a shroud is especially interesting, as it would mean I could put the tuner quite close to the radio, but I don't like the idea of the short loop through the first spreader. Also it would be hard to get the copper strip to a through hull from there.

....

Yes, that was an advantage for me too. All gear is quite close together with no long runs. Tuner is in an interior locker within 2' of the chain plate where attached.

And for me (cat), no short loop via spreader, since there is no spreader...just a big fat long side stay.