Originally Posted by jurgen99
Its exactly the same as a backstay fed antenna. The only difference is that your feedpoint is up in the air. Coax feeding such a reactive antenna with coax produces high losses. the only real advantage of your antenna is that it might have slightly less ground loss. This however will be offset by the greater losses because you are operating a coax into a very HIGH VSWR. I am sure it works like everything else in radio. Even a Miracle whip works. Marine
antenna tuners cant handle such a low impedance only because of the high circulating RF currents. You antenna tuner probably can handle this antenna however it is very high loss.. If you get a proper low impedance tuner designed for tuning marine
short whips like those from Harris they can tune an antenna like yours without any problems. Another such antenna tuner is one designed for aircraft antennas. If we were going to have a antenna shootout, my money
would be on the normal backstay antenna with a SGC230 as the winner. If you antenna was fed with open wire feedlinese it will win if you use the mast
as a ground for the feed point.
Don't we all just love it when an arm-chair quarterback drags out some text book theory and tries to "do in" something that works superbly in the real world?!?
Look Jurgen99, I'm sorry you can't appreciate the real world advantages the sloper has over the backstay antenna. I can agree with SOME of what you are saying but a lot of it simply indicates your misunderstanding of real world applications. Perhaps I should have written a deeper explanation but my post was getting very wordy as it was. I'll try to keep it short as I can here, but this is a complicated subject, and the OP is really overwhelmed by the tech talk going on when all he wanted to know was how to "hook it up".
The sloper and the backstay are NOT identical at all. There is a huge difference moving your current
loop to the masthead with the sloper. The backstay on the other hand has it's current
loop in or near the lazarette at deck
level. (I'll get to all the rigging
that is in the backstay's way next)
I hope you can realize that by feeding the backstay at the top and grounding the braided shield side of the coax cable to the mast
, that this method causes the mast and rigging to work WITH your sloper antenna as the grounded side of this unbalanced antenna, and NOT against it as with the bottom fed backstay...This works out to be a HUGE difference in performance that I have personally tested with hundreds of hours of "on the air" use. Conversely the bottom fed backstay has the mast and rigging in it's WAY, and the mast and rigging is absorbing, or blocking if you will, some of the backstay's signal by being so close physically to a bunch of grounded metal, and the close proximity of grounded metal is the LAST thing you want with a bottom fed backstay. And where is the counterpoise for your bottom fed SGC tuned backstay? Well...the SGC tries like hell to find it in the boat somewhere and seldom does because of the HF wavelengths involved. The sloper on the other hand doesn't really care that much about a counterpoise in the boat. It's got the entire grounded rig working WITH it, not against it. (You admit later in your post that if I used ladder line instead of coax the sloper would indeed be a better antenna....well, since the sloper is NOT a balanced antenna, in essence the coax braid or shield does just that, after all, just how do you think ladder line works feeding an unbalanced antenna? Does it remain "in balance"?
Also I should mention that in polar curves run both with antenna analyzing software
AND real world tests that I have conducted personally, the sloper is somewhat directional off the back of the boat. It also (in theory) exhibits approximately 3 db of gain in this direction. On the down side there are a couple of pretty sharp nulls off the bow at approximately 10 o'clock and two o'clock. These nulls in practice are so narrow that even during testing while sailing I could not keep a signal nulled at those positions because the boat simply moves around too much. So we can consider the nulls to be negligible in real world use.
Your assumption that my sloper has a very high VSWR is simply incorrect because I did not mention (my fault...for space reasons) that I cut my backstay length to a formula, and use the lowest quarter wavelength of the band I want to use. The formula is a simple one for a half wavelength, (then divide that answer by two for your backstay's insulated length). The formula is: ( 468 divided by the frequency in MHz). In my case I always choose the 7 MHz ham radio band (over marine SSB 8 MHz as a personal preference) and this works out to an insulated backstay length of roughly 33 feet) AND....because this is a quarter wavelength I CAN RUN MY TRANSMITTER INTO MY SLOPER ON THE 7 MHz FREQUENCY BAND WITH NO ANTENNA TUNER AT ALL!! So there is no high SWR on this band, period. And for the other bands the tuner comes into play nicely. If I had more time/room I would also explain how to select the coax feedline length so the coax does not become a resonant and radiating part of the antenna---a very undesirable situation on a boat---and something you failed to mention. (BTW, and while I'm thinking about it, the MFJ-269 antenna analyzer unit works like a charm setting up the intricacies of the sloper)
In real world use there is much that text book theory just can't account for when constructing a sloper antenna aboard a sailboat, there are simply too many variables, and before you put any money
on your SGC tuner and backstay winning in a shoot out comparison, I'd like to inform you that I've already shot down MANY a backstay tuned by an SGC (and others) And I shot one SGC down so badly that the owner of the SGC came over to my boat from his dock
to witness for himself that I was actually talking with his friend that could not hear him with his SGC/backstay setup. Here's exactly how it went down and exactly what happened with this accidental test that wasn't even planned:
Some years ago when I was living aboard
in Marina del Rey, CA this friend of mine installed a very nice Kenwood TS440S transceiver and an SGC tuner with an insulated backstay antenna. His boat was three docks away from mine. One morning I heard him calling his friend (on schedule) that was on a boat down in Puerto Vallarta
. They simply couldn't make contact, but I did hear his friend in Puerto Vallarta just fine. So, I broke in and called his friend asking if I could relay any messages for him. His buddy in Puerto Vallarta
came right back to me, and was shocked when I told him I was only three docks away from his buddy. He couldn't believe it! The next thing I know the guy from three docks over is knocking on my hull so he can come aboard and see if I was telling the truth and was actually in contact with his friend in PV. I wish you could have seen the look on his face when I simply handed him the microphone and he and his buddy had a nice long conversation, much of which, not surprisingly, was about sailboat antennas and what I was using---the sloper.
So...there you have it. I'm sure you'll say it was just conditions....but is that really good science when the competing boats are only three docks away from each other? It is true that our boats were less than 100 yards apart. The funny
thing is that my boat at that time was a Rawson
, with a very short rig. My mast was only about 35 feet high and I had the shortest rig on my dock
. I was surrounded by sailboats with taller masts.
So there you go---a real world shootout: Sloper 1 -- SGC/Backstay 0
(in fact, it was embarrassingly not even close)
Anyone interested in how to set one up feel free to PM me.