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Author Topic: Raised vertical  (Read 1110 times)
N8TI
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Posts: 115




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« on: February 14, 2012, 04:50:13 PM »

I do understand that even if you have a vertical that is only 50% efficient, you are still putting out a lot of power into the air, given that we all know that QRP works very well a surprisingly large amount of the time. 100 watts into a 50% efficient antenna still puts you a few S units ahead of the QRP guys if you are running 100 watts out.  Heck, a 10% efficient antenna still allows you double the power of the normal QRP. 

That being said, still it would be nice to run my SB 220 into a fairly efficient vertical.

I've been reading all the posts about verticals.  If you have one fed at the base sitting on mother earth, you need about 90 radials to get good efficiency (according to the ARRL Antenna book).   If you have a raised vertical and feed it above mother earth you only need something like four radials to get the same performance. (!)

I live on a small farm that has space.  So, I have decided to put up a raised vertical  for 40 meters CW, with the radials raised up enough to get the tractor under.  The question is how high to I raise it?  I was thinking of going over to a lumberyard or Home Depot and get the longest 4x4 or 4x6 they make, (maybe 20 feet), throw it in the back of the truck, bring it home and put it in a 3 or four foot deep hole (obviously long end up, Hi).    Then, I would put a 33 foot fiberglass pole at the top with a 12 ga. wire taped to it to act as the radiator. I would then run the four radials straight out, tying them off to trees, or other posts stuck in the ground. I would have 33 foot long radials. If I recall, slanting the radials changes the impedance, but I need to not worry about guys as we have dogs and horses and me running around out there.

I might use an aluminum antenna if it was the better way to go and did not weigh that much.

Do you think it would work well enough to go through the trouble?  Or is getting the base up say, 12 or 16 feet not good enough? It just seemed easier and perhaps cheaper than laying out 90 radials.



Joe N8TI



 
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WX7G
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Posts: 5908




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« Reply #1 on: February 14, 2012, 06:13:43 PM »

Your proposed antenna will work well. Radials raised 15 feet are said to be high enough for 160 meters and for 40 meters that works out to 4 feet. Your tractor-height radials will be great.

You will want to decouple the feedline with a 1:1 current balun at the feedpoint so that most of the counterpoise current returns via the radials and little returns via earth (up the outside of the coaxial cable shield).

Ground mounted, twenty 1/8 wavelength radials will get you to roughly 70% radiation efficiency. That is 1/4 of an S-unit down from 100%. On 40 meters the radials need be only 18' long. See the N6LF radial papers for more on this.
« Last Edit: February 14, 2012, 06:17:35 PM by WX7G » Logged
WA2ONH
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Posts: 246




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« Reply #2 on: February 14, 2012, 06:26:54 PM »

... See the N6LF radial papers for more on this.

That's "Antennas by N6LF" at: http://www.antennasbyn6lf.com/

Series of QEX articles on ground system experiments are most helpful.

Good Luck!
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73 de WA2ONH dit dit    ...Charlie
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"No time is ever wasted that is spent LEARNING something!"
N3OX
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« Reply #3 on: February 14, 2012, 10:41:24 PM »

I've been reading all the posts about verticals.  If you have one fed at the base sitting on mother earth, you need about 90 radials to get good efficiency (according to the ARRL Antenna book).   If you have a raised vertical and feed it above mother earth you only need something like four radials to get the same performance. (!)

For what it's worth, 90 radials is way, way more than you really need to get within a dB or so of perfect.

Just for an example, I have 27 radials that go out to the edges of a space that is at most 50 feet square.  Many radials terminate prematurely because they run into my shed or the fence or something.  They range from maybe 10 to 35 feet long.

I built a 10 foot tall 40m vertical and compared it to a full size quarter wave over this radial field

http://www.n3ox.net/projects/n3oxflex

The shortened antenna with around 5 ohms (maybe a bit less) radiation resistance was only a little more than 1dB worse than the quarter wave.  The shortened antenna had a base impedance of 6 ohms and the taller one a base impedance of 25 ohms.

If I look at the field strength readings and the impedance readings I can try to figure out my ground loss resistance.  The results are pretty consistent with about 1.5 ohms of ground loss, with about 4.5 ohms radiation resistance for the small antenna and 23.5 ohms radiation resistance for the vertical (radiation resistance of verticals over real ground is complicated, hence the uncertainty)

But let's say I have about 1.5 ohms ground loss resistance.  This explains both the base impedances AND the field strength difference between the two antennas.  If my ground resistance is that low, the quarter wave vertical will be about 0.27dB away from PERFECT with a miscellaneous field of 27 radials from less than 1/8th wave up to 1/4 wave.  This is a far cry from 90 1/4 wave or 1/2 wave or whatever but as far as ground return currents go, it's almost perfect for a quarter wave and so good that a 10 foot low impedance high base current vertical works almost as well as a 1/4 wave vertical.

I think people go crazy about radial recommendations for on-ground radials.  It's important, and really sparse systems on the ground can be AWFUL. But it doesn't take many radials to stop having a really sparse awful system. 

Quote
Do you think it would work well enough to go through the trouble?  Or is getting the base up say, 12 or 16 feet not good enough? It just seemed easier and perhaps cheaper than laying out 90 radials.

I ran a 40m 1/4 wave with 4 straight-out elevated radials at about 15 feet at my first QTH.  It was a fine antenna and I was very happy with it.  It may be that you would rather drive the tractor UNDER rather than OVER your radials.    I think 15 feet is way more than enough.  N6LF has shown that if you're careful even a foot off the ground is a big help.

But keep a couple things in mind.  You should use a good choke at the feedpoint for elevated radials.  You can't use them as easily for multiband use.  And on-ground radials are NOT the dire loss situation that many people make them out to be.  You do not have to copper plate your field to get good efficiency.

I'm still digesting N6LF's latest article on elevated systems and need to look a little more but I think that if the elevated system is much effort compared to an on-ground system, a modest on-ground system might be better for many reasons.  90 radials isn't just overkill, it's probably nonsense at least in terms of establishing a low impedance ground return.  Big enough dense enough systems have some effect on pattern but that's not usually what people are talking about when they recommend huge radial systems.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W1FBV
Member

Posts: 25




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« Reply #4 on: February 15, 2012, 06:33:30 AM »

I have seen a study (maybe in QEX as noted above) indicating that the optimum height above ground for the lowest angle of radiation for an elevated vertical is 1/4 wavelength.

Jim, W1FBV
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W6QW
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Posts: 21




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« Reply #5 on: February 15, 2012, 04:29:00 PM »

Here's my $0.02 based on real-world experience.  I have a 43 ft fiberglass vertical (I am stuck with this because of CCR's).  I have run tests using it as a 43' vertical with matching LC at the base with about 30 1/4wave radials on the ground.  Over a course of three months, I compared my reports to Europe on the long-path with other US west coast stations.  Results were very good - comparable to 2 element yagi's at 50' or less (most reports on the long path to Europe - a sizable path length - were at par or 1 S-unit less than the yagi's.  As expected, yagi's at higher elevation would result in an 1-2 s-unit advantage, BUT many times I was at par with them).

I subsequently changed the vertical configuration where I raised the radial plane to 10' above ground and base feeding the remainder of the vertical (33 feet) at 10' above ground.  I have 20 raised (at 10') 1/4wave radials + isolation balun at the feed point.  The result, as measured by almost a years worth of comparison reports to Europe via the long path from California, is that I easily compete with 2 element yagi's - that means that I usually receive the same signal report in Europe as the 2 element yagi's; at times an s-unit less, on occasion, an s-unit more.  This assessment is based on a couple of hundred DX long-path contacts to Europe and middle east.   I have to say that every 3 element yagi that I have competed with inevitably received a couple of s-unit better reports.  I continue to receive comments from DX stations that my signal is loud and most are surprised when I tell them that I am using a ground plane vertical.

What is most striking are the comments received from competing stateside stations that have a hard time comprehending the cost and effort of their yagi installations compared to my simple raised-radial vertical.  There are lots of literature that articulates the attributes and shortfalls of vertical antennas using both ground and raised radial configurations but my real-world experience galvanizes the fact that, for long distance communication, a properly designed vertical is a good performance alternative to a lot of steel and aluminum (read $$) in the air.
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N8TI
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Posts: 115




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« Reply #6 on: February 15, 2012, 05:43:12 PM »

I had an R-7 and went with the tower and quad and there was not much difference. I could not get India with the R-7, but did get it with the quad. That was the only real, measurable difference. So, I definitely intend to do the raised vertical. I will also run some additional radials under the inverted L, as I have three radials now, 59 feet long, and starting off on the ground, but after 20 feet, running on top of six foot high plastic fences.

De Joe
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K4RVN
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Posts: 757




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« Reply #7 on: February 17, 2012, 07:34:08 PM »

Joe,
I'll be looking forward to see how the raised vertical works for you. The 2 element quad should have made a great difference in gain over the R 7 as it has more than twice the gain if installed and fed properly. The specs on a R 7 read 3DBI, while a 2 element cubex quad is 7 to 9 DBI Also it is highly directional with good front to back not omni directional so I'm sorry to hear you did not notice much difference. My 3 element is like day over night after replacing a Hustler 4BTV vertical years ago. I hope that you will keep us posted on your new vertical project.

Frank
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