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Antenna Solutions portable dipoles, verticals, multiband antennas for life on the road.



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  #1  
Unread 10-21-2016, 09:45 AM
N3LYT N3LYT is offline
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Default Antenna mount

Ground mount screw driver antenna. All though my screw drive antenna worked OK up on a pole behind the camper it was fairly narrow band and required a good bit of retuning. $16 worth of parts 2 sticks of 1/2" EMT, 6 5 1/2 carriage bolts 6 1/4X20 wings nuts a touch of welding and a left over piece of aluminum. Tried it yesterday several European contacts with 50 watts and a battery.
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  #2  
Unread 10-22-2016, 12:53 AM
ke0me ke0me is offline
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Am I correct that there is not an earth ground ( ground rod) and there are just the 3 met pipes for the "missing " half of the vertical?

I very interested in this as I have a screwdriver and am looking for various ways to mount.

Do you have the std 54 in whip or the longer one they sell?

Nice looking arrangement.

Are the 3 "radials" 10 feet in length?
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  #3  
Unread 10-22-2016, 09:38 AM
N3LYT N3LYT is offline
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I call it a "ground loss eliminator" Each leg is 5' being cheap I only bought 2 sticks of EMT. so 1 1/2 for the legs what was left I used for the mounting legs they are 5 1/2" tall welded (no loss of connection) to the legs with a carriage bolt through the center held to the aluminum plate with wing nuts they are welded also. It is very light and comes apart so it takes up little space. The whip is 6' it was a 8' whip but storage in the camper is only about 7' 6" bottom line the whip could be 12' tall it still would tune fine. It probably would not tune on 6 meters but it's not some thing I use portable. The antenna is stored in a custom frame so the plate and the High Sierra mount don't take up much space. The HS antenna uses a strange mounting so it has to have it's mount and not a 3/8" stud which would have been a good bit easier
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  #4  
Unread 10-22-2016, 02:28 PM
Roadrunner Roadrunner is offline
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Looks great. I have been using a tripod mount for my HS with four #20 radials about 20' long. It works pretty well except for 10 meters where the SWR tends to climb quite a bit. I think I'll give this idea a try.
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  #5  
Unread 10-22-2016, 08:03 PM
N3LYT N3LYT is offline
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Unlike a non tuneable antenna radial length is not real important when it comes to switching sticks because they do not have to be cut for the bands because the screw driver will tune any way. I’m not sure beyond a certain point if length would have much if any effect. I chose 5’ because it’s half a stick of EMT not real scientific but greater than the portable offerings I have seen for over $200 more than I have in it. In effect it’s a 10’ circle is it enough? I don’t know but it has greatly improved the ability to present a match over it being mounted 6’ off the ground with no counter poise.
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  #6  
Unread 10-23-2016, 10:16 PM
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NN5I NN5I is offline
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Probably they will have virtually no effect.

There are two quite different things that people call radials, or radial systems.

One kind is (typically) λ/4 radials, usually in free air. These, because they are open-circuited at the free end and 1/4 of a wavelength long, are in principle short circuits at the attached end, and (by shorting them out) prevent RF currents from flowing down the outside of the feedline toward the transmitter. That is, they're an RF choke, or a kind of balun. The RF short circuit reflects energy flowing down the outside, sending it back up to the antenna so it'll do some good. You see them on what are misnamed ground plane antennas. Your radials, at five feet, would work this way, perhaps, at about 49 MHz (or odd harmonics of 49 MHz) but nowhere else.

The other kind of radials are long wires (typically several wavelengths long) placed on the surface of the ground, or slightly below, or a bit above. These provide, essentially, a reflective surface that is less lossy than the local dirt is. Think of them as shielding the dirt from your signal so it'll be reflected toward the horizon (or the ionosphere) instead of being absorbed to heat the dirt and cook the worms. Most verticals will work much better with nice long radials of this kind -- or a good natural reflecting surface such as salt water. If you're on baked clay, you need radials. If you're over salt water, you don't. But five feet won't help much.

My old friend Whit Griffith, N5SU (SK), designed and built Voice of America and Radio Free Europe HF broadcast stations for a living and was a true expert. He once experimented by building an HF vertical and gradually adding radials, measuring the efficiency after adding each radial (they were many λ long). He calculated the radiation resistance (since he knew the vertical's length and his transmitting frequency), and measured the antenna's impedance at the feedpoint. If the measured impedance (or the resistive part of it) was, say, twice the radiation resistance, then the efficiency would be 50% since the power delivered to the radiation resistance would be equal to the power dissipated by the ground-system resistance that was in series.

Anyway, he'd add radials, measure the efficiency, and repeat. He found that eight or so radials were not too bad, but could be improved quite a bit, but there wasn't much point in putting down more than, say, 120 radials.

It was also clear that radials shorter than several wavelengths were a pure waste of copper.
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  #7  
Unread 10-24-2016, 10:44 AM
N3LYT N3LYT is offline
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Well they work pretty well on the back of a truck and 10' is a good bit wider than a truck. So if the ground loss is doable on a truck than 10 ' should be an improvement. Alpha Delta makes a tripod arrangement that has 1 meter legs so either they are doing a good job pulling the wool of ones eyes or there is some thing to it. If one has a good look at a screwdriver antenna there is no way that should work either but they do. A return path is needed a 10' circle of conductive material has to be better than a 10' circle of dirt.
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  #8  
Unread 10-24-2016, 08:15 PM
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Quote:
Originally Posted by N3LYT View Post
Well they work pretty well on the back of a truck ... A 10' circle of conductive material has to be better than a 10' circle of dirt.
A 10' circle wouldn't do anything. It's everywhere transverse to the radio wave. A 10' crowfoot, which is what you have, could be better than a 10' circle of dirt, but only for some kinds of dirt. But the crowfoot will never, I think, be any worse than the dirt, since the surrounding dirt is still there and perhaps helping. Besides, the crowfoot keeps the antenna from tipping over.

Comparing an antenna (any antenna) with something you can put on the back of a truck is placing an extreme bias on the comparison. Mobile HF antennas never come close to good performance. They can't, possibly, unless the modern understanding of physics is all mirrors and pipe dreams. But actually the physics of antennas is well understood, though perhaps not everywhere. I understand maybe a tenth of it myself.

Your installation may well outperform the same antenna mounted on a vehicle, but that will depend on the nature of the local dirt, not on that nicely executed crowfoot support.

Maybe I'll build one just like yours. It really looks very good.
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Unread 10-25-2016, 08:07 PM
N3LYT N3LYT is offline
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Correct me if I'm wrong but even at 10 meters the wave length looks at that as a circle. Granted a screwdriver at 100 watts is still QRP but I have over 60 countries mobile with my little Tarheel on the back of my truck. My biggest benefit so far with short tests has been broader tuning it is not as narrow. I understand a fixed tuned antenna probably needs resonate radials because they do not have the option to tune them self’s but a screwdriver will match pretty much anything, will it be efficient? Doubtful but the radio is happy so it will provide full power to an inefficient antenna. My main plan was portability and to get the best I could whilst camping and ease of storage and assemble so far what I have seen it's better on the ground than 8' up on a post behind the camper even with a grounded aluminum roof. If this proves effective my next plan is counter poise just below the antenna on the post I have used in the past.
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  #10  
Unread 10-25-2016, 08:26 PM
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It will probably be inefficient. Most antennas, and especially most small antennas, are.

That's not as bad as it seems, though. Suppose an antenna system radiates only 6.3% of the RF energy fed to it, and wastes the other 93.7%. That's a 12dB loss, which is only two S-units. If a perfect antenna would get you an S9 signal report, you'd still be S7. Big deal. And there are no perfect antennas anyway, though N5SU once got over 85% efficiency out of a very big array. That was a VOA station, I think he said, with a gas-pressurized coaxial feedline nearly a foot in diameter, and many radials extending hundreds of feet. He showed photos, and they were very impressive.

That's why big antennas for point-to-point communications (as opposed to broadcast) are usually directional. If you can double the energy going in the direction you want, at the expense of other directions, that's 3dB right there; and many directional arrays can do much better than that. I once had a six-element 20m beam that probably was 15dB better than any vertical. When I switched to a vertical (not a very good one, either), I thought my receiver was dead.
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