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11-19 of 19 messages
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RE: 43' VERTICAL- DON'T SHOOT THE MESSENGER
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by N5LRZ on December 22, 2008
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I quite agree with VK1OD...
To change the basic design of any antenna and then write a review post changes on the the basic antenna is definately NOT the trasmission of facts.
Reviews should and ought to state the facts of the antenna as designed by the manufacturer, constructed per manufacturer instructions, installed per manufacturer instructions and not post adjustment of the user unless otherwise stated quite clearly in the writing giving both per manufacturer compared to post modifications comparisons in a clear fashion.
The obvious extensive alterations made to the original design of the manufacturer makes the new configuration a completely new design and renders null the old manufacturer design.
I agree with OD in his statement.
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RE: 43' VERTICAL- DON'T SHOOT THE MESSENGER
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by W8JI on December 22, 2008
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The bigger question is how anyone could sell hundreds or "thousands" of an item that is a terrible configuration and get a steller review from everyone when the antenna would be a virtual dummy load in many cases on some bands.
It is more of an indictment of eHam reviews than the manufacturer.
What reference antenna did you compare the vertical to?
I've worked VK6 longpath on 40 meters SSB from my mobile several times, and have cracked dozens if not hundreds of pileups while mobile. I suppose I could say my mobile antenna really rocks and is a world killer, but my mobile antenna is on average ten dB down from a G5RV or a dipole on 80 through 20 meters.
I wouldn't trust an eHam review. Where is the beef? What was the reference antenna?
73 Tom
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RE: 43' VERTICAL- DON'T SHOOT THE MESSENGER
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by AC5UP on December 22, 2008
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I raised an eyebrow when I read that switching to the unun improved the signal strength by ~25 dB... 23 dB is a 200:1 differential and to my eyes that's HUGE.
Either the balun had issues or the unun got a better electrical contact to something when it was installed. ;)
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RE: 43' VERTICAL- DON'T SHOOT THE MESSENGER
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by N3OX on December 22, 2008
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"I raised an eyebrow when I read that switching to the unun improved the signal strength by ~25 dB... 23 dB is a 200:1 differential and to my eyes that's HUGE. "
Maybe not so much.
The *outer shield* of the coax forms a transmission line stub with whatever is underneath it, and that transmission line stub shorts out one winding of the balun-wound coil. Let's say that there's a ground wire that runs in parallel with the coax line that bonds the radials and some ground panel on the wall of the shack.
Let's say that VA3GVS had 150 feet total of coax, but exactly 1/2 wavelength of it was between the antenna and a ground point, to allow for a little indoors.
If the "common mode transmission line stub" formed by the shield of the coax and the ground wire was low loss and about 1/2 wavelength long, it would nearly short out a winding of the balun.
If I model a 43 foot unloaded vertical (over 32 or so 32 foot radials over EZNEC Real/Hi Accuracy ground) fed by 150 feet of RG-213 to a perfect 4:1 transformer to the antenna, I get that the antenna's gain is about -1.75dBi (ground losses), and the feedline loss looking into 4-j59 (low side of the transformer) is about 5.9dB. The equivalent gain is -6.7dBi.
But if I put a parallel load across the low side of the transformer of 0.83-j0 (about what I get from estimating a rough impedance of the "common mode stub" using VK1OD's two-wire transmission line calculator with 10mm conductors and a dead short on the far end), the impedance on the low side of the transformer drops to 0.8-j(tiny) ohms, the coax loss goes up a tad to 6.5dB and the antenna gain goes down to -32dBi because of the tiny resistor short circuiting the high impedance antenna. That's -38.5dBi and nearly 32dB worse than if the short weren't there.
Now a real 1/2 wavelength "common mode line" running along the earth is going to be lossy.
Let's terminate the common mode line in 10 ohms instead of a dead short (1e-8 ohms I think I used)
It's an impedance repeater, so you get about 10 ohms on the far end.
If I put 10 ohms across the balun low winding, the coax loss goes down to 1.1dB but the antenna gain still clocks in around -21dB. So you're at something like -22dBi equivalent antenna and that's a little worse than 15dB down from the perfect UNUN feeding the real antenna.
So the loss could be 15dB it could be 30dB... and although the half-wave-stub problem is limited to a single band or its harmonics, I could imagine someone grounding their coax shield to their radial system really close to the antenna, because ... why not? Could have a switch box there, could just be concerned about lightning, etc, etc. Then you have a few tenths of an ohm RF resistance shorting out one side of your transformer.
And besides, even if we took the less horrible condition and took my 10 ohm example, 15dB down from S5 on my radio is S0, so let's not necessarily take "5 S-units" to mean "25 full dB"
;-)
73
Dan
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RE: 43' VERTICAL- DON'T SHOOT THE MESSENGER
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by W8JI on December 22, 2008
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Why would you raise an eyebrow with a 23 dB difference between a fair feed system..... and one that that puts anywhere from most to nearly all of the RF back on the shield of the coax, and uses the shield of the coax as the antenna??
I would expect my signal to be 20 dB down when I load up the shield of my feedline as an antenna, as opposed to having all of the power radiate from something actually suspended in the air.
The question everyone should really be asking is, "How did so many people miss this critically important point?".
The answer is, because when people give reviews they often have no idea at all how what they are using actually works. They never really compare it to anything else, or even understand how it actually works.
73 Tom
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RE: 43' VERTICAL- DON'T SHOOT THE MESSENGER
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by W9OY on December 22, 2008
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I put up a stock HF-2 butternut in 2004 after the hurricanes blew through. I worked 100 countries on 80M in 3 weeks. I put a wire inv-L on 160 hanging out of a tree, (50ft vertical) in Nov of the same year and worked 100 countries by spring. No baluns, no magic. Both antennas were resonant and both antennas were over the same 3000 ft of radials.
The key to getting a 1/4 wave vertical (or its resonant equivalent) to work is
1. You have to get the greatest amount of current flowing up the antenna as you can.
2. You have to get the greatest amount of current from the "ground" back to the feedpoint as you can.
It is what it is. It's a very simple antenna.
73 W9OY
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RE: 43' VERTICAL- DON'T SHOOT THE MESSENGER
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by K4SAV on December 22, 2008
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..."1st MOD= The addition of a 60ft #14 insulated str. wire (running 30 ft from each side of antenna with 2" of insulation removed from center of wire). This wire is then clamped to antenna, 15 ft up from base, and loosly strung in a slight upward vee to any 2 supports, 30 plus ft away, in a straight line to antenna."....
Maybe I'm misinterpreting what you are trying to say, but two wires attached to the antenna at the 15 ft point on the antenna is a lot different from top loading the antenna, and also different from what the previous posts have assumed you did. This might make a small improvement on 160, but nothing like the improvement you can get from attaching a long wire to the top of the antenna.
Jerry, K4SAV
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RE: 43' VERTICAL- DON'T SHOOT THE MESSENGER
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by AD4U on December 22, 2008
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Well, if you are finally happy with this antenna, then I am happy for you. Sooner or later when you put up a really good antenna, I think you will see what you have been missing.
Dick AD4U
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RE: 43' VERTICAL- DON'T SHOOT THE MESSENGER
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by K0OD on December 22, 2008
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VA3GVS says: "what they said wasn't what I wanted to hear after investing the time and money to set up this Antenna and radial system."
Also applies when the real antenna performance differs from the expectation.
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