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Old 19-12-2008, 20:02   #1
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SSB Antenna Does anyone know why this works

I was helping a couple out with the installation of a new SSB the other day and can not figure out how come it works. But it does and very well. They have a steel boat and have just installed a new Icom 710 with an external tuner. The strange part is their antenna uses a backstay with only one insulator. When I measured the resistance with my digital metre it measured only 0.2 Ohms to ground different than my meter leads shorted together. So I explained to them I didn't think it would work but they were adamant the previous radio did. So being careful and cautious I approached it by first disconnecting the antenna and hooking it to a dummy load. Sorted out the issues of power etc proved the radio was ok. Then connected to their antenna and it worked as good as my dummy load i.e. full 100 watts out very little reflected back and SWRs almost perfect. Their battery system is isolated from the hull but not really as they connected the ground output from the back of the radio to the hull. It works better than my rig and am wondering why. So if anyone can educate me I would appreciate it so I do not say it won't work before trying again.

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Old 19-12-2008, 20:56   #2
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If it is that the backstay is fed from the antenna tuner above an insulator near the back stay's lower end then the grounded mast (and standing rig) is acting as a shunt fed vertical.

This is, for example, a fairly common and effective way on land of feeding grounded antenna towers as vertical antennas.
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Old 19-12-2008, 21:26   #3
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The shunt-fed vertical (antenna grounded at one end, and the feed attached some distance from the grounded end) will work well on some frequencies, but the tuner may not be able do the impedance-match at other frequencies. Which frequencies depends on the dimensions of the shunt. The stand-off distance between the feed-wire and the backstay will also have some effect on the tuning, but the location of the feedpoint is the main factor. Also, some tuners have more matching range than others.

So, I would suggest that the antenna be tested on all bands.
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Old 19-12-2008, 23:33   #4
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Along with the OP, I am waiting to know WHY it works but then again, I wonder why any antenna works.
Yes, I did study the antenna principles many years back and while I knew what they said, I don't think I have ever fully understood the actual transfer of RF electrical power into electromagnetic radiation.

Add to that some strange antenna designs that certainly do work (like short ground shunts or even notch antennas), I just believe as a principle of faith. And yes, I have been an RF tech for a lifetime .
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Old 20-12-2008, 00:13   #5
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As for why it works, and why RF can seem so mysterious, you need to consider the propagation velocity of the electrical fields involved. It takes time for a signal to travel along a wire, and when it hits a discontinuity (such as an open or short circuit), the signal reflects (bounces) back towards the source. This velocity is some fraction of the speed of light, perhaps 75% in a coaxial cable. When the reflected signal meets the transmitted signal it can add or subtract (think sine waves and polarities), depending on the frequency (wavelength) and distance.

This can make a short circuit look like an open circuit, or an open circuit look like a short, or anything in between. This is how the shunt-fed antenna works, and is kind of how antenna tuners work. The shunt-feed converts the short-circuit to the higher impedance that the transmitter is designed to drive.

And while the impedances are being converted, current is flowing through the antenna wire, creating an electromagnetic field that radiates.

(this is simplified, of course)
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Old 20-12-2008, 04:52   #6
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An RF ground is not the same as a DC ground and impedance does not equal resistance.
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Old 20-12-2008, 09:37   #7
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Originally Posted by Tropic Cat View Post
An RF ground is not the same as a DC ground and impedance does not equal resistance.
If you are commenting on my posting, hey, I did say it was simplified!

I was trying to give a gut-level explanation of how shunt-feeding a grounded vertical could provide an impedance match to the transmitter, without resorting to complex impedances, vector math or Smith charts.

Absolutely true about grounds and impedance. A basic familiarity with propagation delay and reflection can help someone understand why this is so.
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Old 20-12-2008, 11:13   #8
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OK, here's another over-simplified metaphor. But it is related to antenna matching and boats!

Consider a swell coming in and hitting a sea wall. It reflects off the seawall and travels back towards the source. As it travels backwards, it meets the next swell coming in. Where the forward swell peak meets the reflected swell peak, the combined swell height can be double. Where the forward peak meets a reflected trough, the swell can be cancelled at that spot.

These constructive and destructive meetings are similar to the not so surprisingly named Standing Wave Ratio (SWR) that we are trying to minimize when we are matching a transmitter to an antenna.

Perhaps you've seen breakwaters or groins that have perpendicular or isolated sections at a harbor entrance? These are used to reflect and partially cancel the energy from incoming waves. A shunt-fed vertical antenna combines the reflections from the grounded and open ends of the antenna in a way that provides a good impedance match for the transmitter.
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Old 21-12-2008, 00:16   #9
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Hey guys thanks for your replies. I have the why it works somewhat in my head now. But I will be really cautious I think trying to figure out how to check this out on any config still in the future. I was worried this time I was going to short out their tuner and hoping the tuner self protection would save me. But like I said I listened to them tell me how it worked really well. And now know that I don't know so much!!!

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Old 21-12-2008, 08:00   #10
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Originally Posted by SariTimur View Post
Hey guys thanks for your replies. I have the why it works somewhat in my head now. But I will be really cautious I think trying to figure out how to check this out on any config still in the future. I was worried this time I was going to short out their tuner and hoping the tuner self protection would save me. But like I said I listened to them tell me how it worked really well. And now know that I don't know so much!!!

Cheers
I guess if you really gotta know the impedence, this is the meter you need.

Sandford Associates – Technical capabilities of the HP 4815A Vector Impedance Meter.

John
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Old 21-12-2008, 09:49   #11
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Thanks Paul, Midland and Wot. I have never heard it described so succinct and clear. What you guys said created a nice mental image of what is happening.
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Old 22-12-2008, 02:47   #12
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Gentlemen, the OP seems satisfied with the replies so may I indulge in a little thread drift .
This may be splitting hairs but I believe most of you have described HOW the antenna works rather than WHY. Or perhaps I have it backwards - you have described WHY and I wanted to know HOW .

Leaving aside impedances, standing wave ratios, vector maths, velocity factors, series/parallel inductance and capacitance etc etc (all of which I understand), what I have never really understood is how/why voltages and currents moving back and forth at RF rates along a particular type conductor (ie antenna) ends up being radiated as EMR. Just what is happening to those electrons (and holes) moving around in the antenna that cause the electrical power to be transferred into EMR.

Perhaps I just can't visualize the fundamental nature of the radio wave whereas I can visualize the nature of voltage and current flow.

Follow up question: is light really photons or waves - just kidding, that is too much drift.
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Old 22-12-2008, 03:59   #13
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...what I have never really understood is how/why voltages and currents moving back and forth at RF rates along a particular type conductor (ie antenna) ends up being radiated as EMR. Just what is happening to those electrons (and holes) moving around in the antenna that cause the electrical power to be transferred into EMR.
Well don't feel bad about that . A few guys like Faraday (for DC) and Maxwell (for everything) have described that it happens and the properties of the electromagnetic fields produced but as far as I know there is still a Nobel prize waiting for the person who explains WHY?
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Old 22-12-2008, 07:03   #14
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Thanks for the encouragement Mid1.
I was sort of hoping that Gord was going to explain it all with one or two succinct lines and an easy to read link.
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