You say you have a 'happy meter'. You need to rephrase this to 'my meter is totally stoned'. If not, i'd like 20 meters of your RG8U over unity coax please.My thru-put notes for my feed lines shows: Back in the days I tested them every time I'd take them down.
Clear Jacket RG8x 106" feet = 100 watts IN 31 watts OUT due to water damage, but after the water dried out the line seemed to work fine, low noise, low SWR, very broad bandwidth. It just didn't make a big signal.
As Henry would say...hehe
RG8U Foam - 3 lines from 50' to 75' feet = 100 watts IN and 100 watts OUT
Belden Mini8 RG8x line 160' feet = 100 watts IN and 84 watts OUT
Davis Bury Flex 213 line 120' feet = 100 watts IN and 92 watts OUT
TW LMR 400 91' feet = 100 watts IN and 100 watts OUT
TW LMR 400 50' feet = 100 watts IN and 100 watts OUT
I have a dummy load with a happy meter, but it is close when compared to and inline meter with a jumper.
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ASTROPLANE best vertical antenna ever?
- Thread starter nosepc
- Start date
Eddie,
It does not matter how happy / accurate a watt meter is it should still show attenuation through long lengths of coax,
When i first swapped from the rg58 i had been using for a while to rg213 i measured the loss on each with a double male connector between meter & radio then replace the double male with the coax,
rg58 160-170ft, 100w in = 42w at the antenna,
rg213 about 210ft, 100w in = 62w at the antenna,
both were inline with the published loss/foot which surprised me as the rg58 was old cb shop crap,
the rg213 came from the Royal Navy, its nothing like the cb shop fake rg213 sold in europe.
It does not matter how happy / accurate a watt meter is it should still show attenuation through long lengths of coax,
When i first swapped from the rg58 i had been using for a while to rg213 i measured the loss on each with a double male connector between meter & radio then replace the double male with the coax,
rg58 160-170ft, 100w in = 42w at the antenna,
rg213 about 210ft, 100w in = 62w at the antenna,
both were inline with the published loss/foot which surprised me as the rg58 was old cb shop crap,
the rg213 came from the Royal Navy, its nothing like the cb shop fake rg213 sold in europe.
Eddie, there is losses in any series oscillator because of the nature of the components we use. There are losses in the electronics of the front end and the finals, the coax, the antenna and back to ground. If you measured the losses from the oscillator all the way round and back again it is quite significant. 100% lossless coax would imply one of two things. First it would imply zero impedance and zero impedance always means open circuit and since our circuit is a series circuit then it can't be open circuit, secondly it would imply infinite impedance which is largely misunderstood. Infinite impedance is not always regarded as an infinitely high impedance but an immeasurable one which can also mean infinitely close to an open circuit. Because coax is 50 ohms matched to the oscillator and antenna impedance then by definition it cannot be infinite because '50' is a finite figure which is measured. So we are left with a finite number which represents resistance at a specific frequency. Coax though, even though it's ohm rating is a given value, that value has very little to do with it's efficiency, it's efficiency is how well it copes with transmission line currents, it's resistance to factors in the environment which can effect it's performance and it's velocity factor. Cheap RG58 and cheap RG213 might well read 50 ohms but the construction of the coax is poor so it just becomes a 50 ohm dummy load in effect with little current getting to the antenna. No coax will ever become 100% efficient at room temperature, one reason is because the velocity factor will never be 100% and it can never be 100% because of the amount of resistance to the flow of current in the coax and secondly because it has a finite ohmic value. Ohms mean loss which ever way you look at it and infinite ohms is just an imaginary figure that scientists play with that doesn't really exist.
Here is my testing using a new 50' foot Times Microwave LMR 400 coax, never been in service. I will do the same with my 91' LMR coax later and post the results. Unfortunately, that little bit of work zapped all the energy I had for today.
Bob, the KW 570D actually shows 116 watts OUT with a dummy load connected directly to the radio. So when the radio power is set at 100 that is noted in the image that is 100% not 100 watts. Kenwood claims the radio makes 116 watts OUT. So to be precise the Daiwa meter shows 116 watts OUT with 16 watts of loss at the antenna end.
So you're right there is loss in my numbers and I misstated the facts.
Bob, the KW 570D actually shows 116 watts OUT with a dummy load connected directly to the radio. So when the radio power is set at 100 that is noted in the image that is 100% not 100 watts. Kenwood claims the radio makes 116 watts OUT. So to be precise the Daiwa meter shows 116 watts OUT with 16 watts of loss at the antenna end.
So you're right there is loss in my numbers and I misstated the facts.
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Now, getting back to the argument what the AP is. I agree that it is some form of dipole but what kind?
Now, we know that a bottom fed loop antenna presents an high impedance at the given frequency at the top of the loop where the current is minimal effectively cutting the antenna in two at that point electrically. This produces two halves of a dipole that has current maxima in the middle at the feed.
Looking at the AP we see the feed point center wire go downwards from the SO239 into the short leg, across the basket at the bottom and up the large leg to the top hat. Meanwhile the outer shield of the coax connects to the bracket and the mast but also connects to the large radiator electrically.
Bob85 tells me that at the given frequency this dead short between the ground and the large vertical radiator is actually not a dead short in reality but an high impedance node similar to the high impedance node at the top of a loop antenna.
So first things first I would regard it as a loop antenna in practical terms but there is a slight problem: According to Avanti and indeed Bob85 the current maxima is from the middle of the large radiator and how can you have a high impedance low current node where the RF current maxima is located? How does it isolate the ground from the radiator with high impedance when in an high current zone?
Here is what I think: I think the high impedance node which separates this loop into two halves of a dipole is not located between the bracket and the large vertical radiator in the current maxima zone, I think the node is where the bottom of the large radiator meets the basket.
I think the ground side (braid) of the coax is radiating on the large element with the top hat and it's electrical length finishes at the bottom near the basket where current is minimum and an high impedance node appears the exact same as a loop antenna. The center feed of the antenna radiates on the short leg its connected to and across the bottom of the basket where it meets the high impedance node.
So the AP is a half wave dipole with the braid radiating on the large element and the center of the coax radiating on the short leg and the basket with the high impedance node separating the two electrically where the current is minima.
This is the only way I see it working.
Now, we know that a bottom fed loop antenna presents an high impedance at the given frequency at the top of the loop where the current is minimal effectively cutting the antenna in two at that point electrically. This produces two halves of a dipole that has current maxima in the middle at the feed.
Looking at the AP we see the feed point center wire go downwards from the SO239 into the short leg, across the basket at the bottom and up the large leg to the top hat. Meanwhile the outer shield of the coax connects to the bracket and the mast but also connects to the large radiator electrically.
Bob85 tells me that at the given frequency this dead short between the ground and the large vertical radiator is actually not a dead short in reality but an high impedance node similar to the high impedance node at the top of a loop antenna.
So first things first I would regard it as a loop antenna in practical terms but there is a slight problem: According to Avanti and indeed Bob85 the current maxima is from the middle of the large radiator and how can you have a high impedance low current node where the RF current maxima is located? How does it isolate the ground from the radiator with high impedance when in an high current zone?
Here is what I think: I think the high impedance node which separates this loop into two halves of a dipole is not located between the bracket and the large vertical radiator in the current maxima zone, I think the node is where the bottom of the large radiator meets the basket.
I think the ground side (braid) of the coax is radiating on the large element with the top hat and it's electrical length finishes at the bottom near the basket where current is minimum and an high impedance node appears the exact same as a loop antenna. The center feed of the antenna radiates on the short leg its connected to and across the bottom of the basket where it meets the high impedance node.
So the AP is a half wave dipole with the braid radiating on the large element and the center of the coax radiating on the short leg and the basket with the high impedance node separating the two electrically where the current is minima.
This is the only way I see it working.
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Its not a dead short when looking into the 1/4wave transmission-line formed by the mast and the radiator side leg of the basket in transmission-line mode, a dead short at the end of a 1/4wave line looks like an open circuit,
current maxima is up around bracket where the coax connects like any 1/2wave dipole ,
current maxima is where most radiation comes from,
The avanti advertising lies about where ground plane antennas radiate most from, they show what looks like a contemporary 5/8 ground plane, the longest antenna legal to install above your property back then,
they all radiate most @ current maxima 1/4wave down from the tip if they have unloaded radiators,
the advertising claim that ground-planes radiate most down near the radials is only true for 1/4waves & starduster type antennas,
while the astroplane does have an advantage in height of current maxima over other antennas when mounted at the maximum legal height, the illustration of beaming over obstructions is grossly exagerated,
current maxima is up around bracket where the coax connects like any 1/2wave dipole ,
current maxima is where most radiation comes from,
The avanti advertising lies about where ground plane antennas radiate most from, they show what looks like a contemporary 5/8 ground plane, the longest antenna legal to install above your property back then,
they all radiate most @ current maxima 1/4wave down from the tip if they have unloaded radiators,
the advertising claim that ground-planes radiate most down near the radials is only true for 1/4waves & starduster type antennas,
while the astroplane does have an advantage in height of current maxima over other antennas when mounted at the maximum legal height, the illustration of beaming over obstructions is grossly exagerated,
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Eddie you can check your measurements against the published attenuation/foot
http://www.timesmicrowave.com/calcu...uency=27.385&runLength=50&mode=calculate#form
http://www.timesmicrowave.com/calcu...uency=27.385&runLength=50&mode=calculate#form
Impedance, in and of itself, does not require loss to function. Mathematically, you can have whatever impedance you want with no losses. The loss that we see in the "real world" is simply because we don't have the ability to create a perfect reactance (be in inductive or capacitive). In other words, the losses we see are not because of the impedance itself, but because of the materials we are limited to using. If we ever achieve the perfect conductor, and the lossless dielectric material, then we may well see actual lossless inductors and capacitors, and by natural extension lossless impedance may become a reality.
If layed out straight, you would have an electrical 1/4 wavelength on one side and an electrical 3/4 wavelength on the other side. The description for why I say this is a bit complex and the key to it you are asking about later in your post, so I'll cover that later.
Actually, in a loop antenna, the point furthest away from the feed point is always a high current (and by extension low impedance) point. As you pointed out above (and quoted below), however, this antenna doesn't actually have an electrical loop in its design. Sure a DC multi-meter will show a short, however, RF is not DC. Again we will get to this below.
The easier to understand explanation to your question is their isn't enough voltage at either side of the wire to drive current onto it. This isn't the only antenna this happens with either, a lot of yagi antennas have connections that work exactly like this, where the elements attach to the boom. Current doesn't flow from the elements to the boom for the same reason.
I don't know that I would necessarily agree with Bob85's terminology here, I wouldn't necessarily call it a high impedance node. I guess it is as good a name as any as the antenna is effectively treating this section like it has a high impedance. Its not, however, because their is a high voltage on this wire, but because of the wire's position, that the antenna is mostly ignoring it. I may have used his term to describe it in the past myself, but after doing some thinking on it, I wouldn't say that what is happening is because of impedance, or that that wire has a high voltage. But yes, according to my model, this part of the antenna definitely exists, and I have discussed it at length on this forum in the past, and I also posted some current distribution images of the this antenna showing just this. We actually called the wire with low current on the "blue wire" for reasons I'll show you below.
I thought it was somewhere in this thread, but scanning through the pages, I am not seeing it. It must have been in another thread on the topic...
Here is what we were talking about, right next to the feed point, look for the wire that is in blue.
I have an image somewhere with a closer up view but I can't seem to find it at the moment... Anyway.
That small blue wire that is next to the feed point, because it has only a small amount of current flowing on it, I removed it to see what would happen to the antenna. Here is an image of the same antenna with that wire removed from the model.
As we can see, this antenna doesn't need that "blue wire" or part of the bracket to be electrically connected to work. With the models above, gain was about the same (actually slightly higher) after removing said wire. The antenna also tuned closer to an SWR match at resonance. This antenna definitely does not need that connection to function, and can actually function better without it. This should also answer any questions about the loop that you see in this antenna, electrically speaking at the antennas designed RF frequency, their isn't one.
However, that isn't to say that that "blue wire" doesn't benefit the antenna in any way either. On further investigation, that higher gain and lower SWR only occurred with a very limited number of isolated mast lengths. What ended up happening is that, with the "blue wire" present and mast lengths that were not optimal length, or perhaps we should call them unlucky lengths, some current would flow through this wire (instead of continuing up towards the capacity hat) and compensate (to some extent) for the different mast lengths, keeping the impedance that the feed point sees, as well as gain, stable and consistent over a very wide range of different mast lengths.
The DB
DB why do you think removing the blue wire causes current distribution to change so much ?
im thinking the current changes like that because the blue wire looks like open circuit through a 1/4wave transmission-line,
if Im right you should be able to tweak frequency to minimize blue wire current,
if that works id also expect removing the wire to cause maximum current change,
i can't think of any other mechanism where removing a short conductor that normally has very little current flowing on it could cause such a change in current distribution.
im thinking the current changes like that because the blue wire looks like open circuit through a 1/4wave transmission-line,
if Im right you should be able to tweak frequency to minimize blue wire current,
if that works id also expect removing the wire to cause maximum current change,
i can't think of any other mechanism where removing a short conductor that normally has very little current flowing on it could cause such a change in current distribution.
Thanks for the link Bob, but my intentions for this coax test was to do a simple RF check on the feed lines using a dummy load. This was to see if the line suffered any physical damage due to wind and weather while in service. I was not looking for technical information.
I recently took my Gain Master down that was destroyed by lightening some time back. The following link goes to my post about the event. Sirio Gain Master and lightning don't mix well.
My 91' foot LMR 400 was attached when the lightening struck, so for a long time I have been curious if the line was damaged.
I did a continuity check on the line and it showed OK, but I have not checked the line under power. Maybe tomorrow I will feel OK and check this longer run for the record.
Here is an image of my model with no Blue line. I agree this wire is not needed for the model, but I did have to retune the match on removing the Blue line in my model below.
Eznec explains this issue well in their Manual.
This is not a quote, but the manual basically says: "...any wire with very little current flowing may be removed from the model with minimal consequences." I take this as pretty standard fair and I don't know if it is a big deal or not. One thing is sure the radiating element would have to be secured some other way.
Sorry to get off track here, but I was reminded of the following issues about Avanti engineers not understanding what they were doing. Some of the talk seems to me to suggest...they didn't know what they were doing either.
The Avanti Patent #3587109 also talks about the loop working whether the loop is closed or open. So when you hear talk from some mentors here on the forum saying that these guys at Avanti back in <1960's did not understand the AstroPlane, IMO were talking CBBS.
The fact is, the folks commenting erroneously, saying that Avanti engineers did not understand the A/P, were reading an article about the Avanti Astro Beam. When in fact the reference in that article basically said these engineers were puzzled by how well the AstoPlane, being used as a driven element, seemed to make a better coupling for their 3 element beam...thus it showed more gain and rejection than the competition. IMO, this admittedly sounds like product puffing and the boss promoting a new found product also making a vailed claim of a new break-through with antenna technology...here to fore not understood.
Just to hear the other side of the story. Sorry!
I agree with DB, but I'm more amazed that this antenna can be fed at just about any point around the mounting bracket. I can match its feed point at the base to the shortened radiator with a top hat or no top hat. I can match this model using DB's scheme using a short wire with a single segment at the opposite end of the mounting bracket from the radiator. It is also possible to feed this model at the top of radial #2 in my image above. IMO this suggest the center of a CFHW dipole and that is about all this antenna really is.
Until I finish modeling this new slant on the A/P I tend agree with DB here.
I also agree with DB here...the previous information posted earlier does not seem to support a continuous loop idea based on DB's points above.
I have an idea how this A/P works too, but I'll wait on that for now. That said, I have changed my mind on this particular antenna several times...so there is no foul with having an opinion.
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All you should have to do to re-tune is adjust the length of the mast. As I stated above, without that wire present, the antenna becomes much more sensitive to changes in the mast length. Its much easier to get a, as I mentioned above, unlucky mast length.
This makes sense, if their is no current flowing on a wire, weather it is there or not will make little difference. Their are, of course, exceptions to this as well.
I've seen discussion to this extent, I don't know that I necessarily agree with it. When people start talking like this I generally just ignore them. At the very least someone had to understand this antenna at least to some extent as their are enough little things that are just so that I find it highly unlikely that dumb luck played a part in its design, although I suppose it is possible. This isn't the only antenna I have heard such stories about, and with a close examination I don't seem to have any trouble modeling and understanding. The hentenna and descriptions on sites like hamuniverse and a few others come to mind...
When it comes to the Astrobeam, we both got better results for the astroplane over a dipole in freespace. As yagi style antennas are driven with a dipole (or sometimes some variant thereof), if you replace that dipole with with another antenna that has more gain, it doesn't surprise me that the resulting beam would have a little more gain as well. It also wouldn't surprise me if that extra gain, in a yagi layout, would be better focused by the director and reflector elements giving the effect of amplifying said difference to some extent. That being said, I haven't modeled the Astrobeam antenna, so I haven't really played with it either. The thing that would concern me would be the width of the Astroplane antenna overall as a dirven element, I played with using a starduster as a driven element for a yagi antenna once, and it didn't work very well, and I came to the conclusion that the angled radials were the main cause of loss in gain as well as other weaknesses I saw. Really, any claim I would make with the Astrobeam as a beam antenna at this point is based on nothing but speculation so I'll stop here.
I've seen your work on this, and while I'm not really interested in moving the feed point my self it is interesting that you can get it to tune as easily as you seem to be able to. That being said, with the feed point at the number 2 location in your file (I think it was, the one with the feed point at the base of the vertical element that ends in a capacity hat) I think this would be a very different antenna, even if it does perform near the same.
When it comes to this antenna being a center fed half wavelength antenna, I disagree. I would say its functionality is electrically closer to a so call "open stub" j-pole antenna. I know it is hard to see just by looking at the antenna, but that is the closest I have come to another antenna when looking at what is happening electrically.
The DB
I have tuned the mast inside the radials to 187" x 1" showing me the best gain at a low angle. I did the same thing a long time ago when Bob started talking about how this part of the mast system on the A/P effected the tune and performance, but without your tricks...that alone was of little value concerning the match. Without these tricks I see the R value being a bit high even when I could fix the value of X.
I will work on this length idea you speak of again using the model you call the Blue Line. That will be another difference no doubt. How sure are you that these tricks in matching are working as intended for these modeling programs?
This almost seems too easy with antennas that don't have a terrible match to start with.
I might try it on my full 1/4 wave model as well...I'm really not sure about that idea I posted earlier.
Have you checked out making the space for the middle of the 2 radials closer or farther from the center of the mast?
Does 4Nec2 allow for tapper at wire connections? I use to see Henry using tapper with his Eznec Pro+ models, but of late he has stopped showing as much of his modeling work.
I see the sign of a J-Pole in this antenna...except I question how different it would be if the bottom hoop was made into a straight wire...like we see on the J-Pole. I believe that difference needs to be checked out and discussed.
Thanks for sharing.