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1-6 of 6 messages
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SkyWave TDR, possible to test NVIS efficency?
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by KI6JJS on August 4, 2009
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Hello Fellow HAMs
I have in my heap of stuff a 4 trace O-scope that I knew how to operate some 20+ years ago.... and was a network technician back then. We used to 'ping' a cable and look at it on the o-scope, estimate the distance to fault, and go fix it. Time Domain Reflectometry, or how ever it is spelled ;-).
I am playing games with setting up to write a definitive guide on NVIS antenna styles, and how well they work. My theory is to set up a 'reference' receive antenna with the o-scope on it, and vary to other antennas to see which one make the largest number of electron volts come back down. There are some pretty obvious issues here, since I want to get a group together to help, I want to be in the same vicinity to set things up. So A) I have to not blow up the o-scope when 35W key down happens, and B) the results will not be valid day-to-day as the conditions will differ. But that is not the point I am after. I would like to get to a point that has the best possible signal from each of the following type of NVIS setups over poor soil (I live in a desert...), all on 40-80-160:
Native dipoles, test to see if the 10'-15' flat-top is as good as it sounds
Tee-Pee tent style, single feed point. Also see how much better 'droopy' on 160 is, articles and empirical results look like it is great. (also want to adjust feed cable on this, it seems inefficient to feed it all without respecting a feed line length of 1/2 VF compensated lambda...)
'Road-side' flat tops, the articles about placing the antennas on top of a guard rail/fence (maintaining the 10'-15' from the metal) seem interesting.
OCF windom in an extended Z, that is to make 90deg bends at each band edge, thus trying to avoid the overhead null that these antennas create when operating higher than their native frequency.
... and so on.
I am certain that someone out there has a good idea on how to do true measurements like this, and that they would be happy to see a ?QST? article on getting the most out of your Near Vertical Incidence Skywave antenna.
Hope to collect lots of good ideas
73-Per
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RE: SkyWave TDR, possible to test NVIS efficency?
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by WB6BYU on August 4, 2009
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Hi Per,
I have a whole file of NVIS antenna designs, especially
for 160m, if you want some other ideas to try. I've been
developing them to maintain NVIS coverage around Oregon,
especially during the winter months at the bottom of the
sunspot cycle. (And the required angles vary around the
state as well - California has the same issue due to
distances.)
What you are trying to do is very much like ionospheric
sounding, which is how the critical frequency is measured.
A string of RF bursts are sent out on different
frequencies and the results plotted, showing which ones
return to Earth and which don't.
For your specific application, however, the simplest
thing to do might be to have the test antennas set up
at one site and the receiver at a reasonable distance,
like 100 miles away or so, out of ground wave propagation
range, so the only signals shown should be skywave.
At the receive site you could use a single trace scope
on the AGC lead of a receiver with a slow sweep, and
have the transmitter switch back and forth between the
antenna under test and a reference antenna. Actually,
a more sophisticated approach would be to have the
transmitter send a continuous carrier and have the
receiving station switch back and forth between the
two antennas, which can be synchronized with the a
display to read the difference. (Some DF sets use this
principle to display the relative output of two antennas
on a meter.) The circuitry is pretty simple: you have
two antennas coming to the box, an internal switch that
sends the signal from one or the other antenna to the
rig, then a wire from the S meter (or other detector -
that can be a whole different discussion) back to the
original box where it is used to charge one of two
capacitors, depending on which antenna is being received.
The result is two voltages, each a function of the
signal from one of the antennas, that can be compared,
plotted, etc.
Or you could use a more primitive method and connect
a digital voltmeter to the AGC lead and record the
exact reading for each of the two antennas, going back
and forth to average the signals over time. You can
calibrate the S meter voltage using a step attenuator
to quantify the difference in dB.
By always using a standard reference antenna (probably
a dipole at a fixed height over ground) you eliminate
the problems with varying ionospheric response over time.
That should get you started, anyway. Let me know if
I can help.
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RE: SkyWave TDR, possible to test NVIS efficency?
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by KI6JJS on August 4, 2009
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Hi WB6BYU, et al.
>I have a whole file of NVIS antenna designs, especially for 160m, if you want some other ideas to try.
[Per] I do! if this idea has any ground-swell at all, the best possible response would be that others in a ~200 mile radius would want to adhere to deploying one of many antenna types, and we could ave a "NX" event one weekend, like testing for a 12 hr period... How can we get access to these files?
>What you are trying to do is very much like ionospheric sounding
[Per] that is indeed where I got the idea, I have watched the IPS radio sounding system (http://www.ips.gov.au/HF_Systems/4/3) with great interest. Since I live in a small radius of the San Francisco based station, I have a special ability to hear the soundings, when the band is open... thus I got the idea that we could invent a way for any HAM to do this at their leisure, and so create a system that could be replicated 'at will'.
I'll Have to paraphrase the rest of your post, as it is worthy of much introspection, but I would like to get the general concept out, your proposal was to 'flip' the situation, and have the receive end be the one that is experimented with, leaving the transmit end to be a reference source. This idea has much merit, a beacon station using a modified set of frequencies from the PC-ALE software, could provide the needed 'soundings' all the time, as the receiver end could have the larger staff allocation playing with configuration changes.
Then comes the idea that there is a reference antenna at the receive end, as well as the antenna under test. I admit to not understanding what the purpose here is, as my intent was to adjust any particular configuration to maximize its potential value, so plotting each configuration against its self would be my reference, can you help clarify more on this point for me?
For the rest, I will cogitate on and hope that we can continue this interesting discussion.
73-Per, KI6JJS
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RE: SkyWave TDR, possible to test NVIS efficency?
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by WB6BYU on August 5, 2009
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The point of the reference antenna is to level out changes
in propagation due to time, distance, etc. Perhaps the
reference antenna is a dipole up 20 feet. While the
performance of such an antenna will vary from one location
to another due to ground conditions, the RELATIVE performance
of that antenna compared to the antenna under test will
allow us to rate all the various antennas in dB above or
below the same reference, even for data not taken at the
same time. So we should be able to say that one antenna
is XdB better than another (at that particular site) by
how they both compare to the same reference.
The reason for putting the two antennas at the receiver
rather than at the transmitter is because it is easier
to switch between them quickly at low power. The DF
systems that use this approach often switch between
antennas at 100Hz, which may be faster than necessary
for this application but does tend to reduce differences
due to fading. You want to be able to synchronize the
switching with the measurements, and this is more
difficult (though still possible) when the transmitter
is the one being switched.
Also, you don't need a full-time beacon transmitter.
Any station at the proper distance can key up a CW
transmission when someone is ready to do a test. The
test itself should take less than one minute to run.
Regarding the antenna designs: drop me an email and
we can discuss this further.
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RE: SkyWave TDR, possible to test NVIS efficency?
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by K0RGR on September 1, 2009
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I think you might get a better test of NVIS capability by placing the receiver close in, but someplace blocked by natural terrain. QRZ lists your QTH as Oakland - placing the receiver on the other side of Mt. Diablo would be a good test! I can tell you it's not real easy to work into the Diablo Valley from the Bay Area on HF.
Around here, my 'test range' for NVIS includes a couple deep river valleys just a few miles outside of town. Without NVIS, there is no propagation into those valleys.
Another thought is that either end of the test circuit could be remoted via the Internet. If you remote the receiver, you would want to change the antennas at the transmitter site.
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RE: SkyWave TDR, possible to test NVIS efficency?
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by KI6JJS on September 2, 2009
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Hi K0RGR, et. al.
Your points on eliminating ground wave are !Right On!. What has transpired in off-line conversations since the original posting should probably be summarized at some point, but alas, I find that I did not buy the winning lottery ticket, so am spending too much valuable time 'working for the man'.
I do have some interesting lessons that even if incomplete may be of broader use to people. so here I go:
Modifying the basic configuration to have 40-80-160M from the design listed here:
http://www.arrl.org/FandES/ead/materials/Fixed-Site-NVIS.ppt
was not as successful as I had originally hoped. the SWR had erratic, but cyclical behavior. But, I have run across one of the many fine articles produced by antennex: http://www.antennex.com/Sshack/type_s/type_s.html and my next attempt will be done with a single coax-feed, but each band spaced apart to reduce interaction.
To the test set up Dale (WB6BYU) has been EXTREMELY helpful in suggesting ideas. We are now down to the point of being able to think in terms of a unknown signal source being able to be measured at the back of a modern transceiver with a DVM, and an antenna switch to go between the reference antenna (must be installed the same way every time so must be pretty easy) and an antenna under test. This might work well enough by just using the AF out pin on most ACC socket of the modern rig. The use of a switching circuit and a scope to compare would add accuracy, but my fear is that it would eliminate many groups who would otherwise wish to get a 'good enough' reading.
There is much interaction with earth in these antennas, so the setup is unique to a minutia. I have high hopes that one could do some sort of ground analysis, and compensate with mild ease. Say for instance, drop a 30M dipole right on the ground under the intended placement, get a reading of X Kc off from perfect, by using your antenna analyzer and adjusting for min SWR, and make some look up table for how to vary the length of the main wires, and 'voila you *should* have a perfect match in mere minutes. that idea needs work too....
But baby steps, I have to get a reliable source and destination setup that will put most of it's energy up. To your other point, it is quite interesting to see how different distances are affected by antenna design and placement, for example, if one had a 'perfect 90 deg take off angle, the 300 mile rule would shrink significantly. Conversely an 80 deg takeoff angle would reduce near-zone at the enhancement of far-zone communications. (not that this near of a zone could not be hit with a normal 2M HT, but that is out-of-scope for this conversation) So when we get a system that is really reliable, it will be most interesting to add the dimension of lateral distance from reviver. Thus we could not only get to a point where we as a community could test and design systems that were able to be expressed in the same terms to each other, but that we could have a way of making adjustments that were ?reliably reproducible? to get to our most important communication reviver... for me, OES HQ is out side Sacramento, for our partners in Telluride, that is a different story. So they may be able to use the same antenna, but at a different elevation in relation to earth-ground at XXX season. (I cannot imagine how to get it set up in that much snow, but would love to be given the opportunity to try... ;-) )
Enough for now, I am sure we can make this conversation grow as we do.
73
-Per, KI6JJS
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