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Registered Member #1329
Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53
Hello Folks, It has been a long time since posting, and my first post on this forum. I have been trying to transmit a signal on 8.97 KHz with no success.
I put together an overview of my setup and wonder if anyone might comment. Not looking for any distance records here just detection of the signal to another hams house. Read below if you can and any comments are welcome Thank you PauLC W1VLF
My 1st goal is to make a VLF 8.97 KC contact, or at least a detection of signal to my friend Jay W1VD who lives only 5 KM distant from me. Jay has a quite effective listening set up and can audibly hear the Russian Alpha signals as well as graph them on Spectran.. etc.
So it is my thinking that his RX is more than sensitive enough to copy my signal, should I actually be able to radiate one. He has looked many times for my signal with no success. Both he and I are using GPS locked HP 3336B level generators as signal references to insure frequency accuracy and stability.
I will describe my transmission set the best I can, then my observations, and then if you please give me your opinion??
The antenna: 50 meters of #12 wire. The wire is terminated near the top of a 40 meter tall tower. There is a nylon insulator 4" long at the top, and the wire is suspended away from the tower after that by rope. The wire runs to a point near the shack 2 meters about the ground. The wire has THHN insulation and runs through a small PVC tube into the shack. I assume about 250 pf of capacitance??? The antenna forms a 45 degree angle with the earth.
The inductor: The inductor is a series of 4 coils. Each coil is comprised of 3 round ferrite rods in series. for a total length of 24" of ferrite X .625' in diameter. The 3 cores are inside of a small diameter PVC pipe of 17" long. About 3.5" of core material sticks out of each end coil. The 17" of PVC pipe is tight wound with a single layer of #24 AWG wire.
This yields 110 mHy and a DC resistance 4.5 ohms each segment of the inductor. The entire set of 4 coils is 450 mHy and DC resistance of 18 Ohms. These values are measured.
Not much is known about the cores, except the above the dimensions and they are new and were used in welders as an "RF Impeder" A noise filter for arc welders?? Each core has grooves on the length as if to provide cooling? No "Q" is known for the inductors.
The matching transformer: 2 Ferrite cores stacked each 2.40" in diameter and AMidon #77 Material. The primary that goes to the RF source, has 10 turns inductance of .370 mhy The secondary has 60 turns for an inductance of 14.5 mHy and a turns ratio of 6 to 1
Amplifier: Mcmarten audio amplifier with about 10 watts output 8 ohms output impedance.
RF generator: HP3336B GPS locked Level Generator.
Observations: When monitoring the RF Voltage using an oscilloscope from the antenna lead to ground the voltage rises sharply at 9 KC and is approx. the 1.5 KV with about 5watts of drive power. I have a 100 to 1 current transformer that is used to monitor for antenna current. It indicts 20 milliamps of antenna current. The voltage across the antenna displays a very clean sine wave with approx 100usec for 1 cycle so it is actually a 9Kc signal I use an SLM with a cable running to a short wire in the back woods as a signal monitor.
All three indicators peak signal coherently at resonance of antenna.
A small arc can be drawn off the antenna terminal when a metal screwdriver is touched to it and pulled away. A sustained arc will just cross a gap of .050 this is approx. Also a purple arc can be drawn off the ferrite rods themselves. No wires are touching the cores but still a arc can be drawn. I am not sure what to make of this?? Are you?
450 mHy and 250 pF of capacitance indicate a resonant freq of the antenna and inductor of 15 KHz this I also do not understand.
Is my antenna much higher in capacitance than calculated? Because of the 45 degree angle to the earth.
With all these indications of high voltage at the antenna there is still no radiation. At least not enough to be discernable at 5KM in QRSS 30.
Do you have any ideas why radiation is so poor or non existant?
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Years ago I repaired an instrument used by surveyors which was a loop tuned to a VLF submarine station. The instrument measured the inclination of the plane of the coil for maximum signal where it was being used becuase certain ore bodies caused anomalities in that angle. The surveyor was in British Columbia and the station was very far away.
You've got such a small power, why not try a dedicated resonant loop as big as you can make it in various orientations on or under the earth's surface. Make a 9kHz gaussmeter, to probe fields away from loop.
Watt, A.D, VLF Radio Engineering, Permagon Press, Oxford 1967.
An 'Earth Telegraph' might throw some light of the ground you have.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I understood that amateur communications below 10kHz were performed by earth currents, rather than direct radiation.
The properties of the earth between the sender and receiver no doubt has a great deal of influence on the distances that can be achieved, but distances of 10km or so have been reported.
The radiation resistance of your antenna must be ultra low at 9kHz.
Russian military VLF antennae are tens of kilometres long, and suspended from giant pylons. I believe that the RF is produced directly by huge electro-mechanical generators in the megawatt range.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, your antenna is totally wrong for the frequency. It needs to be about another 200 miles longer.
Seriously, you need to get as much wire as high in the air as you can. Adding a huge capacitance hat to the top of your current tower might help.
Another kilowatt or ten of power would probably help too. Any practical sized antenna is going to be hopelessly inefficient at 9kc, so you need to throw as much power into it as your budget and conscience permits.
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
Very very few amateur VLF/ELF stations use wire antennas. They're almost all massive inductive creatures. The difference in aperture size between a bloody huge coil of wire and 1,000' of wire in the trees when you're talking about wavelengths exceeding 5KM is really pretty negligible. "Ferrite Stick" antennas are quite common on 137 and 186KHz.
Chris Russell and I kicked this very idea around a number of times, but decided the power requirement would be so huge, and the bit rate so low, that it would result in an insane electric bill. 10kW for 24 hours straight would certainly get the meter spinning.
I currently have a wire antenna resonant around 400KHz (500' Inverted L up 80'). I'm considering trying to get on one of the 500KHz experimental licenses this year. That would be a good step towards getting down into the really low frequencies.
Paul, I'm definitely looking forward to seeing the results!
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
At 9kHz you need a magnetic loop, not an electric dipole.
I've often mused on the idea of using inexpensive 3 core mains cable as the basis for a large area multi-turn loop. Buy 100m of it, sling one end over the roof, have the two ends come down on opposite sides of the house, come in through doors or windows and meet together in the basement. Connect the ends electrically to make a 3 turn loop, and add shunt capacitance to resonate at 9kHz.
If you tried this scheme, I suspect that all the ferrous white-goods like fridges and ovens in the house might act like lossy shorted turns and lower your Q, but it's compact in that it doesn't need any more space than you already occupy. In addition, your co-habitees might weeble about exposure to radiation. Just make sure your Gran doesn't switch her hearing aid to "telephone" setting!
A higher Q use of the principle would be to hoist the top edge of the coil into the trees. Mains cable insulation is good for a couple of kV between cores, as it has to withstand mains-borne transients.
Question - a horizontal coil could be easier to make much bigger than a vertical coil - how much does the shorting turn of the earth affect the antenna? And presumably the polarisation would not match that of the receiver?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Dr. Slack wrote ...
- how much does the shorting turn of the earth affect the antenna? And presumably the polarisation would not match that of the receiver?
There is an interesting article, HOW DOES AN EARTH DIPOLE WORK? which considers the question of induced polarization. You can find it, together with details of many VLF and ULF experiments, on the excellent Italian VLF site here:
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
Dr. Slack wrote ...
At 9kHz you need a magnetic loop, not an electric dipole.
I've often mused on the idea of using inexpensive 3 core mains cable as the basis for a large area multi-turn loop. Buy 100m of it, sling one end over the roof, have the two ends come down on opposite sides of the house, come in through doors or windows and meet together in the basement. Connect the ends electrically to make a 3 turn loop, and add shunt capacitance to resonate at 9kHz.
If you tried this scheme, I suspect that all the ferrous white-goods like fridges and ovens in the house might act like lossy shorted turns and lower your Q, but it's compact in that it doesn't need any more space than you already occupy. In addition, your co-habitees might weeble about exposure to radiation. Just make sure your Gran doesn't switch her hearing aid to "telephone" setting!
A higher Q use of the principle would be to hoist the top edge of the coil into the trees. Mains cable insulation is good for a couple of kV between cores, as it has to withstand mains-borne transients.
Question - a horizontal coil could be easier to make much bigger than a vertical coil - how much does the shorting turn of the earth affect the antenna? And presumably the polarisation would not match that of the receiver?
I've kicked around similar ideas, but unfortunately, here's what I came up with.
For a loop, radiation resistance can be calculated as: Rrad=(320)(pi^4)(number of turns)(area of the loop^2)/(wavelength^4). Imagining I could get a three turn loop that's roughly a square, 15 meters by 30 meters, and transmit at exactly 9kHz, the radiation resistance is about 15 nanoohms. If that run is made with a total of 270 meters of 12 AWG house wire, the total real resistance is about 1.44 ohms (ignoring skin effect for now). So, the overall radiating efficiency is about .000001%, or 10kW in, 1mW radiated. You might lose another 50% or so due to ground proximity losses but I suspect another 3dB at this point is not worth worrying about.
Omega stations, before they went offline, operated at 10-14kHz and could be heard over good distances with 10kW ERP. 1mW is 70dB weaker than that, but one can make up around 30dB by switching to a very slow data mode, like QRSS60. So, you could expect to be heard to about the range where Omega stations were very strong, about 40dB above the noise floor. I'd suspect that's enough for a few hundred km or so, but probably far short of a transatlantic crossing. Sort of a letdown for something that's going to consume a month's worth of electricity in a day.
Registered Member #1329
Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53
Wow .. Thanks a lot for all the interesting and creative answers to my questions. Yes I do realize that my antenna is infantesimally small comparatively speaking. Also significantly more power would be needed to make any kind of contact. There is a fellow who has, along with the help of the VLF community in Europe, has been able to have his signal detected at over 500 KM. This of course is much higher power and a longer antenna. See the attachment.
I am just interested in a local 5 KM contact at this time so I thought significantly less ERP would be needed
I have worked up the numbers for a 500 Mhy coil that is 1.200 meters long and .600 meters in Dia. with a "Q" in excess of 250. It will take 22 lbs of #21 wire. Before I do that and abandon the ferrite core materials I was wondering f there is a problem to use these ferrite core and why were they arcing.
Ground dipole may be another way to go.. intersting suggestion.
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Trying to transmit at 9 Khz
