Re: VLF: getting on 8.970kHz

[Ward K7PO <wmwheaton@aim.com>]

All,

I am reading these posts with interest. Within the next 2 weeks, I will be able to transmit on ~8 khz from SW US (Arizona). My antenna is the one in use currently for WH2XXP on 630m and 2200m. It is a 30m vertical with 16 20m top loading wires, and 200 radials from 30 to 90 meters long on ground. The loading coil/variometer is finished. A 'coil house' is being built now that will keep everything weatherproof.

Do I need to be concerned with corona? Initially, power will be ~100W to the antenna, with higher power later if successful. Any other advice is appreciated. My intention is for this station to be permanent and available for schedules.

Thanks,

Ward K7PO/WH2XXP

-----Original Message-----
From: Rik Strobbe <rik.strobbe@kuleuven.be>
To: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Sent: Thu, Sep 7, 2017 2:22 am
Subject: Re: VLF: getting on 8.970kHz

Hi Jack,

I agree with Stefan that a loop is not a very good choise to transmit on LF, for the simple reason that the radiation resistance (Rrad) of a loop drops with the 4th power of the frequency, while this is 'only' 2nd power for a short vertical antenna.

The calculated Rrad for your loop (11m by 27m = 297m^2) is 2.2e-9 Ohm at 9kHz while your MF antenna would be arround 5e-4 Ohm.

Of course losses will be lower for the loop, but this will not compensate for the over 200000 times smaller Rrad (assume that the losses for the L-antenna are 10k Ohm, the losses for the loop would need to be as low as 0.04 Ohm to get 'break even' with the loop).

73, Rik  ON7YD - OR7T

​

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Van: owner-rsgb_lf_group@blacksheep.org <owner-rsgb_lf_group@blacksheep.org> namens DK7FC <selberdenken@posteo.de>
Verzonden: donderdag 7 september 2017 10:40
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: Re: VLF: getting on 8.970kHz

 

Hi Jack,

Ok, so i'm continuing to write on the reflector. It is not at all a waste of time to read the conversation. Most potential newcomers are in a similar situation like you and they may be interested to see that there are more stations considering to get on the air down there, world wide! For example, IK1HSS tried to receive my VLF carrier transmission, but no one knows, except me. Sharing thoughts can never be a waste of time. Don't we all have loads of time available anyway? As long as someone own a TV, there must be more time than needed :-)...

Now, forget the loop for VLF transmissions. Your inv-L will give a very good radiator for VLF! Important parameters are height and capacity, you know. In the first steps you don't need QRO. 100 W from a normal audio PA is sufficient because the voltages will be very high! At 250 W on 8270 Hz i'm getting about 30 kV across the wire. Your wire and the isolators have to withstand that voltage. If the wire is to small, there will be partial discharges. It makes no sense to run 1 kW then. It would rather make sense to use a bigger wire or even more wire (capacity).... The first thing to concentrate on is a stable signal generator using 1 PPS.
Buy that one for example: https://www.amazon.com/XCSOURCE-Locating-Aircraft-Controller-TE624/dp/B01N2GK1YK/ref=sr_1_6?ie=UTF8&qid=1504772304&sr=8-6&keywords=neo-6m

You seem to have a high antenna capacity which is a big advantage. It is very promising that you can radiate a very strong signal. There is no time to loose!
For the coil i suggest to use a single layer coil using 0.4 mm or even 0.6 mm diameter wire on a tube with 0.3 m diameter or higher. It depends on the space you have for placing the coil. It should stay dry all the time!
You need patience to wind it but it is very relaxing. :-)

I recommend to build an analog amperemeter covering 0...1 A antenna current. It is trivial to build it. Just use a bridge rectifier out of fast switching diodes such as UF5404 and connect the meter to the DC side. The AC side is connected between coil and ground.
The coil is for compensating the capacitive component of the wire. The residual resistive component is transformed to the PA output impedance using a big ferrite transformer (toroid) which also provides galvanic decoupling.

Now, just order the GPS module, the wire and search for a suitable coil body :-)

73, Stefan

PS: With that antenna, you can become the strongest amateur VLF station!


Am 06.09.2017 17:12, schrieb JACK ASKEW:

Hi Stefan

Thanks for the reply.  Sorry to hear that my ERP will be so little with this setup.  A few 10dBs is a lot, I had no idea it would be that much.  I just assumed I could make up the difference by running a much larger audio amplifier.  It would no be hard to put 1KW amplifier on with the availability of some cheap 12V car amps now available.  But maybe this might not be enough power.

For MF I'm currently using a inverted L ~73m long, 24m of that has 3 wires as a top hat, but hopefully will be increasing that soon.  The height of the antenna is ~24m at one end and >36m at the farthest end.  The Tx is a HB set up with 4 FETs running class D, capable of 1KW but I have not pushed it that far as yet.   I have been limiting it to ~300W for local contacts between here and stations South of the Island.  I use an Icom R75 receiver for receiving, it seems to work very well on MF. Best heard DX so far on WSPR is ZL2BCG and VK4YB.

I was hoping you would say that QRSS240 would work with this limited budget set up. It doesn't sound like I need to proceed much further unless I can find a much larger audio power amplifier, a lot of patience, a lot of luck and a good GPS receiver.  I did look at a few of the NEO-6M GPS units, but not sure which one on Amazon you were referring to.

I agree maybe I could go on the reflector with our conversations, however since using even moderate size loop on this dreamer's band does not look realistic, perhaps it would be a waste of everyone's time?

I'll play with it for a few more days if things don't improve I'll pack it all up.

Thanks for your help Stefan.

73, Jack - VA7JX

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From: "DK7FC" <selberdenken@posteo.de>
To: "Jack Askew" <va7jx@shaw.ca>
Sent: Tuesday, September 5, 2017 3:16:05 AM
Subject: Re: getting on 8.970kHz

Hi Jack,

Can we do the conversation on the reflector maybe? There is little activity and each conversation may help to rise some inspiration to lead to an increased activity... Your decision, just reply to the reflector in the next answer if you like.

Well, i understood the procedure you're describing and this is of course the way to go for getting a resonance on the desired frequency.
But, the ERP will be so little with this setup, i guess you are a few 10 dB away from leaving a trace on VE7SLs site.

Which antenna are you using for LF and MF? A typical top loaded vertical antenna will give you the best results on VLF as well. What is the capacity of that antenna, in pF?
But i understand, a loop is much easier to match on that frequency, easier than winding a large coil :-) But if you want to cross 186 km, which is a serious distance already, you will need to large coil and several kV across the antenna!
Furthermore you won't manage QRSS240 on that band. QRSS 240000 is more realistic! That means, you need a receiver and transmitter that has the necessary stability. The cheapest and best way to go is using a GPS module like NEO-6M, which is available from Amazon. It provides a PPS pulse which must be fed into the mic input of the soundcard. Together with SpecLab you can generate a perfectly stable tone then, as well as DFCW, QRSS, EbNaut, etc.... The same must be used on the RX site.

It is also helpful to do local tests, maybe with a notebook, in 1...2...3...4...5...6 km distance, just to get an impression...

What do you think?

I would say you can build the system from junk box parts mostly, that the advantace to the activity on the other end of the spectrum :-)

73, Stefan

Am 04.09.2017 22:42, schrieb JACK ASKEW:

Hi Stefan

I'm having problems finding your right email, so if you get this twice I apologize. 

I have been trying to get a signal on 8.970kHz but with little success.  Since I'm on a limited budget, I have decide to try a large coaxial loop instead of a large coil. I chose RG-59 coax (center and shield) since it has a low resistance as appose to copper wire. The bottom of the loop is only ~10m off the ground but the top side is ~21m up, circumference is ~76m. The mid point goes down to a matching box consisting of a pc board with HV caps and a FT-240-78 core with a sec and pri winding. The primary is 11T of of #14 ins wire and a secondary of #18 awg enamel wire with 55T which connects to a chassis mount SO239 for the coax which goes back to the Tx amp ~30m away.  The HV caps totaled 2.86uF and is in series with one side of the coax loop. The loop measured 100uH, so the combined capacitors plus the 100uH loop seemed to be close to the calculated values, but I think it could have been higher. The Tx is a simple 140W audio amp driven by a Windows free downloaded program called Audio Measuring Systems.  I measured 2A of RF using a calibrated RF ammeter. The same meter was used to select the capacitor values for maximum O/P.  I tried using a scopematch designed for .475 MHz but I guess the toroid across the internal coax line is too lossy as it gets warm and besides the current reading was very low compared to the voltage. 

I don't know how far I can Tx yet.  Steve, VE7SL has tried to receive me, he is ~186 km and used a Perseus receiver with ~24Hz BW and QRSS 240 but no luck so far.

We think we might have too low of a EIRP for that distance.

Is there any other way of optimizing the RF O/P besides using a RF ammeter?

Your thoughts on this would be much appreciated.

Thanks in advance.

73,

Jack Askew - VA7JX