|Subject:||Re: VLF: getting on 8.970kHz|
|From:||Ward K7PO <[email protected]>|
|Date:||Thu, 7 Sep 2017 13:43:26 -0400|
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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.
From: Rik Strobbe <[email protected]>
To: rsgb_lf_group <[email protected]>
Sent: Thu, Sep 7, 2017 2:22 am
Subject: Re: VLF: getting on 8.970kHz
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
Van: owner-[email protected] <owner-[email protected]> namens DK7FC <[email protected]>
Verzonden: donderdag 7 september 2017 10:40
Aan: [email protected]
Onderwerp: Re: VLF: getting on 8.970kHz
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 :-)
PS: With that antenna, you can become the strongest amateur VLF station!
Am 06.09.2017 17:12, schrieb JACK ASKEW:
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