Re: LF: AW: 8.97kHz - Near field and radiated signals?

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Stefan,

On windy days the large field behind my house does have people flying kites in it. Yes, my young grandchildren would love this! 

At the moment I want to avoid using a PC on RX as it only takes a few minutes to leave the house, do the "by ear" walk tests, and return home. When I am reasonably sure the RX is working pretty well then I shall certainly try QRSS or WSPR using a PC to see just what range can be achieved. Double the audible range would be my expectation based on 18dB additional path loss for doubling distance and this sort of RX improvement factor using QRSS.

The idea of putting a 100m earth electrode baseline TX in the field is a good one as increasing the baseline would certainly have a direct impact on range. This will need a second person helping: people walk dogs in the field, so someone would have to keep an eye on the TX set-up for health and safety reasons.

Thanks for your encouragement to keep experimenting.

73s
Roger G3XBM

2010/5/24 Stefan Schäfer <schaefer@hst.tu-darmstadt.de>

OK Roger, very nice!

If you would go to use a PC instead your ears, you could immediately get some "gain" of say 30 dB and thus much more distance. I also like and normally prefer CW instead of ear reception. Normally i say " a PC has not to be a part of a amateur radio station", very conservative, hi. Even my logbock is hand drawn since i do not like having a PC running during the QSOs. But on (LF)/VLF it is the best way to communicate. On 137 kHz it is rather due to the activity that is more in QRSS than in CW but...
So, if you would try SpecLab or Argo on a PC and use a stable VFO i think it is no problem to reach those 3kms in say DFCW-600. Than, it is much more easy to improve your system since you can see how a change of your setup improves the reception. If you can improve some dB, maybe even 10 dB, you could go to DFCW-60 and so on. And you can really try a TV transformer! That would really make sense in some kms at small antennas.

Now, as i saw your QTH i get the idea to ask you if you don't want to buy a kite ;-) It's just funny to have it in the air, even without antennas mounted on it. Also your grad children would enjoy it ;-) And you would be one of the top 3 of your VLF dx list ;-)

I think i will ask Bernd/DF8ZR if he is interested to do some ground electrode tests with me. He is in 16km distance and if we would try DFCW-600, maybe it is possible. Also it would be interesting to see the QRN/QRM daily time dependency... All this is interesting stuff.
Go on with your tests and report us :-)

Best wishes :-)

Stefan/DK7FC

________________________________

Von: owner-rsgb_lf_group@blacksheep.org im Auftrag von Roger Lapthorn
Gesendet: Mo 24.05.2010 18:36

An: rsgb_lf_group@blacksheep.org
Betreff: Re: LF: AW: 8.97kHz - Near field and radiated signals?

Thank you for your fast reply Stefan.

In answer to your questions:


*       Currently, my TX electrode spacing is 10m, which is the best I can manage in my small garden. Both electrodes are as far away from the mains and other utility services as I can manage.
*       I have not tried /P transmit testing yet.

*       My nearest ham who will listen/look is 3kms away, but using beaconing I am able to go out myself and see how far I can reach. This is what I have done so far, going out into the fields behind my house with electrode pairs or a loop receiver. Best (by ear reception) DX is 0.35kms, but the system has much scope for optimisation still.
*       The largest single turn loop I could erect in my garden would be about 50-70 sq m (about 15m top and 4m-5m high). The longest antenna length would also be 15m horizontal with some vertical section(s) too.
*       The best DX targets are quite modest: I would like to exceed 1km initially with the TX power I currently have (around 4W). If I could reach G6ALB in the next village 3kms away I would be delighted. If I got any further I would be both amazed and VERY happy!


73s
Roger G3XBM



2010/5/24 Stefan Schäfer <schaefer@hst.tu-darmstadt.de>


       Hello Roger,

       For your ground electrodes, ground electrodes may be the best "antenna" in some 10...10m. I'm not sure how to calculate field strengths but i just know about the 1/d^3 dependency, so twice the distance means 8 times the power for the same signal reception, in the near field. But it is the question what are the electrodes since there is ever a coupling to the mains earth and all the metallic stuff in regions where people are living so this dependency might not be valid in your tests. As mentioned here, you could benefit from these conductors...

       But if you want to reach more distance with an antenna of the same size (e.g. of your garden)and the same power, you could try such a TV transformer. Take a wire, as long as possible and as high as possible, and apply some kV on it by this transformer. It can even be resonated by adjusting the air gap between the both ferrite parts. That results in a maximum output voltage and thus "ERP". At least this would be a good alternative to your ground electrode tests. You could easily compare the reachable distances. Here i would use a E field antenna for reception in the near field.

       How far is your next receiving amateur radio station away? And have you tried to do such tests outside the village/city, just like Jim does it with his loop antenna? This could allow you to increase the distance of your reception. Here, space is not limited and you could try 2x10m, 2x50m, 2x100m and so on and tell us the difference in the feed point impedance at various frequencies and your ODX :-) Maybe i will also do some ground electrode tests in summer but i am still not sure. The complete receiver of Jim is a really good idea to check the QRM situation at a given QTH. Probably i will also buit his loop design for such tests... Would be interesing what is the maximum distance for a CW reception of my transmitted signal (with the kite antenna and 6mW ERP) ;-)

       I will also do some near field tests in this week, to compare the performance of my 2 grabbers. This will be done here: http://www.google.de/maps?f=q&source=s_q&hl=de&geocode=&q=heiligenberg+heidelberg&sll=47.820666,9.312449&sspn=0.106733,0.219383&g=heiligenberg&ie=UTF8&hq=&hnear=Heiligenberg&ll=49.419391,8.704101&spn=0.001616,0.003428&t=h&z=18 in about 1,8km distance to my grabbers. Power will also be below 100W and the antenna wire will be mounted on a tower in abt 15m height. I will transmit with this TV transformer on a 40m wire.

       Roger, what size is your garden? What is the maximum wire length that you can apply and in which height? What is your goal (best DX, a reals QSO with the next equipped station or just do some funny tests?)

       So, wish you fun and success with your experiments!

       73, Stefan/DK7FC

       PS: What is your locator / QTH?






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