Hello Roger, Dreamers,
if we define near field as < 2 lambda we are talking about everything below 60 km.With this distance I woul be extremely lucky.
I also do "walkaround tests" near my garden. But I bet only in the flat field one can see what is possible with small antenna an without a big loading coil.
Out from my garden I tried my 10h/20t Marconi as well as a 30m earth base. The Marconi worked better, but not good enough. A TV transformer (must be flyback of course) made it much better.
But all the trees and houses are coupling the radiated signal to earth and also the AATIS-RX is very much influenced by them.
I also assume that the horinontal parts of a wire do the same. So only a vertical should work.
For operation in city environment smaller antennas at the roof top (old CB antennas) will possibly work better than the bigger vertical in my garden between trees and houses at ground level. It might be worth a test.
One of the next tests will be with my gig earth base. I have collected about 800m of surplus wire. Possibly this is another way to even radiate more then near field. But I would be very surprised it this was good for any DX.
I bet Stefans kite antenna will at least be the only way to DX operation. Unfortunately this is just portable and just temporary.
vy73 Horst DO1KHS/DI2AN
Von: Roger Lapthorn <[email protected]>
Gesendet: 24.05.2010 18:36:28
An: [email protected]
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 <[email protected]>
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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