Yes, the loop will null the signal when turned 90 degrees but the signal was strong enough yesterday to be copied, just, when the loop was in its worst orientation some 300m away from the TX. In my view this supports the theory that the grounded electrodes act as a loop in the ground.
Things I need to try are:
- Improving the loop receiver to reject 50Hz interference better and provide narrower selectivity.
- Elevating the connecting wires to the TX electrodes to see if this increases the loop area making the signal stronger in the distant near field.
- Replacing the TX electrode pair with a single turn loop with as much area as possible and check if signal levels increase. I wonder what size loop gives the same signal level at maximum range?
- Seeing if the signal can be received as strong by grounded electrodes at 0.4km, or greater, range.
- TXing /P with a longer baseline electrode pair (100m or more) in the nearby field.
- Trying for reception with a voltage probe antenna.
- Increasing range using an up-converter or direct conversion RX with better 50Hz rejection and selectivity.
- Plotting the signal strengths in a full 360 degree sweep around the TX to get some better idea of the signal levels in other directions including in the line of the electrode pair as well as orthogonally.
-
Increasing the TX power by at least 10dB.
- Trying a vertical antenna and feeding it via a TV transformer.
- Seeing the impact on range of moving to 8.97kHz.
- Reception with PC software, TXing with QRSS.
- etc etc....
So, for something so basic there is a LOT still to explore!
From Friday I have our grandson staying for over a week, so experiments will have to stop during this period.
73s
Roger G3XBM
On 25 May 2010 08:00, Rik Strobbe <[email protected]> wrote:
Hello Roger,
as you mention the setup is far from perfect, so it is quite facinating that you can hear it with very simple means up to 400m.
I guess that by increasing power, optimize antenna matching, moving to 9kHz and improving the RX you can improve the signal by many 10's dB.
I am still wondering how an earth current antenna really works. I have read an article that claimed that such an antenna was an underground loop. In that case changing the height of the wire connecting the ground stakes (eg.
from 20cm to 2m) should have very little effect on the signal strength.
Did you try to "null out" the signal with the RX loop ?
73, Rik ON7YD
Verzonden: maandag 24 mei 2010 22:53
Aan: [email protected]
Onderwerp: Re: LF: AW: 8.97kHz - Near field and radiated signals?
Yes Rik, my current earth-mode tests are around 1kHz so the signal is audible in my crystal earpiece. My TX consists of a TDA2003 fed by a 1kHz oscillator keyed every second, so I hear a distinctive "beep, beep, beep..." at the RX end.
I measured the DC resistance between the (original) TX electrodes as around 40-60 ohms and I use a toroidal step-up transformer to achieve rough matching. At the RX end the loop is untuned. When using an up-converter or direct conversion receiver (such as M0BMUs)
I will try at 8.97kHz as well. I still have a LOT to do to optimise the receive system before I try QRSS or WSPR to extend range.
It's all basic, low tech, stuff but it is such fun, HI.
73s
Roger G3XBM
On 24 May 2010 21:37, Rik Strobbe <[email protected]> wrote:
Hello Roger,
interesting.
Did I read it correct that your transmit frequency is 1kHz instead of 9kHz ?
I gues that also ground antennas become more efficient at higher frequencies.
How did you match the antenna ?
73, Rik ON7YD - OR7T
Attached is a map showing how far I managed on 1kHz earth mode this evening. Green is strong RS56 signals and orange is just copied RS33 signals, by ear. TX power 4W + 20m base electrodes, RX 80cm loop.
73s
Roger G3XBM
On 24 May 2010 20:36, Roger Lapthorn <[email protected]> wrote:
Thanks Horst.
I'll try a Marconi antenna at some point, although my space is limited to little more than the antenna I used on 500kHz (a top loaded 5m vertical).
This evening I extended the TX electrode spacings by connecting one end to the copper pipes in the house, almost doubling the effective TX baseline to about 20m. With this, my "earth mode" signals were
much stronger and I was able to cover further than before in a "by ear" walk-about test around the fields. Ultimate range tonight was 0.4kms using 4W (probably less as the TDA2003 was hot and probably turning down the power) and receiving on an 80cm
loop. active HPF, small AF amp and crystal earpiece. Range was limited by 50Hz mains hum and background sferic noise. I must try this set-up with my HF up-converter and FT817 this week which has good rejection of 50Hz (and its harmonics) mains hum and a
narrow CW filter.
73s
Roger G3XBM
On 24 May 2010 18:18, Horst Stöcker <[email protected]> wrote:
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
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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G3XBM GQRP 1678 ISWL G11088
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G3XBM GQRP 1678 ISWL G11088
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http://www.youtube.com/user/g3xbm
G3XBM GQRP 1678 ISWL G11088
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