Hi Stefan yes that could be a problem. I have a gut feeling that being short
(in a vertical sense) they will be almost isotropic. You may need to get
someone to drive around at about 5km to check that out. The size relative
to a wavelength might confound this idea.
All good stuff and L(otsa) F(un)
Alan G3NYK
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Tuesday, October 12, 2010 11:26 PM
Subject: Re: LF: Re: Questions and resonating tests and measurements on a
700m wire antenna at 137 kHz
Thank you Alan for your reply.
One more difficulty could be the different radiation pattern of the two
configurations. So the signal difference at a specific location must be
seen with care. If the signal decreases by say 6...10 dB at different
RX stations (in N, S, W, E) one could get a doubtless information which
configuration is better. But probably the results will not be so clear ;-)
73, Stefan
Am 13.10.2010 00:02, schrieb Alan Melia:
> Hi Stefan. I believe this antenna is through trees and foliage. This may
> mean the measurements maybe not what you think they are. The voltage on
the
> grounded end configuration will probably be lower. this may mean that you
> have less of the power going though the tree foliage. In the ungrounded
> state you have a lot more of your power leaking away from the foliage
before
> it gets to the end. It might be that this shunt path lowers the apparent
> resistance. The only way to test the efficiencies of the two
configurations
> is to get a relaible comparative field strength measurement. It probably
> doesnt really matter if the receiver is not accurately calibrated provided
> you can get a a reading of the dBs difference. This doesnt affect the
> resonance tests you have done to determine how to get the most current
> flowing in the wire.I dont think you can sensibly calculate ERP in this
> sytem. Of course the length is now a significant portion of a wavelength
(~
> 3/8th ) so this has implications too.
>
> Interesting experiment I await the recieve measurements with interest.
>
> Alan G3NYK
>
> ----- Original Message -----
> From: "Stefan Schäfer"<[email protected]>
> To:<[email protected]>
> Sent: Tuesday, October 12, 2010 9:32 PM
> Subject: LF: Questions and resonating tests and measurements on a 700m
wire
> antenna at 137 kHz
>
>
> Dear LF,
>
> Today i have done some measurements on my "earth antenna" at 137 kHz.
> The antenna was used as a ground loop antenna (far end grounded) and as
> a inv-L antenna (far end ungrounded).
>
> Winthin the last days the antenna was optimised. Now the first 250m are
> about 8 m above ground (average). The wire was replaced by a olive-green
> military wire (steel-copper-silver, UV resistant insulation) which makes
> the antenna rather invisible. The rest of the antenna is still in a
> height of about 4m (average). I assume these 4m height difference could
> make a significant difference in the efficiency on LF (not so much on
VLF).
>
> The H Bridge PA was used for the tests. This PA can fed the antenna from
> 10 Hz (!) to 200 kHz as seen today. The applied power (DC input) in the
> tests was about 25 W. A 5 pole low pass filter was applied on the PA
output.
>
> First i wanted to measure u(t), i(t) and the phase by an oscilloscope
> but this is not really running sufficient when supplied by the generator
> (trigger and display problems). So i choose the LF tuning meter (ON7YDs
> website) which i built in 2003.
>
> When configuring the wire (abt 700m length and abt 600m electrode
> spacing) as a ground loop antenna, it is resonated with a series L (not
> C!) of about 800 µH. Then, its Z = 840 Ohm. No matter what the radiation
> resistance is. Field strength measurements on my LF grabber make
> probably no sense due to the low distance. I have to compare both
> configurations in a further test, maybe this weekend.
>
> After disconnecting the ground rods at the far end the antenna (now
> inv-L config) was resonated by using a L of just 250 µH (estimation).
> The Impedance Z was then measured to be 440 Ohm! (U=73,3 V * I=166mA).
>
> What does these values tell us? I have not yet completely understand the
> whole dependency i think. _So i am looking forward to your ideas and
> comments!
>
> _It seems as if the losses have been reduced in the inv-L case. But what
> about the radiation resistance? How can we compare those different
> antenna types?
> The ground losses on VLF and down to 10 Hz (quasi DC) are about 700 Ohm,
> today. Since the antenna is _not_ short against lambda (i.e. it is even
> longer than lambda/4), the antenna acts not like a pure loop and not as
> a usual back garden LF antenna. Additionally, the ground losses are very
> high here! The ground electrode on the fed point could be about 350 Ohm,
> so 50% of the total loop losses on VLF. But, theoretically, the
> radiation resistance could be some Ohms (?) due to the antenna length.
> If it will become longer (...lambda/2), the ground electrodes current
> decreases, and so the losses? Quite difficult and interesting, isn't it?
>
> My idea is just to do further tests, hoping to be copied at many LF
> stations, since this is the final goal :-) A simulation seems to be very
> difficult, due to the many unknown parameters...
>
> I want to find what is the best configuration (loop or inv-L) and i want
> to increase the antenna to 1000 m length! :-) Then i will build a tuner
> with fixed components. This will be an easy job since the voltages and
> currents are moderate! :-) The impedance should be transformed to 50 Ohm
> (sincei have a 25m long RG58 cable to the class E TX that should be used
> later) and i hope i can improve the RX for that antenna.
>
>
> 73, Stefan/DK7FC
>
>
>
>
>
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