Message-ID: <F4839781003A4ABDA02E5B6D4CCE9329@MJUPC>
From: "Mike Underhill" <mike@underhill.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <500EB973.8080005@iup.uni-heidelberg.de>
In-Reply-To: <500EB973.8080005@iup.uni-heidelberg.de>
Date: Tue, 24 Jul 2012 19:37:53 +0100
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Subject: LF: Re: Pretests for the 630m band dipole
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Stefan

As a long time 'lurker' on this LF group I am particularly interested in 
what orientation you plan for your low height 630m band dipole antenna and 
what azimuthal radiation pattern you find.  A few years ago I did some work 
(on a contract) on low height, on-the-ground and underground antennas in the 
1 to 10MHz region.  The underground antenna was an end fed wire about 45m 
long in a 10cm 'land drain' in damp clay soil at a depth of about 45cm. 
(The wire was put in the drain using five sets of 10 drain rods.)  These 
antennas radiated quite successfully with received and transmitted signals 
generally about 10 to 30dB down on a reference horizontal 83m loop at 15m 
height.  Some of the tests were performed  and winessed on a regular Sunday 
morning net on 3722kHz.

Ocasionally broadcast signals in the 31m and 25m bands were found to be be 
stronger on the underground antenna than on the reference antenna.  So local 
field strength measurements were then made just above ground on the 
under-ground and on-ground (dipole) antennas.  A 35cm tuned loop antenna on 
an AOR hand-held scanning receiver was use as the field sensor.  This could 
sense the field polarisation as well as the field strength.  The conclusions 
were that such antennas "radiate and receive best from the ends with 
vertical polarisation".  This was confirmed earler this year at the IET 
Ionnospheric Radio conference in York in a conversation with the conference 
chairman Les Barclay, G3HTF.  He had found similar results some years 
earlier.

The interesting question is why this might be so.  The following is now 
offered as a possible explanation:

Electric field lines above the antenna extend to a height that is a 
substantial fraction of (the square root of)  a wavelength and terminating 
on the two ends of the antenna.  This electric field creates displacement 
currents as half loops above the antenna from one end of the antenna to the 
other.  The half loops of displacement current radiate with vertical 
polarisation with maxima in the direction of the ends of the dipole antenna. 
As the antenna height is raised the E-field lines and displacement currents 
become established below the antenna and above the ground.  The currents are 
in the same direction  as the currents above the antenna and so they start 
to cancel the loop radiation from the upper half  loop(s) as the antenna is 
raised.  At some critical height the conventional dipole mode takes over and 
the loop mode becomes suppressed.  The critical height has been found to 
proportional to the square root of wavelength in accordance with the 
properties discovered for 'electromagnetic coupling'.  (Measurements have 
been made for antennas at different heights and form the minimum horn size 
variation with frequency for the Goubau single wire transmission line.) 
>From measurements at higher HF frequencies I estimate the critical 
change-over height to be about 10 to 15 meters at 470kHz or 500kHz.  At 
136kHz these figures are increased by the square root of the freqency ratio, 
which is about 1.9.  Real height of the antenna is important!

Whether this hypothesis or suggested theory is correct depends on real and 
not simulated measurements.  So you can guess the reason for my interest.

Looking forward to your results with interest, so I can go back to lurking 
either with a warm glow of satisfaction or with a red face of embarrassment 
and disappointment.

Mike - G3LHZ

----- Original Message ----- 
From: "Stefan Schäfer" <Stefan.Schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, July 24, 2012 4:04 PM
Subject: LF: Pretests for the 630m band dipole


> MF,
>
> This week i intend to start my tests with a full size dipole for the 630m 
> band. All the equipment for this /p is available now.
>
> Since the dipole is VERY close to ground (relative to Lambda) there will 
> be a capacitive component and the Z=R at resonance will be lower than 36 
> Ohm of course.
> I have just done some simulations in EZNEC and found that it is useful to 
> feed the dipole assymetrical, i.e.  not in the center. The simulation 
> tells that a wire length of 315m will be necessary. The resonance 
> frequency does not change by different feed points. I hope this applies in 
> practice as well :-)
> The electrical parameters will depend on the soil conductivity and epsilon 
> r, which i don't know exactly of course. But if the settings in the 
> simulation are reasonable, then there will be a real component of Z=R 
> (=resonance)of 46 Ohm. So i would not even need a transformer to match the 
> antenna to the PA.
>
> 85% means that one wire is "just" 47m. So if someone lives close to a 
> forest, there would be a chance to build such a TX antenna and run the 
> major part of the antenna outside the garden. Just a short rest has to be 
> inside the own property. For a 90% feed, R would be 96 Ohm. This could be 
> matched to 50 Ohm very easily and the shorter part of the antenna would 
> only be 31.5m...
>
> I have a bundle of old telephone wire (once found a 1800*0.4mm  x 10m 
> length cable) and so i can connect several pieces in series for the test. 
> So i can make a table of measurements. The modified MFJ-259B antenna 
> analyzer will help, hopefully.
> Looking forward to that test.
>
> Due to the permanent number of OP4 listening stations, this mode may 
> actually help to get an impression about the efficiency and directivity.
>
> 73, Stefan/DK7FC
>
>
>