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Re: LF: Re: Pretests for the 630m band dipole

To: [email protected]
Subject: Re: LF: Re: Pretests for the 630m band dipole
From: Stefan Schäfer <[email protected]>
Date: Tue, 24 Jul 2012 22:51:39 +0200
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Hello Mike,

Thank you for your answer. I am sure there are plenty of 'lurkers' on THIS reflector :-))

Am 24.07.2012 20:37, schrieb Mike Underhill:
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.
Well, i still don't know the actual location where i want to test the antenna. I have some ideas though. I want to try to get a antenna orientation beaming to G and PA, where the most RX stations are present. Anyway, the tests will be temporary, i.e. i will remove the antenna after a few hours of testing. So if the project looks promising after the first tests i will certainly repeat it at different locations. I also want to try different locations because i want to see the effect of different ground conductivities, so one test will be done close to a lake (Neckar, Heidelberg, maybe this location: http://no.nonsense.ee/qthmap/?qth=JN49HK81MN&from=JN49HK80RG) and another on a stony hill. There will be many tests and hopefully the radiated signal is acceptable so it is an alternative for many OMs with limited estate... I will also try different heights above ground, starting from 0 to maybe 2m or 5m AGL.


Ocasionally broadcast signals in the 31m and 25m bands were found to be be stronger on the underground antenna than on the reference antenna.
A stronger S/N or a stronger signal level?

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 may be an idea to test on MF too. Elevated ends as a compromise of effort compared to a completely lifted antenna. But first i want to see if the efficiency is at least in thge range of 0.5%, otherwise it is not worth the effort i find. 100W RF power may be the absolute limit for a /p setup...

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
Oooh i think there will be an excessive discussion on this description by other amateur here in thge group. Or not? I can imagine a positive effect from lifted ands since the voltages and the impedance is maximal at the ends. So the negative effect of the conductiong soil, imagined as a parallel loss resistor will be reduced by that. But things may be much more complex on LF and MF compared to HF, due to the skin effect. I must admit that i am rather focused on practical tests than by reading in the literature. Also there are some very good teachers in THIS group.

I wonder if this interesting discussion will be copied by someone to yahoo to suggest that the real LF/MF discussions are done there, whereas the "getting rid of..." messages from the "live and let die"-minded fraction are permanently copied here... (Sorry).

change-over height to be about 10 to 15 meters at 470kHz or 500kHz.
That's not so easy but possible anyway.
BTW it is not at all easy to evaluate the differences of such different antenna arrangements. I have never seen such a QSB on any band. Probably it is the best to have a local RX station and do the tests on groundwave distance. Probably the antenna radiates in a very high angle But the difference (of the simulation) between 90 deg and 20 deg is just 5 dB while the antenna gain is stated to be -5 dBi...


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.

OK Mike, go back lurking, HI. And thank you for the nice words. I'm glad to be able to pull some lurkers out of the background from time to time :-)

As i said the results may be very different to your observations on the 31m and 25m band. But there will be results :-)

Best 73 and i hope we can discuss the results in a QSO on the band in 2013!

Stefan/DK7FC




Mike - G3LHZ



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