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RE: LF: Small MF ferrite antenna games

To: "[email protected]" <[email protected]>
Subject: RE: LF: Small MF ferrite antenna games
From: Rik Strobbe <[email protected]>
Date: Tue, 2 Jun 2015 06:05:54 +0000
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Thread-topic: LF: Small MF ferrite antenna games

Hi Stefan,

 

getting a cardioid antenna using a magnetic ferrite rod (or loop) antenna and an small electrical antenna is very simple and done for ages in 80m ARDF (foxhunt) receivers. Sure it can be done with SpecLab, but it maybe overshoot.

Just to be sure: you will need to sum the signals of both antennas, not mix (multiply) them.

The most critical factor to get a deep null is to ensure that the ferrite rod  / loop antenna is properly shielded in order to avoid that it does pick up any E-field.

Effective shielding is rather easy with a ferrite rod antenna: just put the rod inside a 'sliced' aluminium tube.

 

73, Rik  ON7YD - OR7T

 


Van: [email protected] [[email protected]] namens Markus Vester [[email protected]]
Verzonden: dinsdag 2 juni 2015 0:11
Aan: [email protected]
Onderwerp: Re: LF: Small MF ferrite antenna games

Stefan,
 
>> Is it possible to 'build' a cardioid antenna by combing the signals of E and H within SpecLab? It is only a mixing of these two signals with a different 'weight', right? This could be done in the 'left to right output amp', right?
 
Yes, this should be possible, although I never actually tried it that way. The weight factor can be made positive or negative for pointing the main lobe eg. west or east. Note that you will have to adjust the analog hardware such that the phase difference between the whip and the loop channel is either 0° or 180°, so that the signal from the unwanted direction can cancel.
 
> I assume i will need a relay to select one of the two loops to feed it to the RX, but what happens if i use the combined loop signal to try that?
If you use the 90° combined signal, you still get a null in the back side of the cardioid - this is what can be selected as "single, sharp cardioid" in SpecLab's RDF menue.  Despite of having only two input channels, you can conveniently point the antenna in any direction by software. The difference to the "classic" cardioid pattern is that the "other" loop antenna also contributes signals from the sides, effectively reducing the forward gain and making the null sharper ( |cos| instead of cos^2 pattern). 
 
Usually that sharper null is more of a disadvantage, for example if you want to suppress QRN from a range of angles around the back side (eg. statics from southeast Europe). But occasionally sharpness can help to null an undesired signal coming in from a direction near that of the desired signal.
 
All the best,
Markus (DF6NM)
 
 

From: DK7FC
Sent: Monday, June 01, 2015 1:39 AM
Subject: Re: LF: Small MF ferrite antenna games

Hi Markus,

Yes yes, great ideas. I will go on step by step. First i need to configure the link in stereo mode and see if it runs stable. The mono-stream needs about 220 kBit/s. It's availability is fine now over 2.4 km even in slight rain. I assume it will work. If this runs stable then i can build a stereo-RX.
Is it possible to 'build' a cardioid antenna by combing the signals of E and H within SpecLab? It is only a mixing of these two signals with a different 'weight', right? This could be done in the 'left to right output amp', right? I assume i will need a relay to select one of the two loops to feed it to the RX, but what happens if i use the combined loop signal to try that?

73, Stefan


Am 27.05.2015 12:32, schrieb Markus Vester:
Yes Stefan, bring colour into your life ;-) Even after more than 10 years, I still sometimes enjoy just sitting and watching things roll by on the colour-RDF screen.
 
With crossed loops and an E-field antenna you have a number of options.
 
1. The colour-DF scheme takes the E-field on one channel and a 90°-combination from two loops on the second, providing omnidirectional reception with 360° RDF functionality. The combiner is simple to make, and you'll have to add a second RX channel. A stereo soundcard can be used, and SpecLab has everything needed onboard. Having a 475 kHz directional grabber available in Heidelberg would surely be very useful to the community.
 
2. Then you can hardware-combine whip and loop for a unidirectional cardioid pattern. This is equivalent to K9AY or similar antennas, which on LF have provided outstanding results for a few observers in quiet locations. The crossed loops can be used with electronic steering or a small goniometer, which allow you to virtually rotate the cardioid in the shack. Note that the phase-based colour-DF scheme still works if the cardoid antenna instead of the whip is used as intensity source. I have tried this on LF, but in my location SNR gain was not up to expectations because my loops are too much affected by noise from underground cables.
 
3. The next step would be downconverting and digitizing all three channels separately, allowing you to steer the cardioid by software and even monitor different beam headings simultaneously. But a three channel digitizer is not so easily available, and you'll need dedicated software for postprocessing. Paul Nicholson can do this at VLF, using three stereo soundcards augmented by GPS ticks, and a posteriori synchronisation in his own software.
 
All the best,
Markus (DF6NM)
-----Ursprüngliche Mitteilung-----
Von: DK7FC
<[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Mi, 27 Mai 2015 2:13 am
Betreff: LF: Small MF ferrite antenna games

MF, 

Today i've played a bit with a small ferrite antenna for 630m. 
It is 14 cm long and has 8 mm diameter. 
Without any calculations i wound a winding of unknown number of turns  
around the rod and quickly found C about 200 pF at resonance (475 kHz). 

Then i added a few turns for a 50 Ohm output (measuring the voltage of  
an induced signal at resonance. When applying 50 Ohm, the voltage has to  
decrease to 50%) to the RX. 

https://dl.dropboxusercontent.com/u/19882028/MF/small%20MF%20ferrite%20antenna.jpg 

As expected the input signal is to low, it does not rise the noise of  
the RX, so i'm missing about 20 dB. It can't be much more though because  
i can receive IQ2MI/B clearly in QRSS-30 on 476.180 kHz with 25 dB SNR  
in 122 mHz, see attachment. 

The antenna is hanging in 6m AGL in a tree in my garden... 

I'm playing with the idea of a rotary H-field RX antenna for MF, maybe  
in combination with a stereo receiver and a E-field antenna. Oh and  
maybe with a colour spectrogram as known from DF6NM. A lot of things to  
do :-) 

73, Stefan 

 PS: Yesterday, in a CW QSO with DK6SX/p on the loop he once completely  
disappered into QSO which never happend before when receiving on a  
omni-directional antenna. So i guess the colour changes in such a  
spectrogram must be quite interesting and informative... 

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