MW-antenna directional during the nighttime, omnidirectional in the daytime

Useful information regarding antennas for SDR products.
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MW-antenna directional during the nighttime, omnidirectional in the daytime

Postby OH2BUA » Thu Jun 22, 2017 6:09 am

There exists several directional mediumwave receiving antenna concepts based on an idea that the directionality is optimized by 'tuning' an end-resistor for the best achievable Front-to-Back (F/B) ratio, and after that usually replacing it with an equivalent fixed resistor.

For example Beverage, EWE, Delta Loop and Double-KAZ, to name a few.

I chose D-KAZ (tks Neil Kazaross and Mark Durenberger) to go with at my remote countryside QTH in southern Finland (locator grid KP30), for listening North-American MW signals. (Several antennas i have there, this is one of them.)

I built it heading 325 degrees. At backside i have the strongest megawatt-level flamethrowers of the world in Eastern Europe, Caucasus and Middle-East.

I don't go here into details of D-KAZ, you can easily find a ton of information from the internet.

But pay attention into the 'null-pot resistor'. That is the critical thing to achieve directionality. Make it zero ohms, it works ok, but then it is omnidirectional. Leave it open-ended, it works ok, but again is omnidirectional. Tune it perfectly - and your antenna will have a great F/B ratio, over 40dB at best.

Very nice. But, at 325 degrees i don't have about anything else than North-America from October to March, and from sunset to sunrise only. Almost no use out of that.

Would be nice also to have a 'daytime' antenna for listening Europe.

Ok, now we are going into the subject:

Let's build it for being directional in nighttime, and omnidirectional in daytime. Let's also use an LDR (Light Dependent Resistor) when making the termination resistor.

Basic things good to know:

- Hand-optimized (for best possible F/B ratio) termination resistor for D-KAZ is usually somewhere between 800-1200 ohms.
- LDR resistance is about 100 ohms in daylight (>1000lx), but several megaohms in darkness. (Several megaohms means 'nothing' in this case.)

I carefully 'nulled' the potentiometer for Mid-Eastern backside signal at 145 degrees, on 1440kHz.
Then i measured the resistance. The multimeter read 1100 ohms.
Then, i replaced the potentiometer with a fixed 1100 ohm resistor. And, added an LDR (epoxy GL5528) in parallel with that fixed resistor. I installed the LDR heading to 230 degrees, which is the sunset direction here in mid-winter. Or well, actually i wanted it for watching away from the first rays of the rising sun.

What happened: The resistance in darkness is still 1100 ohms, but about 90 ohms in daylight.

The result? The antenna is directional during the nighttime, but omnidirectional in daytime. I can now use it also for listening Europeans at daytime, and for recording stateside overnight, in winter.

In my case the antenna is connected into the Hi-Z port of SDRplay RSP2, using 450 ohms ladder line, without balun. I think resistor-terminated RX antennas are pretty suitable for RSPs in general, as they (ants) do have quite low forward gain, meaning not too much signal usually.

73, Jukka oh2bua

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