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
Useful information regarding antennas for SDR products.
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